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Journal of Bacteriology, January 2001, p. 171-177, Vol. 183, No. 1
0021-9193/01/$04.00+0 DOI: 10.1128/JB.183.1.171-177.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.
Component of the Rhodospirillum centenum Photosensory
Apparatus with Structural and Functional Similarity to
Methyl-Accepting Chemotaxis Protein Chemoreceptors
Ze-Yu
Jiang and
Carl E.
Bauer*
Department of Biology, Indiana University,
Bloomington, Indiana 47405
Received 24 May 2000/Accepted 22 September 2000
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ABSTRACT |
Photosynthetic bacteria respond to alterations in light conditions
by migrating to locations that allows optimal use of light as an energy
source. Studies have indicated that photosynthesis-driven electron
transport functions as an attractant signal for motility among purple
photosynthetic bacteria. However, it is unclear just how the
motility-based signal transduction system monitors electron flow
through photosynthesis-driven electron transport. Recently, we have
demonstrated that the purple photosynthetic bacterium Rhodospirillum centenum is capable of rapidly moving swarm
cell colonies toward infrared light as well as away from visible light. Light-driven colony motility of R. centenum has allowed us
to perform genetic dissection of the signaling pathway that affects photosynthesis-driven motility. In this study, we have undertaken sequence and mutational analyses of one of the components of a signal
transduction pathway, Ptr, which appears responsible for transmitting a
signal from the photosynthesis-driven electron transport chain to the
chemotaxis signal transduction cascade. Mutational analysis
demonstrates that cells disrupted for ptr are defective in
altering motility in response to light, as well as defective in
light-dependent release of methanol. We present a model which proposes
that Ptr senses the redox state of a component in the photosynthetic
cyclic electron transport chain and that Ptr is responsible for
transmitting a signal to the chemotaxis machinery to induce a
photosynthesis-dependent motility response.
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INTRODUCTION |
Quantity and quality of light are
important environmental factors that all photosynthetic organisms
respond to. Plant cells use light as a signal for controlling many
photomorphogenic processes such as chloroplast and seedling maturation
and development. Motile unicellular microorganisms such as algae,
cyanobacteria, and anoxygenic (non-oxygen-evolving) photosynthetic
bacteria are capable of altering swimming behavior in response to
light. These cells utilize photosensory behavior as a tool to migrate
to locations that are optimal for photosynthetic growth.
Photosensory behavioral responses displayed by microorganisms generally
fall into two categories (reviewed in reference 28). One response is phototaxis, which is movement toward or away from a
light source using a mechanism(s) that involves measuring the direction
of propagation of a specific light beam. A second motility behavior is
the scotophobic response, which involves a random alteration in the
direction of movement when motile cells experience a decrease in light
intensity or cross a light/dark border. Algae and cyanobacteria
typically display both phototactic and scotophobic responses, while
anoxygenic photosynthetic bacteria usually display only a scotophobic
response. The process of phototaxis has been extensively studied in
several different algal species where specific light-absorbing
photoreceptors are used to mediate photosensory responses
(28). Detailed genetic and biochemical analyses of the
archaeon Halobacterium salinarium also indicate that this species uses specific light-absorbing photoreceptors to mediate a
scotophobic photosensory response (reviewed in references
13 and 28).
No detailed molecular genetic or biochemical characterization of the
phototactic or scotophobic photosensory apparatus from prokaryotic
cells has been published. There is evidence that the scotophobic
response exhibited by prokaryotic purple bacteria is tightly coupled
with photosynthesis-driven electron transport. For example, studies
with Rhodospirillum rubrum (12),
Rhodobacter sphaeroides (1, 2), and
Rhodospirillum centenum (18) have established
that a functional photosynthetic electron transport chain is required
for a scotophobic response. Studies with chemical inhibitors of the
electron transport chain also indicate that R. sphaeroides
responds to a change in the rate of cyclic electron transport of the
photosystem rather than to alterations of membrane potential or proton
motive force (9, 10). It is also known that the chemotaxis
cascade in purple bacteria constitutes a downstream signaling pathway
that transduces the photosynthesis-driven photoresponse to the
flagellar motor (16, 17, 26). This indicates that the
purple bacterial photosensory response involves integrating information
of photosynthetic electron transport with that of chemosensory
responses. It also suggests that one or more components of the purple
bacterial photosensory machinery may have structural features in common
with chemotaxis receptors.
