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Journal of Bacteriology, February 2000, p. 1016-1023, Vol. 182, No. 4
Department of Biology, Wesleyan University,
Middletown, Connecticut 06459,1 and
Department of Biological Sciences, University of Sussex,
Brighton BN1 9QG,2 and Department of
Biological Sciences, University of Warwick, Coventry CV4
7AL,3 United Kingdom
Received 9 August 1999/Accepted 24 November 1999
Interspecies genetic exchange is an important evolutionary
mechanism in bacteria. It allows rapid acquisition of novel functions by transmission of adaptive genes between related species. However, the
frequency of homologous recombination between bacterial species decreases sharply with the extent of DNA sequence divergence between the donor and the recipient. In Bacillus and
Escherichia, this sexual isolation has been shown to be an
exponential function of sequence divergence. Here we demonstrate that
sexual isolation in transformation between Streptococcus
pneumoniae recipient strains and donor DNA from related strains
and species follows the described exponential relationship. We show
that the Hex mismatch repair system poses a significant barrier to
recombination over the entire range of sequence divergence (0.6 to
27%) investigated. Although mismatch repair becomes partially
saturated, it is responsible for 34% of the observed sexual isolation.
This is greater than the role of mismatch repair in
Bacillus but less than that in Escherichia. The
remaining non-Hex-mediated barrier to recombination can be provided by
a variety of mechanisms. We discuss the possible additional mechanisms
of sexual isolation, in view of earlier findings from
Bacillus, Escherichia, and
Streptococcus.
Bacteria from all major taxa are
able to exchange genes across species by homologous recombination
(26). While the various bacteria take up donor DNA by a
diversity of mechanisms, all studied systems of homologous
recombination share at least one homologous feature: recombination
depends ultimately on the activity of the RecA protein and its
homologues (26). Similarly, there is one system that hinders
recombination across species in both the Proteobacteria and
the gram-positive bacteria (3). This is the mismatch repair system encoded by mutS, mutL, and their
homologues. Because some of the molecular basis for interspecies
recombination is shared across disparate taxa, we might expect that
recombination between species is constrained in similar ways throughout
the bacterial world.
In addition, previous studies have shown that the frequency of
homologous recombination decreases with the sequence divergence between
donor and recipient, in a manner that is similar across a wide range of
organisms. In Bacillus transformation as well as in
Escherichia conjugation, the frequency of recombination decreases exponentially with the degree of DNA sequence divergence between donor and recipient (23, 28, 31). A similar
exponential relationship has been observed for the frequency of
intrachromosomal crossovers in Saccharomyces cerevisiae
(5). Nevertheless, the major mechanisms producing
recombinational barriers have been shown to differ in each of the above
cases. In Escherichia coli, the predominant barrier to
recombination is presented by the methylation-directed mismatch repair
system (21). In Bacillus, mismatch repair is only
marginally effective in preventing recombination between divergent
sequences; the most significant barrier is that donor strand invasion
and initiation of strand exchange require near identity between donor
and recipient at both ends of the donor segment (15, 16). In
yeast, the effects of mismatch repair and sequence divergence are both
significant recombinational barriers (5).
Whereas an exponential relationship between interspecies recombination
and sequence divergence appears to be universal, it is striking how
much more mismatch repair contributes to recombination barriers in
Escherichia than in Bacillus. It is not clear
whether the role of mismatch repair in Escherichia or
Bacillus is typical and under what circumstances mismatch
repair is most likely to evolve to be a significant barrier to
interspecies recombination. More comparative work is needed to
understand the evolution of mismatch repair and its role as an
interspecies recombinational barrier. In this paper, we approach this
problem by investigating another system of microbial recombination:
natural transformation in Streptococcus pneumoniae. We test
whether the exponential relationship between sequence divergence and
sexual isolation, observed in Bacillus and
Escherichia, also holds for transformation in
Streptococcus. We also investigate the mechanisms of sexual
isolation between Streptococcus species.
