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Journal of Bacteriology, December 1998, p. 6743-6748, Vol. 180, No. 24
0021-9193/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
Superimposition of TyrR Protein-Mediated Regulation
on Osmoresponsive Transcription of Escherichia coli proU
In Vivo
J.
Gowrishankar1,* and
A. J.
Pittard2
Centre for Cellular & Molecular Biology,
Hyderabad 500007, India,1 and
Department
of Microbiology and Immunology, University of Melbourne, Parkville,
Victoria 3052, Australia2
Received 15 June 1998/Accepted 5 October 1998
 |
ABSTRACT |
Osmotic regulation of proU expression in the
enterobacteria is achieved, at least in part, by a repression mechanism
involving the histone-like nucleoid protein H-NS. By the creation of
binding sites for the TyrR regulator protein in the vicinity of the
70-controlled promoter of proU in
Escherichia coli, we were able to demonstrate a superposed
TyrR-mediated activation by L-phenylalanine (Phe), as well
as repression by L-tyrosine, of proU expression in vivo. Based on the facts that pronounced activation in the presence
of Phe was observed even at a low osmolarity and that the affinity of
binding of TyrR to its cognate sites on DNA is not affected by Phe, we
argue that H-NS-mediated repression of proU at a low
osmolarity may not involve a classical silencing mechanism. Our data
also suggest the involvement of recruited RNA polymerase in the
mechanism of antirepression in E. coli.
| |
TEXT |
The proU operon in
Escherichia coli and Salmonella typhimurium
encodes a binding-protein-dependent transport system that mediates the
active uptake of the compatible solutes glycine betaine and L-proline during growth in media of elevated osmolarities.
Under such growth conditions, the expression of proU is
induced 400-fold at the level of initiation of transcription, but the
underlying regulatory mechanisms are not clearly understood (reviewed
in references 6 and 14).
Analyses of the cis regulatory regions necessary for osmotic
induction of proU have identified an extended sequence (more than 500 bp long) designated the negative regulatory element (NRE), whose proximal end is situated approximately 70 bp downstream of the
70-controlled promoter (P2) and which is required for
the full repression of proU at a low osmolarity (7, 10,
22, 28, 29). Genetic and biochemical data suggest that the NRE
mediates the repressor function of the histone-like nucleoid protein
H-NS on proU (approximately 20- to 25-fold) (7, 10, 22,
29). Nevertheless, the NRE does not serve as a portable cassette
for osmotic regulation when placed downstream of heterologous promoters
(7, 28), indicating that sequences around and upstream of P2
are also required for its function. Furthermore, in mutants lacking
H-NS or the NRE, or both, a residual 8- to 10-fold osmotic inducibility
of proU is observed (7, 10, 22, 28, 29); this
inducibility has been interpreted to represent a second distinct
mechanism acting directly on the cis element(s) in the close
vicinity of P2 (7, 14). Finally a
s-controlled promoter, P1 (situated 190 bp upstream of
P2), has also been identified which, at least in S. typhimurium, is cryptic and whose relevance in proU
regulation is as yet unclear (7, 33).
It has been suggested that H-NS-mediated repression of proU
at a low osmolarity is achieved by promoter "silencing" and that relief of repression at a high osmolarity is the consequence of cytoplasmic potassium glutamate accumulation (6, 28). In the
silencing model, the NRE serves as a position-independent silencer
locus (10, 28, 49) akin to that described for the regulation
of several eukaryotic genes (5, 27). The following features
have been cited in support of this model. (i) H-NS is not a typical
sequence-specific regulator protein (for reviews, see references
3 and 47), nor is the NRE a
typical operator sequence. Indeed, there exist two regions of curved
DNA in the vicinity of proU P2 (see Fig. 1B), one falling
within the proU NRE and the other located about 150 bp
upstream of the promoter (13, 29, 40, 41), to both of which
H-NS exhibits preferential binding (22, 29, 40). (ii) The
separation and phase angle of the NRE from proU P2 can be
varied over a distance of 200 bp without affecting its ability to
mediate repression (10, 18, 28). (iii) NRE-mediated
repression is also observed for several different variants of the P2
promoter (19, 49). (iv) A role for H-NS binding has been
implicated in the only locus (bgl) in E. coli
where silencing has been unequivocally established (26, 38,
39); the protein has also been postulated to silence several other genes in the organism (12, 23). One question as yet unanswered is whether the repressive action, consequent to the binding
of H-NS to proU, is direct (43) or indirect
(18, 19, 29).