Recently, the purple bacterium R. centenum was shown to
exhibit a unique photosensory behavior that allows an efficient genetic screen for photosensing-defective mutants. In liquid growth medium, this species forms swim cells with a single polar flagella that exhibit
a classic scotophobic response. However, when R. centenum is
grown on an agar-solidified medium, the cells differentiate into
multiflagellated swarm cells that are capable of exhibiting a visible
phototactic response en masse as colonies (24, 25). By
visually screening for swarm colonies that are unable to respond to
light, we have been able to obtain a collection of mutants that are
defective in photosensory behavior (18). In this study, we
report the cloning, sequencing, and mutational analysis of a gene,
which we have named ptr (for phototransducer), that appears responsible for transducing a signal from the photosynthesis-driven electron transport chain to the chemotactic signal transduction cascade. Sequence analysis reveals that Ptr has a high degree of
sequence homology to chemoreceptors. A targeted chromosomal disruption
of ptr results in a specific defect in the ability of both
swim and swarm cells of R. centenum to perceive both visible and infrared light.
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MATERIALS AND METHODS |
Bacterial strains, plasmids, and culture conditions.
The
wild-type R. centenum strain ATCC 51752 (8, 25)
and a ptr disrupted strain, ZJF6-4, were used in this study.
Cells were cultured photosynthetically in CENS medium or aerobically in
CENS or PYVS medium (25). Escherichia coli
strain XL1-Blue (Stratagene) was used for routine DNA cloning purposes
and grown in Luria-Bertani medium at 37°C. When needed for plasmid
selection, appropriate antibiotics were used in E. coli
strains at concentrations of 150 µg/ml for ampicillin, 50 µg/ml for
spectinomycin, and 10 µg/ml for gentamicin. For R. centenum strains, spectinomycin was used at 10 µg/ml and
kanamycin was used at 40 µg/ml on agar plates. Light-driven
phototactic colony migration assays were conducted on 0.8% PYVS agar
plates as described previously (25).
DNA manipulation.
All restriction and DNA modification
enzymes were purchased from New England Biolabs and used according to
the manufacturer's instructions. Standard molecular methods were
followed as described by Sambrook et al. (29). A clone of
the mini-Tn5-Spr (spectinomycin resistance
gene)-disrupted ptr gene was constructed by isolating
genomic DNA from R. centenum strain ZJF6-4 as described by
Jiang et al. (18). The chromosomal DNA was digested with NcoI, dephosphorylated with alkaline phosphatase, and
ligated into plasmid vector pZJD7 that was also digested with
NcoI. The ligation reaction product was then transformed
into E. coli strain XL1-Blue with recombinant plasmid pPRCN,
obtained by selecting for both ampicillin resistance and
Spr. The DNA insertion in plasmid pPRCN is about 6.7 kb,
which brings the total length of sequence flanking the interposon to
about 4.7 kb.
One targeted mutant in the ptr locus (ZJC
ptr1) was
constructed using a suicide vector to promote allelic replacement
through recombination (Fig.
1A). A plasmid
containing deletion of ptr (pPRCN1) was constructed as
follows. First, plasmid pPRCN (described above) was digested with
SfiI, which removed the mini-Tn5 interposon Spr omega cassette as well as all of the ptr
coding sequence between the interposon insertion site and the
ptr stop codon. Second, a previously described nonpolar
Spr gene cassette (17) was ligated with
SfiI-digested pPRCN to give rise to plasmid pPRCN1. An
NcoI fragment from pPRCN1 that contained the nonpolar
Spr gene was then ligated to pGmLacZ (17) to
give rise to pGm-PRCN1, which was transformed in E. coli
S17-1 (
pir) for mating with wild-type R. centenum. Transconjugants were selected on CENS plates with
kanamycin, spectinomycin, and
5-bromo-4-chloroindolyl-
-D-galactopyranoside (X-Gal) as
an indicator dye. The putative deletion mutants LacZ
Spr Gms were confirmed by PCR analysis with
relevant primers (data not shown).