S. pneumoniae is a gram-positive bacterium, naturally
competent for transformation. The Streptococcus HexAB
mismatch repair system is homologous to MutSL (3) and has
been shown to correct single mismatches arising during recombination
between otherwise identical substrates. The HexA protein recognizes
mismatches in heteroduplex DNA, and, for transformation by mismatched
DNA, it is able to remove the entire donor strand and thus prevent
recombination (3). However, while the role of the Hex system
in detecting single mismatches is well documented, its effectiveness in
preventing recombination between divergent species is not yet fully
known. There is evidence that the system is easily saturated in the
presence of multiply mismatched DNA and may not be able to provide an
efficient recombinational barrier (13).
In this study, we investigated the extent to which DNA sequence
divergence reduces the rates of recombination between
Streptococcus species, as well as the role that the HexAB
mismatch repair system plays in producing sexual isolation (i.e.,
resistance to recombination across species). We transformed three
recipient strains, two wild-type strains and one mismatch
repair-deficient derivative, with chromosomal DNA isolated from related
Streptococcus strains marked with rifampin resistance.
Consequently, we determined the relationship between transformation
frequencies and DNA sequence divergence, in the presence and absence of
mismatch repair.
Strains.
Strains used in this study are listed in Table
1. We used S. pneumoniae
strains Pn16 (highly competent wild-type isolate) and R6
(unencapsulated laboratory strain) as recipients to determine the
relationship between sequence divergence and sexual isolation in
wild-type strains. We used strain R6
0021-9193/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.
Barriers to Genetic Exchange between Bacterial
Species: Streptococcus pneumoniae Transformation
ABSTRACT
Top
Abstract
Introduction
Materials and Methods
Results
Discussion
References
INTRODUCTION
Top
Abstract
Introduction
Materials and Methods
Results
Discussion
References
MATERIALS AND METHODS
Top
Abstract
Introduction
Materials and Methods
Results
Discussion
References
3, a mismatch repair mutant of
strain R6 with a deletion in the hexA gene, as a recipient in investigating the role of mismatch repair in sexual isolation. The
remaining strains, used as donors, were obtained from the National
Collection of Type Cultures (NCTC; Colindale, United Kingdom) and the
American Type Culture Collection (ATCC; Manassas, Va.).
TABLE 1.
List of strains
Isolation of rifampin-resistant mutants. Strains used as DNA donors were streaked onto plates from glycerol stocks and incubated overnight. A single colony was picked and spread onto a 2- to 3-cm square on a fresh plate. Following a further 24 h of growth, the square was harvested onto a swab and spread over a fresh plate containing 10 µg of rifampin/ml. Resistant colonies appeared within 2 to 3 days.
Isolation of chromosomal DNA. Bacteria were harvested from eight 90-mm-diameter brain heart infusion (BHI)-blood (5%)-agar plates using a plastic loop and resuspended in 1 ml of 50 mM Tris-10 mM EDTA, pH 8.0. After addition of 100 µl of 10-mg/ml lysozyme and 5 µl of RNase (Boehringer), the cells were incubated at 37°C for 30 to 60 min, followed by addition of 150 µl of 100-mg/ml proteinase K and a further 30 min of incubation. Cells were lysed by addition of 100 µl of 20% (wt/vol) sarcosyl and incubated at 37°C until lysis was complete, indicated by the clearing of the cell suspension, subject to a maximum time of 2 h. The resulting lysate was subjected to two extractions with phenol-chloroform and a further extraction with chloroform. The DNA was then precipitated by the addition of 1/10 volume of 3 M sodium acetate, pH 3.5, and 1 volume of isopropanol. The DNA was collected by centrifugation in a microcentrifuge at 13,000 × g for 15 min, and the resulting DNA pellet was washed with 70% ethanol, air dried briefly, and resuspended in 200 µl of deionized water. The quality and quantity of the DNA were determined by spectrophotometry and confirmed by visualization on an agarose gel. All samples had ratio of optical density at 260 nm to that at 280 nm of between 1.8 and 2.0.