In vitro tests of the silencing model are rendered difficult by the
fact that no accepted method for the reconstitution of proU
osmotic regulation in a cell-free system exists. One prediction of the
silencing model, which would help distinguish it from other mechanisms
of H-NS-mediated repression of proU, is that the silencing effect would extend sufficiently upstream of the proU
promoter to interfere also with the recognition of closely linked
binding sites for other DNA-binding proteins. This is the hypothesis
which we have sought to test in this study, by first creating specific sites for binding of the regulator protein TyrR adjacent to the proU P2 promoter and then addressing the question of whether
TyrR binds these sites in vivo at low and high osmolarities. Our
results indicate that TyrR-mediated regulation can be superimposed on osmotic regulation of proU transcription and suggest in
particular that the chromatin architecture in the proU P2
promoter region, even at a low osmolarity, is permissive to the binding
of and activation by TyrR. To that extent, therefore, it appears to be unlikely that a silencing mechanism operates to achieve proU
repression at a low osmolarity.
Overview of and rationale for choosing the TyrR regulation
system.
The TyrR protein in E. coli mediates the
transcriptional regulation of several operons involved in the
biosynthesis and transport of the aromatic amino acids (reviewed in
references 30 and 31). The
protein can act as either a repressor or an activator depending upon
the promoter and the particular coeffector to which it is bound. We
chose to work with the TyrR system primarily because of the fact that
neither the affinity of the protein for its cognate binding sites on
DNA nor the footprint obtained on such binding is altered in the
presence of its coactivator L-phenylalanine (Phe) (2,
4, 31, 35). The need for imposing this constraint on our choice
of system is explained below. In the case of other well-characterized
activator proteins such as CRP (20), AraC (37),
MalT (37), and the LysR family of proteins (36),
the association of the proteins with their respective coactivators leads to an alteration of the DNA-binding characteristics of the proteins.
TyrR-mediated repression is achieved in the presence of the coeffector
L-tyrosine (Tyr). Genes whose expression is repressed by
TyrR often have two adjacent TyrR binding sites (TYR R boxes), one of
which overlaps the promoter. (The TYR R box is 22 bp long, and its
consensus sequence is described in the legend to Fig. 1.) The box
overlapping the promoter has a relatively weak affinity for TyrR (weak
TYR R box) and is bound only when it is close to and on the same face
of the helix as the other box, which has a stronger binding affinity
for the protein (strong TYR R box). For instance, in the case of the
tyrP gene (encoding a Tyr-specific permease), which is
repressed by TyrR-Tyr, the weak box overlaps the 
35 region of the
promoter, whereas the strong box is upstream of and separated from the
weak box by 1 bp. It has been shown that, in the presence of Tyr, the
protein self-associates to form a hexamer and that it binds
cooperatively to both boxes to cause repression.
Transcriptional activation by TyrR in the presence of Phe requires only
the presence of a strong box suitably positioned upstream
of the
promoter. A spacing of 18 bp between the TYR R box and
the
35 hexamer
is optimal for the purpose. The TyrR dimer remains
constitutively bound
to the strong box, and upon binding Phe it
acquires the ability to
activate transcription by the process
of RNA polymerase recruitment
(
15,
32); under these specific
conditions, TyrR has been
shown to increase the affinity of binding
of RNA polymerase to the
adjacent promoter and to stimulate open-complex
formation
(
16) by functioning as a class I transcription activator
(
21,
48).
In the case of native
tyrP, the 1-bp separation between the
two TYR R boxes (which is necessary for repression control) places
the
strong box 15 bp away from the
35 hexamer, which distance
is
suboptimal for Phe-mediated activation (
1). The greatest
activation effect at
tyrP is observed for that template in
which
the strong box has been moved upstream by another 3 bp; in the
latter situation, addition of Tyr also leads to an activation
rather
than a repression of
tyrP expression (
1).
Creation of the TYR R box(es) near proU P2.
In
this study, we chose to simulate at proU P2 the regulatory
features described above for the tyrP gene (1,
2). In order to test whether TyrR could repress proU,
it was necessary (i) to use site-directed mutagenesis to create a weak
box overlapping the 35 region of the P2 promoter and (ii) to
introduce a strong box sequence upstream of and 1 bp away from the weak
box, as is the case in native tyrP. In order to test the
ability of TyrR to activate proU, it was necessary to
introduce a strong box sequence 3 bp farther upstream than in the
previous construct and to leave the remainder of the proU
regulatory region unaltered. For convenience, these two sets of
alterations are referred to below as the repression tester and
activation tester variant sequences, respectively.