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FIG. 1.
(A) Restriction and gene map of the ptr
locus. ORFs are depicted as boxes with arrows. Two partial ORFs flank
the ptr gene. The mini-Tn5 interposon insertion
site in mutant ZJF6-4 is indicated. (B) Hydrophobicity plot map of Ptr.
The two peaks of high hydrophobicity labeled TM1 and TM2 designate two
putative transmembrane domains. (C) N-terminal sequence of Ptr. The two
putative transmembrane domains are boxed, and the putative
b-type heme-binding motif is underlined and in boldface. (D)
Sequence alignment of carboxyl-terminal sequences of Ptr and known
chemoreceptors. The putative methylation sites are underlined, and the
highly conserved domain is boxed with dashed lines. Tsr and Tar,
E. coli serine and aspartate chemoreceptors, respectively;
McpA_R.c., McpA from Rhodospirillum centenum; McpA_C.c.,
McpA from Caulobacter crescentus.
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DNA sequencing.
Double-stranded DNA sequencing was
accomplished by primer walking using an ABI automated DNA sequencer
(Applied Biosystems model 373; Perkin-Elmer). Sequence data were
analyzed and assembled with the Sequencher program (Gene Codes
Corporation, Ann Arbor, Mich.).
Methanol evolution assay.
Release of 3H-labeled
methanol from R. centenum cells that undergo a decrease or
increase in light intensity was measured using a modified volatile
methanol evolution assay as described originally by Kehry et al.
(19) and Kirby et al. (20). Cells were grown photosynthetically in minimal CENMED medium containing 0.2 mM L-methionine to approximately 200 Klett-Summerson
photometer units (red filter no. 66). The cells were then washed three
times with CENMED containing chloramphenicol (0.1 mg/ml) and suspended
in the same medium to a final density of 500 Klett-Summerson units. Six
milliliters of washed cells was then placed in a glass screw-cap tube
containing 60 µCi of
L-[methyl-3H]methionine (80 Ci/mmol, 5 µCi/µl; Amersham Life Science) and incubated for
2.5 h with a tungsten light source emitting a light intensity of
100 µW/cm2. After incubation, the cells were washed with
15 ml of CENMED medium containing 0.2 mM L-methionine and
then loaded into a translucent 32-mm-diameter 0.45-µm-pore-size
Acrodisc (Gelman Science) to form a cell filter paste. The cell filter
paste was then attached to a Pharmacia high-pressure liquid
chromatograph and washed continuously under low illumination (<5
µW/cm2) for 20 min with 40°C CENMED medium containing
0.2 mM L-methionine at a flow rate of 1.0 ml/min. A
step-down light response was achieved by an increase in the infrared
light on the cell filter paste to 100 µW/cm2 for 20 s followed by a shift-down to low-illumination conditions. Uncapped
Eppendorf tubes containing 0.5-ml fractions of the cell filter paste
flowthrough were then placed into 20-ml glass scintillation vials that
contained 8 ml of nonaqueous scintillation fluid (Amersham Life
Science). Volatile methanol released from the Eppendorf tube was then
trapped in the scintillation fluid by incubating tightly capped
scintillation vials at room temperature for 24 h. The Eppendorf tubes were then removed from the scintillation vials, and the scintillation fluid containing trapped methanol was quantified in a
scintillation counter (model TRI-CARB2100TR; Packard Instrument Company, Downers Grove, Ill.).
Nucleotide sequence accession number.
The nucleotide
sequence for ptr in R. centenum has been
deposited in the GenBank database under accession number AF064528.
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RESULTS |
Isolation and characterization of the ptr gene.