Preparation of competent cells and transformation. A single colony was inoculated into 3 to 4 ml of C medium (11a) and grown at 37°C until the solution became slightly cloudy (approximately 3 h). A 100-µl aliquot was then used to inoculate 7 ml of prewarmed C medium, and the mixture was incubated for 1 3/4 h. At 10-min intervals, 425 µl was removed and added to 75 µl of sterile glycerol; the solution was mixed, and a 20-µl sample was removed to a fresh tube. Both samples were frozen on dry ice. After 2 h, sampling was stopped.
Cells sampled at each time point were tested for competence as follows. Each 20-µl sample was thawed and added to 480 µl of C medium, and the mixture was split into two 250-µl aliquots. One microgram of chromosomal DNA from the rifampin-resistant isogenic strain was added to one sample. Cells were incubated for 2 h at 37°C, and 50 µl was plated onto BHI-agar supplemented with 10 µg of rifampin/ml. Competence was induced over a 40- to 60-min period, shown by an increasing number of transformants during the competent phase. The time of onset of competence depended upon the strain and the optical density of the original inoculum. In subsequent experiments, the competent cells were partially thawed on ice and 80 µl was added to 1,920 µl of C medium. The remaining cells were refrozen on dry ice and stored at
80°C. The diluted competent cells were split into 150-µl
aliquots, and 0.6 µg of donor DNA was added (for the negative
control, no DNA was added). Following incubation at 37°C for 2 h, cells were serially diluted in BHI broth and plated on selective
medium. The sexual isolation between a recipient and a test donor was
quantified as the frequency of homogamic transformation (the frequency
at which the recipient was transformed by its own Rifr
mutant) divided by the frequency of heterogamic transformation (the
frequency at which the recipient was transformed by the test donor).
Estimate of sequence divergence at rpoB. We used PCR to amplify two fragments of the rpoB genes from all the strains used in the experiments. The first fragment, extending from base 372 to 1160 (corresponding to base numbers of Streptococcus pyogenes ATCC 700294 rpoB [http://www.genome.ou.edu]), was amplified using degenerate primers 5'-GGG ACG TTC GTN ATH AAY GG-3', for the leading strand, and 5'-CCA AGG TGG TCD ATR TCR TC-3', for the lagging strand. The second fragment, base 1440 to 2380, was amplified using primers 5'-TTG TCA CAR TTY ATG GAY CA-3', for the leading strand, and 5'-TCG CGA GTG ATY TCY TCM GG-3', for the lagging strand. All primers were designed using the rpoB alignment of Bacillus subtilis (2) and S. pneumoniae (C. G. Dowson, unpublished data). PCRs were carried out using Taq DNA polymerase (Qiagen) and consisted of 95°C denaturation for 30 s and 57°C annealing for 30 s, followed by extension at 72°C for 1 min, for 25 cycles. The PCR products were purified using Qiaquick PCR purification columns (Qiagen) and adjusted to standard concentrations. They were sequenced using the external primers (described above) at the DNA Sequencing Facility at the University of Pennsylvania Medical Center, by the dRhodamine dye terminator method with ABI PRISM 373XL and 377 sequencers (Perkin-Elmer, Applied Biosystems Division). The result was a full double-stranded sequence of 510 bp for the first fragment, and 872 bp for the second fragment, for all tested strains. The combined sequence divergence of the two fragments was used as the estimate of DNA sequence divergence in the rpoB region. We have earlier found that, in Bacillus, the above fragments give a good estimate of interspecies divergence across the entire gene (15). In addition, rpoB is located within the highly conserved ribosomal gene cluster. In Streptococcus, rpoB is directly flanked by rpoC and is in close proximity to rpoA (the other subunits of RNA polymerase), along with several ribosomal protein genes (data from the ongoing S. pyogenes sequencing project is available at http://www.genome.ou.edu). Since all the above genes are highly conserved, the sequence divergence at rpoB is representative of divergence within the entire region throughout which the recombinant molecules are expected to extend.
