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Journal of Bacteriology, August 1999, p. 4825-4833, Vol. 181, No. 16
0021-9193/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.
Transformations in Flagellar Structure of
Rhodobacter sphaeroides and Possible Relationship to Changes
in Swimming Speed
Judith P.
Armitage,1,*
Thomas P.
Pitta,2
Margot A.-S.
Vigeant,3
Helen L.
Packer,1 and
Roseanne
M.
Ford3
Microbiology Unit, Department of
Biochemistry, University of Oxford, Oxford OX1 3QU, United
Kingdom1; Rowland Institute of
Science, Cambridge, Massachusetts 021422; and
Department of Chemical Engineering, University of Virginia,
Charlottesville, Virginia 22903-24423
Received 1 March 1999/Accepted 3 June 1999
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ABSTRACT |
Rhodobacter sphaeroides is a photosynthetic bacterium
which swims by rotating a single flagellum in one direction,
periodically stopping, and reorienting during these stops.
Free-swimming R. sphaeroides was examined by both
differential interference contrast (DIC) microscopy, which allows the
flagella of swimming cells to be seen in vivo, and tracking microscopy,
which tracks swimming patterns in three dimensions. DIC microscopy
showed that when rotation stopped, the helical flagellum relaxed into a
high-amplitude, short-wavelength coiled form, confirming previous
observations. However, DIC microscopy also revealed that the coiled
filament could rotate slowly, reorienting the cell before a transition back to the functional helix. The time taken to reform a functional helix depended on the rate of rotation of the helix and the length of
the filament. In addition to these coiled and helical forms, a third
conformation was observed: a rapidly rotating, apparently straight
form. This form took shape from the cell body out and was seen to form
directly from flagella that were initially in either the coiled or the
helical conformation. This form was always significantly longer than
the coiled or helical form from which it was derived. The resolution of
DIC microscopy made it impossible to identify whether this form was
genuinely in a straight conformation or was a low-amplitude,
long-wavelength helix. Examination of the three-dimensional swimming
pattern showed that R. sphaeroides changed speed while
swimming, sometimes doubling the swimming speed between stops. The rate
of acceleration out of stops was also variable. The transformations in
waveform are assumed to be torsionally driven and may be related to the
changes in speed measured in free-swimming cells. The roles of and
mechanisms that may be involved in the transformations of filament
conformations and changes in swimming speed are discussed.
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INTRODUCTION |
Bacteria swim by using the
electrochemical ion gradient (usually the proton motive force [
p])
across the cytoplasmic membrane to drive flagellar rotation. To change
swimming direction, flagellar rotation can either stop or switch
between counterclockwise (CCW) and clockwise (CW), depending on the
species. A change in switching frequency biases the overall swimming
direction of motile bacteria to an optimum environment for growth; for
reviews, see references 1 to 3, 5, 15, and
20. Peritrichous species such as Escherichia
coli swim smoothly when the majority of flagella are rotating CCW.
The flagella rotate together as a bundle, which pushes the cell forward
at about 20 µm s
1. Periodically, the flagella switch to
rotate CW, and the bundle flies apart. This activity reorients the cell
for its next period of smooth swimming. Rhodobacter
sphaeroides, on the other hand, rotates its flagellum only CW and
changes swimming direction by stopping that rotation, although the
driving force, p, remains constant (4). The frequency of
stopping was found to increase when the cells were moving down a
gradient (16). High-intensity dark-field microscopy showed
that a normal flagellar helix was formed during periods of swimming,
but when the cells stopped, the filament relaxed into a
short-wavelength, high-amplitude helix which coiled against the cell
body. The helix reformed from the cell body out, and the cell usually
swam off in a new direction. The "glare" around the cell body
resulting from the high light intensity made it impossible to identify
what was happening to the coiled form next to the cell (4).