In
a previous study, we described the isolation of a collection of
mini-Tn5-Spr mutants that exhibited an absence
of scotophobic and phototactic behavior in response to changes in
infrared and/or visible light intensity (18). One mutant
strain, ZJF6-4, is particularly intriguing since both swim and swarm
cells of this mutant are defective in responding to changes in both
visible and infrared light. Unlike chemotactic mutants of R. centenum (17), this mutant strain exhibits normal
chemotaxis to pyruvate and acetate, indicating that the chemotaxis
machinery is intact. This finding, in combination with the fact that
the strain exhibits normal photosynthetic growth, leads us to conclude
that this mutant strain contains a specific defect in a gene that
governs both visible and infrared photosensory responses. To analyze
the gene disrupted in ZJF6-4, we constructed a genomic library from
which we obtained a clone containing the mini-Tn5-Spr and flanking regions by selecting
for resistance to spectinomycin. Sequence analysis of the cloned
fragment revealed that the mini-Tn5-Spr
transposon disrupted an open reading frame (ORF) containing a 671-amino-acid, 71-kDa protein (Fig. 1A). Since the
mini-Tn5-Spr-disrupted gene appears to
specifically affect the photosensory signaling pathway (see below), we
have named the disrupted gene ptr for phototransducer.
A search of the GenBank database with the translated
ptr
sequence reveals a high degree of homology to a number of eubacterial
and archaeal chemotaxis receptors (Fig.
1). Most conspicuous is
a
highly conserved carboxyl-terminal domain extending from amino
acid
residues 508 to 550 that in chemoreceptors is known to participate
in
binding of the chemotactic signaling molecule CheW (Fig.
1D)
(
23). Flanking this region are two putative CheR/CheB
methylation/demethylation
sites (Fig.
1D). One notable alteration is
that the distance between
the methylation/demethylation sites and
the CheW/CheA docking
site is approximately 45 amino acids shorter in
Ptr than in other
eubacterial methyl-accepting chemotaxis proteins
(MCPs) (Fig.
1D). A hydrophobicity plot of Ptr (Fig.
1B) also indicated
the
presence of two putative transmembrane domains at the amino
terminus,
with one extending from amino acid residues 16 to 37 and the
other
extending from residues 297 to 320. Assuming that Ptr serves a
receptor function similar to that of other known MCPs, the
260-amino-acid
region between the two putative transmembrane domains
could function
as a sensory domain that is localized to the periplasm.
In this
context, it is interesting that the putative periplasmic loop
contains a sequence motif, Glu-Trp-His-Arg (starting at residue
72),
that is nearly identical to a
b-type heme-binding sequence
present in cytochrome
b6 (Fig.
1C)
(
7).
In addition to
ptr, the clone also contains a partial ORF of
400 amino acid residues that has sequence similarity to
3-methylcrotonyl
coenzyme A carboxylase and propionyl coenzyme A
carboxylase from
various plant and bacterial sources. This ORF is
located 600 bp
upstream of, and transcribed in the same direction as,
ptr (Fig.
1A). A second partial ORF (151 amino acid
residues) with an unknown
function is also present in the clone that is
located downstream
of
ptr. This ORF is transcribed in the
direction opposite
ptr transcription (Fig.
1A).
Functional characterization of the ptr locus.
To
confirm that the mini-Tn5-Spr disruption of
ptr is responsible for the phenotype observed with strain
ZJF6-4, we constructed a targeted chromosomal deletion of
ptr using allelic exchange, creating the mutant ZYCptr1
(see Materials and Methods). As observed for the original ZJF6-4
mutant, ZYCptr1 displayed normal wild-type photosynthetic growth and
a normal chemotactic response to pyruvate and serine (data not shown).
To quantitatively evaluate the swim cell scotophobic photoresponse, we
recorded the individual rotational movement of cells attached to a
microscope slide and measured their response to a decrease in light
intensity (17). The observed average percentage of 30 independently tracked cells of wild-type and ZJCptr1 mutant cells
during a 3 min interval is shown in Fig.