Nucleotide sequence accession numbers. The sequences obtained in this study have been assigned GenBank accession no. AF194507 to AF194528.
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RESULTS |
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Abstract
Introduction
Materials and Methods
Results
Discussion
References
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Sequence analysis of donor and recipient strains.
The
sequence divergence values and phylogenetic relationships
among the strains are shown in Table 2
and Fig. 1. We used maximum parsimony
along with bootstrap analysis based on 1,000 replicates to determine
the phylogenetic relationships among all strains, based on the partial
rpoB sequence data. The phylogeny agrees with that
determined earlier using 16S rRNA and sodA1 data (14,
20), with the exception of the clustering within the Streptococcus anginosus group. The analysis of
rpoB shows Streptococcus constellatus to be most
closely related to S. anginosus, whereas according to
earlier data (16S rRNA and sodA1) S. anginosus
forms a clade with Streptococcus intermedius. This
discrepancy is supported by high bootstrap confidence values and might
indicate a recombination event in the vicinity of rpoB.
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3). Among the rifampin-resistant donors, we noted His526
substitutions, also known to confer resistance in Bacillus,
Mycobacterium, and Escherichia (8,
30). We identified Asp526 (S. pneumoniae Pn16,
S. intermedius) and Tyr526 (Streptococcus oralis,
S. pneumoniae strain R6, S. pneumoniae strain
96129) substitutions.
Relationship between sexual isolation and DNA sequence
divergence.
The transformation frequencies for each donor and
recipient, along with the corresponding sexual isolation values, are
given in Tables 3 and
4. The relationship between sexual
isolation and sequence divergence between the donor and the recipient
is shown in Fig. 2. We find that the fit
of the log-transformed sexual isolation data
(R2 = 0.94 for the wild type, and
R2 = 0.93 for the Hex
recipient) is significantly better than a corresponding fit of raw,
non-log-transformed data (R2 = 0.35 for the
wild type; R2 = 0.31 for the
Hex recipient). That is, the relationship between sexual
isolation (
) and sequence divergence (
) in
Streptococcus is close to exponential and can be described
by the formula
|
(1) |
, represented by the
regression slopes in Fig. 1, are 17.82 and 18.39 for the Pn16 and R6
strains, respectively. Analysis of covariance shows that these two
values are not significantly different (F = 0.6; P = 0.45). This suggests that the sensitivity of sexual isolation to
sequence divergence is constant across wild-type S. pneumoniae strains, despite the fact that strain Pn16 exhibits a
baseline (homogamic) transformation rate eightfold higher than that of strain R6 (Table 3).
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The effect of mismatch repair on heterospecific
transformation.
The mismatch repair-deficient mutant exhibits
sexual isolation consistently lower than that of its isogenic wild-type
strain: for the wild type (wt) = 18.39; for
the Hex mutant (Hex
) = 11.70. Analysis of covariance shows these coefficients to be
statistically significant (F = 53.8; P < 0.0001).
Averaging over all donor strains, mismatch repair is responsible for
34% of the observed sexual isolation (i.e., [wt Hex]/wt · 100%). For
every donor, with the exception of S. oralis, sexual
isolation values are higher for the wild-type recipient than for the
mismatch repair mutant. The influence of mismatch repair on integration
frequencies varies across strains, increasing the sexual isolation by
up to 36-fold, for Streptococcus sanguis. Transformation
with Streptococcus adjacens (Abiotrophia
adjacens) DNA, which is 27.4% divergent from that of the
recipient at rpoB, produced no detectable transformants in
the presence of mismatch repair, while the mismatch-deficient strain
produced transformants at a sexual isolation of 3,800. Thus, mismatch
repair plays a role in preventing recombination between divergent
Streptococcus species, across the entire spectrum of
sequence divergence tested (0.7 to 27.4%). However, the effectiveness of the system in detecting multiple mismatches is significantly reduced, compared to its effectiveness against single mismatches (see
Discussion). Our results show this reduction to be most pronounced at
intermediate sequence divergence levels (S. oralis). The
lack of a detectable mismatch repair function in preventing integration of S. oralis DNA may represent the saturation effect
experienced by the Hex system in the presence of partially divergent
DNA, as described by Humbert et al. (13).