The frequency of direction changing, which results from switching or
stopping motor rotation, is controlled by environmental stimuli
signalling through a phosphorelay system. Sensory receptors control the
activity of a cytoplasmic histidine protein kinase, CheA, which in turn
controls a small response regulator, CheY. Phosphorylated CheY can bind
to the motor and cause switching. Interestingly, while the flagella of
E. coli are controlled by one CheY protein, four CheY
proteins control a single, unidirectional motor in R. sphaeroides. In addition, unlike enteric species, R. sphaeroides changes swimming speed independently of changes in
p (10, 18). Any changes in E. coli flagellar
rotation could be dampened by the flagella rotating together, while
R. sphaeroides swimming reflects the behavior of a single
flagellar motor (17).
In this study, we used video-enhanced differential interference
contrast (DIC) microscopy to examine any changes that might occur in
the structure of the R. sphaeroides flagellum during free
swimming and direction changing. In addition, we tracked the swimming
pattern of unstimulated cells by using a microscope which can
accurately track behavior and swimming in three dimensions (6,
8). We found that R. sphaeroides does change speed
while swimming. Using DIC microscopy, we found that the coiled form of
the flagellum can rotate and probably contributes to direction changing. In addition, we also identified a new flagellar waveform which appears to be a rapidly rotating straight or low-amplitude conformation.
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MATERIALS AND METHODS |
Strains and growth conditions.
R. sphaeroides WS8 (a
gift from W. Sistrom) was grown anaerobically in succinate medium under
tungsten illumination at room temperature as previously described
(14). Cells were either harvested and resuspended in sodium
HEPES buffer (20 mM, pH 7.2) or examined directly in growth medium.
Ficoll (2%) was added to increase viscosity and decrease the rate of
rotation of the flagellum for DIC microscopy. E. coli AW405,
a wild-type strain, was grown in tryptone broth to the mid-exponential
phase, harvested, and resuspended at 107 per ml in 20 mM
potassium phosphate buffer (pH 7.0). All measurements involving
E. coli were made at 30°C, while all measurements for R. sphaeroides were made at room temperature.
Video-enhanced DIC microscopy.
Video-enhanced DIC microscopy
was performed essentially by the method of Block et al. (9).
Flagella were detected with a Nikon Diaphot microscope fitted with a
×100 DIC 1.4-numerical aperture (NA) objective and a 1.4-NA oil
immersion condenser. The microscope was illuminated with a 100-W
short-arc Hg lamp (Oriel, Stratford, Conn.). The light was passed
through a water filter (to remove infrared light) and a 475-nm,
long-pass filter (Shott Glass Technologies, Duryea, Pa.) before it was
coupled to the microscope via a 0.48-NA multimode optical fiber
(GeneralFiber Optics, Fairfield, N.J.). Light emitted from the fiber
was collected by a ×20, 0.5-NA, 160-mm-tube-length DIC objective
followed by a relay lens (100-mm focal length). The relay lens focused
the back aperture of the objective onto the field iris, and the lens pair focused the end of the fiber onto the back focal plane of the
condenser, the image of the fiber just filling that aperture. The rest
of the optical train was standard Nikon configuration. Images were
collected with a Nevicon video camera (Hamamatsu Corp., Tokyo, Japan).
The background was subtracted, and two frames were averaged with an
Argus-20 image processor (Hamamatsu). The data were stored on Hi-8 videotape.
Still images from the tape were digitized with a Raptor video board
(Bitflow Inc., Woburn, Mass.). Images were deinterlaced and contrast
stretched with Photoshop (Adobe Systems Inc., San Jose, Calif.).
Three-dimensional tracking.
A tracking microscope was used
to obtain data on the swimming pattern of unstimulated R. sphaeroides, and these data were compared to the swimming pattern
of E. coli. The microscope tracks individual bacteria as
they move in three dimensions (6, 7). Growing, motile
R. sphaeroides cells were diluted in growth medium to
107 cells per ml. This concentration was used to reduce the
problems of cells visually interfering with each other during tracking (13). The bacteria were placed in a tracking chamber on the microscope, and individual cells were tracked in bulk solution, far
from the surface. Cells remained motile at room temperature for several
hours. E. coli cells were tracked similarly, except that the
temperature of the stage was increased to 30°C.
The position of the bacterium being tracked was sampled every 1/12 s,
and these data were collected and analyzed by use of
a Macintosh Power
PC 8100 running LabView 3.1 (National Instruments)
with a 16-bit A-to-D
converter (National Instruments NB-MIO-16XH).