2. After recording rotation direction
during a 45-s interval, we subjected the cells to a 75% decrease in
light intensity by inserting a neutral density filter into the
microscope's light beam. After 105 s, the cells were returned to
the original high light intensity. As indicated in Fig. 2, the
proportion of wild-type cells responding to a reduction in light
intensity increased from a background of ~3% to more than 85%. In
contrast, the percentage of ZYJptr1 cells that reversed remained at
background levels, indicating that disruption of ptr results
in a loss of the scotophobic reversal response. There was also no
increased reversal frequency of either wild-type or ZYJptr1 cells to
an increase in light intensity, which confirms previous observations
that a scotophobic reversal response is observed only during a decrease
in light intensity. However, an obvious increase of speed of rotation
was observed for both strains upon the increase in light intensity (a
process known as photokinesis), which is presumably caused by a
light-driven increase in photosynthesis-generated membrane potential
(11).
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FIG. 2.
Reversal frequencies of swimming cells of wild-type
( ) and ZJCptr1 mutant ( ) R. centenum in response to
a decrease in light intensity ( ) and an increase in light intensity
to the original level ( ).
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We also assayed the effect of deleting
ptr on the
phototactic colony motility exhibited by swarm cells. As shown in Fig.
3,
deletion of
ptr resulted in
loss of both visible and infrared
light-driven colony motility of
strain ZYJ
ptr1. Microscopic observation
of individual cell movement
in swarm colonies also indicates no
effect of visible or infrared light
on individual cell reversal
in ZYJ
ptr1 (data not shown). In
contrast, individual cells in
wild-type swarm colonies undergo a rapid
loss of motility when
subjected to a decrease in light intensity
(
17).
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FIG. 3.
Phototactic colony migration assays of mutants ZJF6-4
and ZJCptr1 along with the wild type and a chemotaxis operon
deletion mutant (17) as controls. Arrows indicate the
direction of light that causes positive (colony as a unit moves toward
the light source) and negative (colony moves away from the light
source) phototactic responses.
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We also performed
trans complementation of ZYC
ptr1 with a
plasmid-encoded copy of
ptr. For this analysis, we
constructed
a replicating plasmid that contained the
ptr
gene along with 570
bp of intergenic DNA upstream of
ptr.
When this plasmid was mated
into ZYC
ptr1, the strain regained normal
scotophobic as well
as phototactic responses to visible and infrared
light during
swim and swarm cell phases, respectively (data not shown).
This
confirmed that the observed lack of light-driven motility changes
in ZYC
ptr1 is a consequence of disruption of
ptr and not
a result
of polarity effects on neighboring gene
expression.
Demethylation of Ptr during adaptation of the scotophobic
response.
Interactions of chemical attractants with MCPs are known
to cause a transient increase in the methylation state of the MCP receptors. The formation of a specific ligand-receptor interaction is
thought to cause a conformational change in MCPs that promotes a
subsequent interaction with CheR. CheR then methylates the MCP at three
to four sites within two conserved methylation domains. The methyl
groups are then released as methanol by phosphorylated CheB, with the
rate of demethylation being stimulated by a decrease in attractant
level (reviewed in reference 31). The presence of
two putative methylation/demethylation sites in Ptr implies that the
light-mediated scotophobic and phototactic responses that are mediated
by Ptr may involve a methylation/demethylation process similar to that
of chemoreceptors.
To assay for Ptr methylation, we conducted a light-dependent volatile
methanol evolution assay based on previously developed
methylation/demethylation assays of chemoreceptors. In this assay,
we
harvested [
3H]methionine-grown cells onto a filter to
form a cell paste, continually
washed the cell filter paste with
unlabeled growth medium to remove
unincorporated
[
3H]methionine, and then subjected it to a step-up
followed by a
step-down in light intensity, with fractions of the cell
filter
paste wash assayed for elution of [
3H]methanol as
described in Materials and Methods. As shown in
Fig.