The template used for the mutagenesis reactions was an M13 phage
derivative bearing a
proU fragment comprising P1, P2, and
the NRE. This 1.26-kb
proU fragment (see Fig.
1B) is
identical
to that earlier described for plasmid pHYD272 (
7)
and is known
to carry all the
cis elements involved in
proU osmoresponsivity.
Recombinant DNA manipulations were
performed essentially as described
previously (
34).
Site-directed substitution mutations were introduced
by the method of
Vandeyar et al. (
44), using a commercially
available kit
from United States Biochemical
Corp.
The sequence of wild-type
proU P2 in the region of interest
is shown in Fig.
1A, sequence i. To
facilitate the introduction
of the strong box sequence, a unique
MfeI site, CA
ATTG, was created
by introducing a
T-to-A substitution (underlined) 14 nucleotides
upstream of the
35
hexamer (Fig.
1A, sequence ii). A weak TYR
R box overlapping the
35
region was then created by site-directed
mutagenesis of a CCAT sequence
to TAAA (Fig.
1A, sequence iii).
(The
35 hexameric sequence itself
was left unaltered, although
there is a natural match with the right
arm of the palindromic
TYR R box consensus at three of six positions
and with the
35
region of
tyrP at four of six positions.)
The introduction of
a strong TYR R box sequence flanked with 5'-AATT
overhangs (shown
in Fig.
1A, sequence iv) into the
MfeI site
of sequence iii in
Fig.
1A led to the creation of the repression tester
variant,
that is, with two TYR R boxes separated by 1 bp. On the other
hand, the introduction of the strong TYR R box sequence flanked
with
5'-AATT overhangs and containing an additional 3 bp (Fig.
1A, sequence
v) into the
MfeI site of sequence ii in Fig.
1A resulted
in
the construction of the activation tester variant, that is,
with the
strong TYR R box positioned 18 bp upstream of the
35
hexamer.
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FIG. 1.
Introduction of the TYR R box(es) near proU
P2. (A) The nucleotide sequence upstream of the 35 region of the
wild-type proU P2 promoter (i) and those following
sequential site-directed mutagenesis to create first an MfeI
site (at position 51 relative to the start site of P2 transcription)
(ii) and then a weak TYR R box (iii) are shown. The MfeI and
the 35 hexamer sequences are indicated, and the mutated base residues
are in italics. Also shown are the pairs of annealed oligonucleotide
sequences (iv and v) that were used to generate the double-stranded TYR
R strong box sequences (identical to that in tyrP) flanked
with 5'-AATT overhangs for construction of the repression tester and
activation tester variants, following insertion into the
MfeI sites shown in sequences iii and ii, respectively. The
22-bp TYR R box sequences (whose consensus is the palindrome
5'-N2TGTAAAN6TTTACAN2-3',
in which the residues shown in bold are invariant) are underlined, and
the invariant residues are in bold. (B) Schematic depiction of the
position of insertion of the TYR R strong box sequences in the two
proU variants, relative to P1, P2, and the NRE. For
nucleotide numbering, the start site of P2 transcription has been taken
to be +1. Shaded bars indicate the regions of DNA curvature to which
H-NS exhibits preferential binding.
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After each step of mutagenesis, M13 phage clones carrying the correct
mutation were identified by appropriate single-nucleotide
sequence
tracking. The complete sequence of the P2 promoter region
for each of
the two variants finally obtained was verified by
automated DNA
sequence analysis (data not shown). The structure
and disposition of
the regulatory elements in the
proU variants
constructed in
this study are schematically depicted in Fig.
1B.
In order to undertake in vivo expression studies, the variant
proU sequences were then subcloned upstream of the
lacZ reporter
gene in the very-low-copy-number trimethoprim
resistance plasmid
pMU2385 (
46). The resultant plasmids were
designated pMU6442
(with the activation tester variant sequence) and
pMU6443 (with
the repression tester variant sequence). As a control,
plasmid
pMU6441 was also constructed as a derivative of pMU2385
carrying
the 1.26-kb wild-type
proU regulatory
region.
Effects of the TYR R box(es) on proU regulation in
hns+ and hns derivatives.