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DISCUSSION |
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Top
Abstract
Introduction
Materials and Methods
Results
Discussion
References
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The relationship between sequence divergence and sexual
isolation.
Our study shows that S. pneumoniae
experiences sexual isolation from related strains and species. Sexual
isolation in Streptococcus increases approximately
exponentially with DNA sequence divergence, as has previously been
observed within Bacillus and Escherichia. It is
notable that Streptococcus and Bacillus share not
only the exponential form of the relationship but also the same level
of sensitivity of sexual isolation to sequence divergence. The
sensitivities of sexual isolation to sequence divergence parameters
() for transformation of wild-type Streptococcus and
Bacillus strains are 18.1 (average for Pn16 and R6
recipients) and 21.37 (15), respectively. These figures are
remarkably close. To further illustrate this trend, the sexual
isolation () between S. pneumoniae and S. sanguis is 1,920 at 15.1% sequence divergence. Within
Bacillus, the corresponding sexual isolation between
B. subtilis and Bacillus licheniformis is 2,344 at 14.5% divergence (15). It is possible that maintenance
of comparable levels of sexual isolation arises from similar
evolutionary pressures on recombination frequencies within each genus.
Possible barriers to interspecies recombination. An interspecies recombination event may be broken down into the following steps: (i) donor DNA is taken up by the recipient cell, (ii) donor DNA escapes the recipient cell's restriction system, (iii) a donor-recipient DNA heteroduplex molecule is formed, (iv) the mismatched heteroduplex escapes the action of the mismatch repair system, and (v) the recombinant strand is replicated by the recipient's DNA replication machinery. We present a short discussion of the relevance of the above steps as mechanisms of sexual isolation in Streptococcus.
The initial DNA uptake step can be dismissed as a genetic barrier in our system. Although in some bacterial species, such as Haemophilus influenzae (26), DNA uptake has been shown to be sequence specific, in the known gram-positive species (Streptococcus and Bacillus) the rate of uptake is independent of the sequence and is hence not sensitive to the sequence divergence between the donor and the recipient (18, 26). Thus, in Streptococcus, DNA uptake is unlikely to be a source of sexual isolation. The donor DNA must then escape the action of the recipient's restriction endonucleases. The recipient strains used in the experiments described in this paper do not harbor restriction/modification systems. Thus, in this particular case, neutralization of donor DNA by the recipient's restriction enzymes cannot control recombination rates. In general, the effect of restriction systems has been shown to introduce a constant reduction in recombination frequencies, independent of the divergence between the donor and the recipient (31). In Bacillus, this effect is small (possibly because the single-stranded donor DNA taken up on transformation is not subject to digestion), amounting to at most a sixfold reduction in recombination frequencies for restricting strains compared to those for nonrestricting strains (31). Similarly, in conjugation between Salmonella and Escherichia cells, sexual isolation is not significantly influenced by the presence of type II restriction (17). Therefore, in Streptococcus, restriction is not expected to be a source of the exponential relationship between sequence divergence and sexual isolation (18). The two subsequent steps in the recombination process, namely, the formation of a heteroduplex molecule and the evasion of the recipient's mismatch repair system, present the most likely barrier to interspecies recombination. A wealth of information on the formation of recombinant DNA molecules is available from both in vivo and in vitro studies of E. coli and S. cerevisiae. In E. coli initial processing of the donor recombinant substrate is often necessary to produce free single-stranded ends. This is carried out by the RecBCD enzyme, which unwinds and digests double-stranded DNA until it encounters a characteristic
sequence. The process results
in the creation of free 3' single-stranded DNA ends. All further
recombinogenic processes depend on the action of the RecA protein in
binding to the single-stranded DNA and initiating invasion into a
homologous and highly conserved (24) region of the
recipient's double-stranded DNA molecule. (Although 5' donor ends can
also be invasive, they cannot be extended and do not initiate
recombination [10, 22].) The 3' recombinant joints are
then subject to extension and editing (see reference 27 for a recent discussion of the above processes).