4, wild-type cells subjected to a
step-down in infrared light
intensity released a burst of volatile
methanol into the wash.
In contrast, the ZYJ
ptr1 mutant strain
challenged with a similar
step-down in light intensity did not produce
methanol (Fig.
4).
Indeed, the methanol elution profile of the
ptr mutant was very
similar to that observed with an
R. centenum cheB-disrupted control
strain that is incapable
of stimulating removal of methyl groups
from chemoreceptors
(Fig.
4) (
17). Also in congruence with motility
assays is
the absence of a release of [
3H]methanol during a
step-up in light intensity (Fig.
4). As discussed
above, a step-up of
light intensity increases the speed of swimming
but does not affect the
reversal frequency. These results indicate
that a step-down in light
intensity, which stimulates the scotophobic
tumbling response, involves
a Ptr-dependent release of methanol
in a manner similar to that
observed with chemoreceptors.
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FIG. 4.
Infrared light-induced release of
[3H]methanol with nonradioactive chase. The y
axis denotes the measured scintillation counts from tritium-labeled
methanol; the x axis denotes the collected fractions. A
upward arrow indicates that the light source was turned on and the
illumination of cell paste was started; a downward arrow indicates that
the light source was switched off. Data for wild-type, cheB
mutant, and ZJCptr1 mutant strains are represented by diamonds,
triangles, and circles, respectively.
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DISCUSSION |
Even though it was observed over a century ago that photosynthetic
bacteria exhibit light-induced alterations in motility (6), there has been little progress in determining how
photosynthetic bacteria control photosensory behavior. Early studies by
Manten (21) and Clayton (4) established that
wavelengths of light that are attractants are also the same as those
that are absorbed by the photosystem to promote photosynthesis-driven
electron transport in purple photosynthetic bacteria (reviewed in
reference 28). The scotophobic response has also
been shown to require a functioning photosystem (reaction center), as
well as components of the photosynthesis-driven electron transport
system such as a functioning cytochrome bc1 complex (18). It has been proposed that the generation of
electrons by photosynthesis is the signal that governs phototaxis in
anoxygenic photosynthetic bacteria (10). This scenario
is in contrast to what is thought to occur in the archaeon
H. salinarium, in which separate retinal
photoreceptors (sensory rhodopsins I and II) absorb light, resulting in
conformational changes of the photoreceptors. Light excited sensory
rhodopsin then interacts with MCP-like transducer proteins that
interact with the Che proteins (3, 13, 27, 30) in a way
not unlike that proposed for Ptr.
To obtain a better understanding of the nature of the purple bacterial
photosensory transduction system, we previously performed a genetic
screen for R. centenum mutants that are defective in light
perception (18). One mutant strain isolated, ZJF6-4,
exhibited characteristics of a disruption of a key component in the
photosensory signal transduction system. Specifically, ZJF6-4 swim
cells were observed to be defective in the scotophobic response, while
swarm cells were defective in both positive and negative phototaxis. In
this study, we cloned an ORF that was disrupted in ZJF6-4 and demonstrated by sequence analysis that it codes for a protein with a
high degree of sequence similarity to bacterial and archaeal chemoreceptors. The observation that Ptr has structural features in
common with MCPs clearly strengthens our earlier conclusion that the
photosensory signaling pathway in purple photosynthetic bacteria
involves the che gene products that are also responsible for
mediating chemotaxis (17). Since only limited number of chemicals have been tested on R. centenum for chemotactic
response, Ptr may serve an additional role as a bona fide
chemoreceptor, similar to the transducer HtrII from H. salinarium, which transmits the blue light signal from sensory
rhodopsin II and also serves as a chemoreceptor for serine
(14). Presumably, a light-dependent signal perceived by
Ptr is integrated into the chemosensory signal transduction cascade via
interactions with CheW/CheAY (Fig. 5). Indeed, integration of a signal from the photosystem (i.e.,
photosynthesis electron flow) with signals derived from chemoreceptors
should allow cells the ability to effectively migrate in response to either light or chemical gradients, depending on which signal is
dominant. This should allow maximum flexibility for migration to
positions conducive to optimal growth.