The
plasmids pMU6441, pMU6442, and pMU6443 were each transformed into a
pair of isogenic tyrR+ and tyrR366
strains, JP7740 and JP8042, respectively, for lacZ expression studies. Both strains are prototrophic, 
lac,
and recA (46). In light of the role suggested for
the H-NS protein in proU silencing, we also transformed the
three plasmids into strains JP10938 (tyrR+
hns-205::Tn10) and JP10939 (tyrR366
hns-205::Tn10), which are the
recA+ hns derivatives of JP7740 and JP8042,
respectively. (The hns mutations were introduced by phage P1
transduction, with strain PD145 [8] serving as the
donor.) The transformant derivatives were cultured in defined low- and
high-osmolarity media supplemented when necessary with Tyr or Phe, and
the specific activity of 
-galactosidase in each culture was
determined by the method of Miller (24). Each value reported
is the mean of at least three independent measurements.
In order to test for the
cis effects of the introduced
sequence variations on
proU osmotic regulation, we first
determined
the values for
lacZ expression in the
tyrR mutant derivatives
(
hns+ and
hns), that is, in which the possibility of a confounding
effect caused by binding of TyrR was excluded. The results are
presented in Table
1. In these
tyrR host strains, supplementation
of the culture medium
with Tyr or Phe had no effect on
-galactosidase
expression from any
of the three plasmids (data not shown).
Under the conditions of growth and assay used in this study, we
observed a >1,000-fold osmotic induction of wild-type
proU expression in the
tyrR hns+ strain (Table
1).
Neither
proU variant (in pMU6442 or pMU6443)
was affected in
low-osmolarity-medium repression in the
hns+
strain; on the other hand, the expression levels in the
NaCl-supplemented
medium were lower than that for wild-type
proU itself (Table
1).
Nevertheless, at least a 250-fold
osmotic inducibility was still
observed for both mutant derivatives in
the
hns+ strain. A possible explanation for this
partial loss of osmoresponsivity
is that the strong TYR R box
insertions in pMU6442 and pMU6443
(27 and 24 bp, respectively)
fortuitously introduce half-integral
turns of the DNA helix in the
region, which in an earlier study
was shown also to be correlated with
reduced expression of
proU at a high osmolarity
(
41).
In the
tyrR hns background, the expression profiles for the
three plasmids were more or less similar to one another (Table
1).
Consistent with the data from earlier studies (
7,
10,
18,
22,
29), absence of the H-NS protein led to a moderate
increase in
proU-lacZ expression in low-osmolarity medium. All
three
plasmids exhibited residual osmotic inducibility in the
hns
strain, although once again the absolute values for
lacZ
expression
from pMU6442 and pMU6443 in the NaCl-supplemented medium
were
less than that for the wild-type
proU control.
We then measured the levels of
lacZ expression from the
three plasmids in the
tyrR+ strains
(
hns+ and
hns) to determine the
regulatory role of TyrR on the mutant
proU promoters. The
results are presented in Table
2. In
concord
with earlier practice (
2), the magnitude of
TyrR-mediated regulation
by the two coeffectors was calculated as the
ratio of
-galactosidase
activity in the
tyrR mutant to
that in the
tyrR+ strain in the presence of the
particular coeffector (repression)
or its reciprocal (activation).
As expected,
lacZ expression from the wild-type
proU regulatory region was not affected by TyrR at low or
high osmolarities
in either the
hns+ or
hns background (compare the values for pMU6441 in Tables
1 and
2). Furthermore, even for the plasmids pMU6442 and pMU6443
in both
the
hns+ and
hns derivatives, TyrR
did not exert any significant regulatory
effect during growth in the
low- or high-osmolarity minimal media
that were not supplemented with
Phe or
Tyr.
In the case of plasmid pMU6442 (bearing the activation tester variant
of
proU), we found that
-galactosidase expression was
activated by TyrR in the presence of Phe, and less so in the presence
of Tyr, in both the low- and the high-osmolarity media (Table
2). In
the low-osmolarity medium, the magnitudes of activation
mediated by
TyrR-Phe for the
hns+ and
hns strains
were approximately 200- and 10-fold, respectively.
The corresponding
values for activation mediated by TyrR-Tyr were
around 22- and
1.5-fold, respectively. The marked TyrR-mediated
activation for pMU6442
could not be demonstrated for another related
plasmid variant
(designated pMU6445) in which the strong TYR R
box was positioned 3 bp
closer to the P2 promoter (data not
shown).