In E. coli, (27) all modes of DNA editing, while
involving several different components, appear to be controlled by the
MutS protein (homologue of the Streptococcus HexA protein).
Hence, the HexA mutant used in our experiments is expected to lack all
mismatch repair activity.
The importance of RecA and HexAB functions in promoting and preventing
recombination between divergent substrates is well documented (18,
26), and we discuss those events in detail, as pertinent to
Streptococcus transformation, in separate sections below. In
fact, we argue that RecA-dependent and HexAB-dependent steps are likely
to produce most of the sexual isolation in Streptococcus.
On the other hand, since during transformation the donor DNA enters the
cell as a single-stranded molecule, no processing by RecBCD is
necessary. In fact, no known homologues of the RecD protein exist in
gram-positive bacteria, while the function of the AddA and AddB enzymes
in Bacillus (RecB and RecC homologues) seems to be limited
to UV damage repair and might not be required during transformation
(31). Thus, it is unlikely that DNA processing prior to
recombination is involved in preventing interspecies recombination in
Streptococcus.
In later stages, postsynaptic events such as branch migration and DNA
replication may influence the frequency of successful recombination.
Branch migration, which extends the initial heteroduplex molecule, is
promoted by the RuvA, RuvB, and RuvC proteins (26). RuvA
mutants of E. coli exhibit a reduced frequency of
conjugational recombination. However, the reduction is relatively small
and appears not to be very sensitive to DNA sequence divergence. Matic et al. (17) demonstrated that, in intraspecific crosses, the recombination frequency is reduced by a factor of 4, whereas in interspecific conjugation (between E. coli and
Salmonella enterica serovar Typhimurium) it is reduced by a
factor of about 10 (27). This corresponds to a sexual
isolation factor due to the RuvA pathway of at most 2.5. In addition,
Majewski and Cohan (16) demonstrated that, in
Bacillus, by providing regions of flanking identity to the
recipient, they were able to force a divergent donor insert to
recombine at a nearly homogamic frequency. Similarly, the RecG protein,
which is involved in the resolution of Holiday junctions, has
comparable effects in intraspecific and interspecific crosses in
E. coli (17). Thus, branch migration is at most
slightly impeded by mismatches and should not contribute considerably
to sexual isolation between species.
Lastly, DNA synthesis may be initiated from the early heteroduplex
intermediates, preventing dissociation of such potentially unstable
joints. Stambuk and Radman (27) showed that mutations in
E. coli priA and recF genes, which are known to
be involved in recombination-associated DNA synthesis, have
differential effects in interspecific and intraspecific crosses. It is
possible that de novo DNA synthesis is necessary to circumvent the
potential dissociation of early heteroduplex intermediates. It is
possible that such DNA replication events may be impeded by mismatched heteroduplex joints. However, in E. coli, disrupting the
RecF pathway reduces recombination frequencies by not more than sixfold in interspecies recombination and about twofold in intraspecies events.
Hence the sexual isolation produced by mismatches impeding the
RecF-mediated processes is at the most a factor of 3. In addition, Zawadzki et al. (31) found no measurable effect of RecF in
preventing interspecific transformation in Bacillus. Thus,
DNA synthesis induced by recombination is likely to be only a minor
source of sexual isolation in Streptococcus.