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FIG. 5.
Proposed model of Ptr that senses the redox state of
cytochrome (Cyt) c in the periplasmic space. LHC, light
harvesting center; AdoMet, adenosylmethionine; MeOH, methanol.
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Sequence analysis indicates that Ptr exhibits a high degree of identity
with other MCPs in the putative cytoplasmic output domain, specifically
in the region that interacts with CheW. One noticeable change in the
output domain is closer positioning of the two putative methylation
domains to the CheW docking domain. In comparison to E. coli
Tsr (5), it appears that Ptr is missing the linker region
that exists between 
helix 7 and the methylated helix 6; 9
helix, which contains the second methylation domain, is also
significantly (approximately 10 helical turns) shorter. It remains to
be seen whether these structural alterations are a unique feature of
Ptr or are common to other MCPs from R. centenum. Like other
chemoreceptors, Ptr does not show a high degree of sequence identity
with other MCPs in the putative periplasmic sensory domain. If the
periplasmic domain of Ptr monitors the redox state of a
component of the photosynthesis-driven electron transport chain, then
it may contain a redox-responsive center. In this vein, it is
interesting that this domain has the sequence REWHRT, which is similar to the heme
b-binding sites (underlined) found in cytochrome
b561 from E. coli
(KSWHET) and cytochrome b6 from Spinacium oleracea
(RSVHRW) (7). Consequently,
ptr may contain a heme in the periplasmic sensory domain. In
some respects, this would be similar to the case for the
heme-containing aerotaxis transducers, a notable difference being that
characterized aerotaxis transducers have no periplasmic domains
(15).
One puzzle is how disruption of Ptr leads to a loss of both positive
(movement toward infrared light) and negative (movement away from
visible light) phototactic colony motility of R. centenum. It is known that positive and negative phototaxes
require functioning reaction center and cytochrome
bc1 complexes, indicating that both processes
involve a measure of photosynthesis electron transport (10,
18). It is also known that both infrared and visible light can
be absorbed by photopigments and drive photosynthesis electron
transport (22). Indeed, the observation that both light types drive photosynthesis and yet lead to such different motility responses indicates that Ptr responds not only to alterations in
photosynthesis-driven electron transport but also to electron transport
as well as to an additional component(s) that distinguishes between
visible or infrared light. Another possibility may be that light
directly modifies the activity of Ptr itself. Based on this study and
previous studies of photosensory responses in R. centenum
and other purple photosynthetic bacteria, a testable model can be
proposed. As depicted in Fig. 5, a component of the periplasmic
photosynthetic cyclic electron transport chain such as a cytochrome
c may reduce a heme bound to the periplasmic loop of Ptr.
This could cause a conformational change in Ptr that affects integration with the chemotaxis machinery. At this early stage of
molecular analysis of bacterial photoperception, we cannot rule out
that other components in the photosystem, such as the reaction center
or cytochrome bc1 complex, may interact with Ptr to transmit signals generated by light perception. Clearly, additional characterization of Ptr and other upstream components is needed to
determine how these two different types of light are capable of
transmitting different signals through Ptr. Direct visualization of
light-driven colony motility of R. centenum swarm colonies makes this a good model system to unravel additional details of bacterial photoperception.
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ACKNOWLEDGMENTS |
We thank members of the Indiana University Photosynthetic
Bacteria Group for helpful comments.
This work was supported by funding from NIH GM58050.
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FOOTNOTES |
*
Corresponding author. Mailing address: Department of
Biology, Indiana University, Jordan Hall, Bloomington, IN 47405. Phone: (812) 855-6595. Fax: (812) 855-6705. E-mail:
cbauer{at}bio.indiana.edu.
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Journal of Bacteriology, January 2001, p. 171-177, Vol. 183, No. 1
0021-9193/01/$04.00+0 DOI: 10.1128/JB.183.1.171-177.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.
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Abstract
Introduction