The data were analyzed to
obtain the instantaneous velocity of
a bacterium at any given point by
use of the method of Berg and
Brown (
8). The velocities were
then used to calculate the following:
the average speed of each
bacterium for the period during which
it was tracked; the average speed
of the population; the standard
deviations of the average speed of the
population; the maximum
speed of each bacterium; the number of times
each bacterium stopped
or tumbled; the average bacterial run time; and
the rate of acceleration
out of a stop or during
swimming.
A cell was considered to have "stopped" when its measured velocity
fell below 10 µm s
1, a speed which correlated well with
the actual behavior examined
by eye and which was the speed measured
for cells subjected only
to Brownian motion. All cells were examined by
eye to confirm
that events considered stops did not look like slow
translational
movement. Bacterial accelerations were calculated by
determining
the average speed of a given bacterium and the standard
deviation
of that speed to give a measure of the extent to which speed
changed
during swimming. Any measured change in velocity greater than
or equal to one standard deviation of the speed occurring over
one time
step was considered a "significant" acceleration. The
accelerations
were described with two categories: "stop to go,"
in which the
bacterium accelerated out of a stop, and "go to go,"
in which the
bacterium was measured as changing speed while already
swimming.
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RESULTS |
Three-dimensional tracking.
Figure
1 shows representative tracks from three
different cells of R. sphaeroides and an E. coli
cell. The direction in which each cell is swimming is indicated in the
legend to Fig. 1. Each point represents 1/12 s; therefore, the closer
the points are together when oriented within a two-dimensional plane,
the slower the cell speed. Increases in speed are shown by increased
distance between the points. In all cases, the images are printed so
that the run between the two arrows is oriented in the plane of the paper. Away from this section, the cells are swimming in three dimensions, and it is difficult to decouple the changes in speed from the effects of projection onto the plane. The average swimming speed of all R. sphaeroides cells was 27 µm
s1; the average swimming speed measured for E. coli was 22 µm s1. Both species showed a random
three-dimensional swimming pattern; periods of smooth swimming in
E. coli were interrupted by tumbles, and those in R. sphaeroides were interrupted by stops. The difference in the
swimming behavior of the three R. sphaeroides cells is obvious. The cell in Fig. 1A swam very rapidly, with few changes in
speed or direction, whereas the cell in Fig. 1B swam very slowly, stopped frequently, and changed direction during periods of slow swimming. The cell in Fig. 1A was tracked for 5 s before being lost, while that in Fig. 1B was tracked for well over 20 s. The cell in Fig. 1C was swimming parallel to the plane between the arrows
and could be seen to accelerate out of a stop.
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FIG. 1.
Three-dimensional swimming patterns of three individual
R. sphaeroides cells (A, B, and C) and one E. coli cell (D). Between the arrows, the plots are oriented so that
the cell is swimming parallel to the plane of view. Each spot
represents 1/12 s. The closer together the spots, the slower the cell
speed. The cells in panels A and D were swimming top to bottom, while
those in panels B and C were swimming bottom to top. The asterisk marks
a stop followed by acceleration out of that stop.
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The speed of individual
R. sphaeroides cells also could be
seen to change between stops. This finding is shown more clearly
in
Fig.
2, where the swimming speeds of
three different bacteria
from the same population are plotted.
Bacterium 1 showed a steady
swimming speed of about 45 µm
s
1 with short stops at 1.5 and 9 s. There was a
brief reduction
in speed at about 5 s. Bacterium 2 continued to
swim at a steady
average of about 38 µm s
1 throughout
the 8 s tracked, but bacterium 3 showed a great variation
in
speed, between about 30 and 50 µm s
1.
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FIG. 2.
Change in swimming speed of three individual R. sphaeroides cells tracked in three dimensions. When the speed
falls below 10 µm s1, the cells have probably stopped,
and the measured speed probably represents Brownian motion, although in
some cases there may be slow movement. The time scales are different,
as the cells were tracked for different periods of time before being
lost from tracking.
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The average swimming speed within a run is plotted in Fig.