Mismatch repair as recombination barrier. Our results show that the Hex mismatch repair system plays a significant role in preventing recombination between divergent DNA sequences. The extent of its significance is intermediate between that observed in Escherichia, where mismatch repair is the major recombinational barrier, and Bacillus, where it plays only a minor part. It has been suggested (15) that the relative ineffectiveness of the Bacillus mismatch repair system may be caused by degradation of mismatch repair proteins, which occurs under starvation conditions (9) necessary for induction of competence. In Streptococcus, competence is not induced under starvation, and thus mismatch repair proteins are probably not degraded (13). This may explain the more significant role of mismatch repair in preventing interspecific recombination in Streptococcus than in Bacillus.
However, the Streptococcus mismatch repair is still a rather poor mechanism of sexual isolation. The Hex system has been shown to efficiently correct specific single-base pair mismatches, known as low efficiency (LE) markers, resulting from transformation, with an accuracy of over 90% (4). It is believed that once a mismatch is detected, the entire donor strand, which may be as large as several kilobases, is rejected (3). Let us consider a simple model, assuming that, in a donor strand containing several (m) mismatches, detection of every mismatch is an independent event. Hence the probability of the entire strand escaping detection is equal to the probability of m mismatches being undetected. We further assume that, on average, one in every four mismatches is an LE mismatch (4, 11), escaping detection with a probability of 0.1, while others are high-efficiency (HE) mismatches, always escaping detection. The probability of a foreign strand escaping the mismatch repair system can then be expressed as (0.1)m/4 or (0.1)L
/4,
where L is the length of the donor strand and is the
sequence divergence. Taking into account that the length of donor DNA
integrated on transformations is often greater than 8 kb (1,
3), we should expect that the reduction in transformation
frequency between 15%-divergent DNA strands, caused by mismatch
repair, should be of the order of 10300.
In our experiments, the Hex system reduces the recombination
frequencies by not more than 36-fold (S. sanguis, 15%
sequence divergence from recipient). It is obvious that the
effectiveness of mismatch repair against multiply mismatched sequences
is greatly reduced compared to its effectiveness against single
mismatches. Hence, the simple model described above is incorrect, and
detection events cannot be considered independent. We present three
explanations for the decrease in efficiency of the Hex system in the
presence of multiple mismatches.
First, it is possible, that a saturation of mismatch repair may be
reducing the effectiveness of the Hex system. Humbert et al.
(13) have previously shown that the concentrations of HexA and HexB proteins become limiting during transformation with divergent DNA. They found that mismatch repair is ineffective in preventing transformation by DNA with intermediate sequence divergence levels (1 to 10%) from S. pneumoniae and suggested that saturation of the system may be responsible. Independent recombination events at loci
other than the selected locus may reduce the number of mismatch repair
proteins available to prevent integration of the selectable marker.
This would further suggest that organisms closely related to S. pneumoniae, such as S. oralis, that are quite likely to
possess a wide range of genes with intermediate sequence divergence are
potentially important donors in the evolution of the pneumococcal genome (6, 25; S. King, unpublished data). This
finding suggests a potential problem for the development of novel
pneumococcal targets for vaccination or chemotherapy that may become
subject to a strong selective pressure (7).
Second, it has also been shown that the presence of a particular HE
marker (namely, a C/C mismatch) in the immediate proximity of an LE
marker reduces the probability of detection of the LE marker
(11). Thus, some mismatches may have a "masking" effect on adjacent mismatches, possibly by destabilizing the DNA helix conformation. In multiply mismatched strands, extended destabilized regions may increase the overall probability of an entire strand escaping detection.
Finally, multiple mismatches may interfere with the search for nicks in
the donor strand. Such nicks allow the Hex system to distinguish
between donor and recipient and correct the appropriate strand. The
nick-directed search needs to extend from the mismatch all the way to
the end of the donor strand. Mismatches present along the way may
impede the search. On the other hand, in Escherichia, mismatch repair uses methylation patterns to distinguish between the
donor and the recipient strands. The search for methylation does not
need to extend to the end of the strand and takes, on average, 256 bases. This may explain why, in E. coli, mismatch repair is
less hampered by multiple mismatches (13).