A moderate level of TyrR-mediated repression in the presence of Tyr
(around twofold) was demonstrated for plasmid pMU6443
(bearing the
repression tester variant of
proU) in the
hns+ strain at a high osmolarity and the
hns mutant at both low and
high osmolarities (Table
2).
Repression in the
hns+ strain at a low
osmolarity could not be demonstrated because
of the very low levels of
basal expression in these cultures.
Repression was rendered more
pronounced (6.8-fold) in the
hns+ strain
additionally carrying a multicopy
tyrR+ plasmid
pMU1065 (
46) (Table
2). As expected, growth in the
presence
of Phe did not repress
lacZ expression from pMU6443 in
the
tyrR+ strain (Table
2).
Absence of correlation between intrinsic promoter strength and
degree of Phe-mediated activation.
The level of activation by
TyrR-Phe of proU in plasmid pMU6442 is at least an order of
magnitude higher than that reported earlier for tyrP or
other genes for aromatic amino acid metabolism (even after optimization
of spacing between the strong TYR R box and the 35 region). We
considered the possibility that this difference (in degree of
activation) merely reflects the fact that the promoter for
proU is inherently weaker than the TyrR-activable promoters of the native TyrR regulon. This hypothesis is rendered more plausible by the data in Table 2, which reveal that even in proU the
degree of activation is most pronounced when the level of basal
expression is the lowest (that is, in the hns+
strain grown in low-osmolarity medium).
We sought to test this hypothesis by creating a down-promoter mutation
in
tyrP and then examining the degree of activation
by TyrR
at the mutated promoter. For this purpose, the A residue
(underlined)
in the
35 hexamer (TTG
ACG) of
tyrP was
converted
to the noncanonical C, which is found in
proU P2
(Fig.
1A, sequence
i), by site-directed mutagenesis. The
tyrP template into which
this mutation was introduced is
identical to one described in
an earlier study (
48) that has
the strong TYR R box situated
18 bp upstream of the
35 region (that
is, at a location optimal
for studying
activation).
The expression of the
lacZ reporter gene on each of two
isogenic plasmids, pMU6449 and pMU2055, carrying the mutant and
wild-type
tyrP promoter sequences, respectively, was then
determined in
transformants of JP7740 (
tyrR+)
and JP8042 (
tyrR). Consistent with the results of earlier
work
(
1), the wild-type
tyrP promoter was
activated 12- and 6.5-fold
by Phe and Tyr, respectively, in the
tyrR+ host (Table
3). The mutant
tyrP promoter
exhibited a 16-fold
reduction in basal expression in the
tyrR strain, but the levels
of activation supported by TyrR
(8- and 4-fold with Phe and Tyr,
respectively) were more or less
similar to those for the wild-type
promoter (Table
3). We therefore
conclude that there is no correlation,
at least in
tyrP,
between promoter strength and the magnitude
of TyrR-mediated
activation.
Conclusions.
In this study, we have successfully designed and
created modified proU regulatory regions that have now
acquired an additional facet of activation or repression control by the
TyrR protein and that still retain substantial osmoresponsivity in the
tyrR mutant background. These results establish, for the
first time, that appropriately positioned TYR R boxes are sufficient to
confer TyrR-mediated regulation on a heterologous promoter in vivo.
Although the
proU regulatory region used in this study
carries two promoters, several lines of evidence suggest that
osmoresponsivity
and TyrR control are both exerted at promoter P2. (i)
As mentioned
above and reviewed earlier (
6,
14), no role for
P1 in normal
proU osmotic regulation has yet been
established. Mutations that
abolish P2 promoter activity abolish
proU expression. Conversely,
rpoS mutations that
abolish P1 promoter activity do not affect
normal
proU
regulation. Furthermore, there is no evidence that
transcription from
P1 traverses past P2 into the NRE region (
33).
(ii) The
placement of the TYR R box(es) in the activation tester
and repression
tester variant plasmids pMU6442 and pMU6443, respectively,
was designed
specifically to exert regulation at the P2 promoter.
(iii) Finally, the
osmoresponsivity of
lacZ expression from plasmids
pMU6442
and pMU6443 was not affected in an
rpoS::Tn
10 mutant (data
not shown),
thereby excluding a role for the P1 promoter in such
regulation.