3 and shows that the speed of the
individual cells within the population
varied greatly. Below about 10 µm s
1, the cells were probably stopped, but there was
slow nontranslational
movement, probably as a result of both slow
flagellar rotation
and Brownian motion. Because the cells were
incubated in high
light and growth medium, the
p of these
photosynthetically grown
cells remained at its maximum level, but there
was still a great
variation in the swimming speed measured between
stops. The average
velocity was 27.3 ± 10.4 µm
s
1, and the average maximum speed was 46.3 ± 9.5 µm s
1 (the maximum speed measured was 62.3 µm
s
1). In contrast,
E. coli swam at an average
velocity of 22.6 ±
3.5 µm s
1 and had an average
maximum speed of 35.0 ± 5.4 µm s
1 (Table
1).
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FIG. 3.
Distribution of swimming speed during periods of smooth
swimming between stops, calculated from 32 tracked R. sphaeroides cells (a) and 11 tracked E. coli cells (b).
Below 10 µm s1, R. sphaeroides cells were
considered stopped. The number of the y axis refers to the
number of periods of smooth swimming, and the speed on the x
axis is the average speed during a period of smooth swimming.
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TABLE 1.
Average swimming behavior calculated for 32 individual
R. sphaeroides cells and 11 individual E. coli
cells by three-dimensional trackinga
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The stopping frequency of individual cells varied greatly, with some
cells stopping every second and others showing no stops
over several
minutes. The average number of stops in an average
measuring period of
11 s was 4.5 ± 4.8. The time that cells remained
stopped
also varied between a fraction of a second and several
seconds; the
average time of a stop was 0.34 ± 0.32 s. In comparison,
an
average
E. coli tumble lasted 0.16 ± 0.06 s
(Table
1). In
137 measured periods,
R. sphaeroides was
found to stop and subsequently
change direction 116 times, change
direction without a measurable
stop 19 times, and stop but not change
direction 2 times. The
mean turn angle was 76° ± 41°.
Fig.
1C and
2 show the rate at which a cell accelerates out of a stop.
Each point is 1/12 s; in Fig.
1C, it takes about 0.3
s to reach a
maximum swimming speed. The rates of acceleration
from stop to go and
from one speed to another (labelled go to
go) during swims were
calculated. The average rate of acceleration
from stop to swim was
about 175 ± 78 µm s
2, but accelerations ranged
from 15 to a maximum of 520 µm s
2 (Fig.
4). Some cells therefore reach their
initial swimming speed
as rapidly as
E. coli, which reaches
25 µm s
2 very rapidly after a tumble, usually within
one time point (Fig.
4b), but others show distinctly slow rates of
acceleration up
to swimming speed.
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FIG. 4.
Distribution of acceleration rates of 32 R. sphaeroides cells tracked from stop to swim ( ) and speed
changes while free swimming ( ) (a) and 11 tracked E. coli
cells (b) (symbols are as in panel a). The frequency with which the
individual acceleration rates were measured (as described in Materials
and Methods) is shown on the y axis. Data show 95%
confidence limits. The averages (mean ± standard deviation) of
the behaviors measured over the entire histogram were as follows: ,
178 ± 18 and 122 ± 10 µm s2 for R. sphaeroides and E. coli, respectively; , 95 ± 15 and 99 ± 10 µm s2 for R. sphaeroides and E. coli, respectively.
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Figure
4 plots the rates of acceleration for cells from a stop to
smooth swimming and also within periods of smooth swimming.
The rates
are plotted as the frequency with which cells accelerate
to beyond one
standard deviation of the starting speed. Greater
accelerations were
seen for cells coming out of a stop than for
cells already swimming.
During a run, many cells accelerated to
over one standard deviation of
the initial swimming speed, an
average of about 99 µm
s
2, but accelerations out of stops averaged about 122 µm s
2. The ranges of speed changes and acceleration
rates were seen
not only between cells but also within the swimming
patterns of
individual cells. In contrast, in
E. coli speed
changes showed
a much narrower range and the frequency with which cells
changed
speed while swimming was much
lower.
DIC microscopy.
Video-enhanced DIC microscopy is able to
reveal individual flagella in vivo. Ficoll was added to the medium to
increase its viscosity and thus slow the rotation of the flagella;
otherwise, rapidly rotating flagella would not be resolvable.