It has been suggested that the mismatch repair system may be modulated
so as to provide a strong barrier to recombination under normal growth
conditions and a weak barrier under stressful conditions, when
recombination may be particularly favorable (12, 28). Our
results show that even under exponential growth, which is necessary for
induction of competence in Streptococcus, mismatch repair is
a poor barrier to interspecific recombination. Moreover, Humbert et al.
(13) have shown that levels of mismatch repair proteins are
not modulated during competence. In view of the above and the data from
transformation in B. subtilis (15, 16), we
believe that the role of mismatch repair as a barrier to interspecific recombination in gram-positive bacteria is minor and most probably incidental to that involved in the correction of errors introduced during DNA replication.
Recombinant joint formation as a genetic barrier. According to data from recombination in Escherichia (27-29) and Bacillus, the reluctance to form mismatched DNA heteroduplex molecules is caused primarily by the scarcity of minimum efficiently processed segments (MEPS), short regions of near identity between the donor and the recipient (15, 16). Once such conserved blocks are located, the adjacent sequences seem to undergo recombination despite a high level of divergence between the donor and the recipient. In Bacillus, recombination requires mismatch-free segments of about 20 bp or more at both ends of the donor strand (16). Data from E. coli indicate that only one highly conserved, invasive end is needed in that system (22, 24). Such sequences are most probably necessary for initiation of RecA-mediated strand invasion, stabilization of the recombinant joint, and possibly the subsequent recombination-associated DNA synthesis.
In the preceding discussion of possible mechanisms of sexual isolation, we suggested that, other than mismatch repair, most intermediate processes are unlikely to have a major sequence divergence-specific effect on sexual isolation. We further propose that the remaining major barrier to recombination between diverged sequences is the availability of MEPS sequences. We can apply the model described by Majewski and Cohan (15) to determine the length of a MEPS in Streptococcus from the experimentally determined sensitivity of sexual isolation to sequence divergence,Hex, in the absence of mismatch repair. The model assumes that a
mismatch-free region of n nucleotides is necessary for a
successful recombination. It can be shown from equation 1 that the
length of such a region may be estimated as n = Hex · ln (10). Thus, in Streptococcus, the minimum length of an invasive end
would be 27 bp, if one such end was needed, or 14 bp, if two flanking ends were necessary (as for Bacillus). More work, for
example, analysis of the recombinant junction produced on
transformation with divergent DNA, needs to be done to determine which
of the above models is correct. We also observe that the resistance to joint formation poses less of a barrier to recombination in
Streptococcus (Hex = 11.72) than
in Bacillus (MutSL = 17.95)
(15). This difference is significant by analysis of covariance (F = 33.4; P < 0.0001).
To conclude, we find that the frequency of transformation in
Streptococcus is an approximately exponential function of
the sequence divergence between the donor and the recipient. As in other recombinational systems, the two major factors producing sexual
isolation are the mismatch repair system and the difficulty in
heteroduplex formation.
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ACKNOWLEDGMENTS |
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We thank J. P. Claverys for the supply of the hexA construct that was used to create the R6 hexA mutant strain.
This work was supported by U.S. Environmental Protection Agency grant R82-5348-01-2 and by the Wellcome Trust.
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FOOTNOTES |
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* Corresponding author. Mailing address: Laboratory of Statistical Genetics, Box 192, Rockefeller University, 1230 York Ave., New York, NY 10021. Phone: (212) 327-7922. Fax: (212) 327-7996. E-mail: majewski{at}complex.rockefeller.edu.
Present address: Procter & Gamble, 65824 Schwalbach/Ts, Germany.
Present address: Zeneca Agrochemicals, Jeallots Hill Research
Station, Bracknell RG42 6ET, United Kingdom.
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REFERENCES |
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Top
Abstract
Introduction
Materials and Methods
Results
Discussion
References
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Journal of Bacteriology, February 2000, p. 1016-1023, Vol. 182, No. 4
0021-9193/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.
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