The striking finding in this study was the 200-fold stimulation of
proU expression at a low osmolarity achieved with TyrR-Phe
in the activation tester variant. The fact that the binding of
the TyrR
protein dimer to the strong TYR R box is constitutive,
that is,
independent of Phe (
2,
4,
31,
35), with the
latter merely
serving to convert the bound protein into an active
conformation for
the recruitment of RNA polymerase, allows us
to make two inferences:
(i) the strong TYR R box upstream of P2
is accessible for TyrR protein
binding even at a low osmolarity,
and (ii) TyrR binding by itself (in
the absence of Phe) has no
effect on
proU repression under
these conditions. Our results
therefore indicate that if silencing does
occur at the
proU P2
promoter, it does not extend to this
upstream TYR R box
region.
Our findings may also be important for an understanding of
antirepression as a mechanism of activation of gene expression
in
E. coli. An antirepressor may be operationally defined as a
factor which promotes transcription by interfering with a system
of
repression. Antirepression may be said to exist when the magnitude
of
transcriptional activation mediated by the factor is higher
in the
presence of a particular repressing condition than in its
absence.
Examples of transcriptional activation by RNA polymerase
recruitment
and antirepression may not be mutually
exclusive.
Several instances in which DNA-binding regulator proteins act as
antirepressors of H-NS in mediating transcriptional activation
are
known. These include cyclic AMP-cyclic AMP receptor protein
for the
divergently transcribed promoters in the
pap locus
(
11)
and perhaps too for
bgl (
26,
39),
CfaD for the promoter of
the
cfaABCE operon (
17),
IHF for the early promoter of phage
Mu (
45), and FIS for the
P1 promoter of each of the rRNA operons
(
42) and perhaps for
the
hns promoter itself (
9). In each
case, it has
been assumed that binding of the specific regulator
protein to DNA
directly alters the nucleoprotein topology in a
manner that renders
H-NS incapable of
repression.
Earlier results obtained with TyrR also suggest that the protein acts
as an antirepressor of HU and IHF in mediating activation
at the
mtr and
tyrP promoters (
48). In the
present study as
well, we found that the magnitude of TyrR-mediated
activation
of
proU in pMU6442 at a low osmolarity, in the
presence of either
Tyr or Phe, is much higher in the
hns+ strain (where H-NS serves to repress
proU expression) than in
the
hns mutant (Table
2). Therefore, TyrR fulfils the operational
definition of an
antirepressor of H-NS in this situation. Yet,
as argued above, TyrR
binding by itself (in the absence of Phe)
does not alter the repressive
nucleoprotein topology at
proU during
growth in
low-osmolarity medium. Therefore, our findings implicate,
for the first
time, recruited RNA polymerase as a component in
the mechanism of
antirepression.
Finally, the results in Table
3 also indicate that the substantially
enhanced magnitude of stimulation at
proU by TyrR-Phe
may
not simply be a consequence of
proU bearing a weaker
promoter
than that of
tyrP. One could speculate, therefore,
that this difference
is a reflection of the relative degrees of basal
repression to
which different promoters, including those of the native
TyrR
regulon (
48), are subjected by the binding of the
nucleoid
proteins.
| |
ACKNOWLEDGMENTS |
We thank all the members of the Pittard laboratory for their advice
and stimulating discussions.
Financial support for the study was provided by the Australian Research
Council and the Bilateral Science and Technology Collaboration Program
(to A.J.P.) and by the award of a CSIR Raman Research Fellowship (to
J.G.). J.G. is an Honorary Senior Fellow of the Jawaharlal Nehru Centre
for Advanced Scientific Research.
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FOOTNOTES |
*
Corresponding author. Mailing address: Centre for
Cellular & Molecular Biology, Uppal Rd., Hyderabad 500007, India.
Phone: 91-40-7172241. Fax: 91-40-7171195. E-mail:
shankar{at}ccmb.ap.nic.in.
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1991.
Importance of the position of TYR R boxes for repression and activation of the tyrP and aroF genes in Escherichia coli.
J. Bacteriol.
173:5079-5085[Abstract/Free Full Text].
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Andrews, A. E.,
B. Lawley, and A. J. Pittard.
1991.
Mutational analysis of repression and activation of the tyrP gene in Escherichia coli.
J. Bacteriol.
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Journal of Bacteriology, December 1998, p. 6743-6748, Vol. 180, No. 24
0021-9193/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
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