Figure
5
shows the changes in conformation seen
for the
R. sphaeroides flagellum. Figure
5A shows various
flagellar conformations:
a coiled form (a), a functional helix (b), a
helix relaxing into
a coil (c), and an apparently straight conformation
(d). Figure
5B shows a straight filament forming a helix (b) and
reforming
the straight conformation (c). On occasion, coiled flagella
were
seen to rotate slowly before reforming a functional helix from
the
cell body out. Figure
5C shows the slow reforming of a coiled
conformation to a functional helix; the shadow on the cell body
shows
the change in orientation during this period. The slow rotation
reoriented the cell during the stop, so that the next period of
swimming was in a different direction. The time taken for the
transformation from the coiled to the helical waveform varied
from
about three frames to several seconds and was dependent on
the rate of
motor rotation and the length of the filament, which
varied between 1 and 6 µm. The time taken also might have been
longer because the
medium contained Ficoll.
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FIG. 5.
DIC images of the flagellar filament of R. sphaeroides. (A) Various flagellar conformations: a coiled form
(a), a functional helix (b), a helix relaxing into a coil (c), and an
apparently straight conformation (d). The cartoon shows the likely cell
size and the flagellar shape. (B) Filament changing between straight
and helical conformations. (C) Sequential formation of a functional
helix from a coiled filament. The two images at the top show the coiled
form rotating. Bars, 1 µm.
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Frequently, the flagellum was seen to transform into an apparently
straight form (Fig.
5A, panel d, and 5B, panels a and c).
This
transformation could occur within one video frame, and the
flagellum
could reform into a helix within one frame. The filament
could
transform directly from a coil to a straight form or from
a helix to a
straight form. The transformation to the straight
form happened from
the motor out, and the filament could be seen
to rotate very rapidly,
as evidenced by the movement of the filament
in the medium. The
straight form of the filament was clearly longer
than the helical form
of the filament (Fig.
5B), eliminating the
possibility that it was a
rapidly rotating, short-wavelength,
low-amplitude conformation. When
the normal helix reformed from
the straight conformation, it did so
rapidly from the distal end
of the filament. The change from coiled to
helical form or coiled
to straight form happened from the cell body
out, while the formation
of the helix from the straight form or the
helix from the coiled
form happened from the distal end of the
filament. The slow rotation
of the coiled flagellum (upper two panels
in Fig.
5C) and the
reformation of a helix from a coiled flagellum
could be seen
clearly.
Several cells which were predivisional and had two flagella were seen.
The flagella clearly rotated independently and were
never seen to form
the equivalent of a bundle; i.e., they never
rotated in phase (Fig.
6). Periodically, one or the other
filament
would stop rotating and would coil against the cell body,
while
the other filament continued rotating and pushing the cell
forward.
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FIG. 6.
DIC images of a dividing cell with two independently
rotating flagellar filaments. The two flagella rotated independently,
even when close together. Bar, 1 µm.
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DISCUSSION |
The data presented here suggest that R. sphaeroides can
change swimming speed under unstimulated conditions, possibly as the result of a change in either motor rotation rate or flagellar conformation. Increases in swimming speed were measured when
free-swimming R. sphaeroides was given some metabolizable
chemoattractants, and this sustained increase was independent of
changes in p (18). Tethered R. sphaeroides
cells have also been found to show a much greater variation in rotation
rate than tethered enteric cells under nonstimulated conditions
(17). Recent work with a quadrant photodiode, sampling at a
rate much higher than that of video analysis, has suggested that the
bias of the tethered R. sphaeroides motor, i.e., the
stopping frequency, is significantly higher than the switching
frequency of E. coli. This finding may indicate the
occurrence of unresolved very short stops, which may affect both
measured swimming speed and flagellar conformation (8a). Tethered cells are under much higher frictional loads than
free-swimming cells. Changes in swimming speed may therefore reflect
short stops or transient transformations in flagellar conformation.
Within a population, there was great variation in the behavior of
individual cells and changes in the patterns of behavior of single
cells during a period of swimming. There were also great variations in
measured stopping frequency and length of stops. When swimming rapidly,
the cells tended to swim a straighter course than when swimming slowly,
when the cells swam in more obvious curves. Bacteria have been shown to
swim in very curved paths when swimming close to surfaces
(13), but the bacteria being tracked here were well away
from the surface. There were also occasions when cells appeared to
change direction after slowing down rather than after stopping. Changes
in swimming speed (which may be the manifestation of a series of
extremely short stops or flagellar transformation), as well as
measurable stopping, may therefore be involved in behavioral responses.
Recent data have shown that the related bacterium Sinorhizobium
meliloti may change direction by altering the speed of individual
flagella within a bundle (5, 19).
It had been assumed that Brownian motion caused the reorientation of
R. sphaeroides during a stop, but calculations suggested that, in the relatively viscous environment in which the bacteria live,
Brownian motion would be a slow and inefficient mechanism for
redirecting swimming given the short average time of a stop. The data
from the DIC microscopy showed that the coiled form of the flagellum
did result from a stop in rotation but that, once coiled against the
cell body, the flagellum often slowly rotated before the helix reformed
from the cell body out. The p was still maximal under these
conditions, and the reduced rate of rotation was presumably the result
of the increased viscous drag on the high-amplitude helix. The slow
rotation of the flagellum may also explain the movement of the cells
measured during a stop.
The rapid rotation of the flagellar motor caused the helix to reform
from the cell body out, producing a functional helix. It might be
expected that, as thrust can be equated with the helical shape of the
filament, the speed at which a cell swims might relate to the extent of
functional helix reformation. Therefore, a long filament would take
longer to form a functional helix, and the rate of acceleration from a
stop to a smooth swimming speed would be slower than that measured for
a cell with a short filament. The wide range of acceleration rates from
stop to swim may therefore partly reflect the difference in time taken
to go from the coiled form to the functional helix conformation. With
DIC microscopy, a long flagellum was seen to take longer to reform a
complete helix than a short filament, and this finding would be
expected to affect speed. The R. sphaeroides flagellum
probably grows continuously, unlike, for example, the Caulobacter
crescentus single flagellum (11). Individual R. sphaeroides cells can have flagella between 1 and 9 µm in length
(21).
On several occasions, cells that were clearly dividing and had two
flagella were seen. These flagella were seen to rotate independently,
sometimes one stopping while the other continued to rotate. They were
not seen to rotate together as any type of bundle, even when apparently
very close. This finding suggests that either a specific hook structure
is required to form a bundle or a critical number of rotating filaments
is needed.
A third waveform
a very rapidly rotating straight formwas apparent
with DIC microscopy. It is tempting to speculate that the
transformation among coiled, helical, and straight forms is involved in
the change in swimming speed seen in the tracking experiments. The
resolution of the DIC microscope is limited to about 200 nm as a result
of the optical "blooms" caused by the techniques. If the filament
formed a shallow waveform with less than a 200-nm amplitude, it would
appear straight. We could not determine, therefore, whether the
filament was really straight or was a low-amplitude helix. However, the
very obvious increase in length compared to the length of the helical
form showed that there had been a major change in conformation, and it
was not to a short-wavelength, low-amplitude helix, such as that which forms during the normal-to-curly transformations of E. coli
flagella when the motor switches rotational direction.
Why and how does this conformation form, and what effect does it have
on the pattern of swimming? Mutations in the flgL gene of
E. coli, coding for the hook-associated protein HAP3, which links the flexible hook and the more rigid filaments (12),
result in flagella which go through the conformational changes seen in free-swimming R. sphaeroides. These so-called sag
mutants show normal swimming behavior in liquid medium but poor
swarming ability in soft agar, probably because the filaments are
forced into a straight conformation by the viscous drag in agar. The
transformations seen in sag mutants were similar to those
seen in R. sphaeroides, with torsion-induced changes
probably occurring with changes in torque, the straight conformation
forming from the cell body out and reforming a helix from the distal
end. A straight filament would naturally rotate faster than a helical
form because of the reduction in drag; however, it would also produce
less driving force. It was suggested that HAP3 is responsible for
maintaining the E. coli flagellar filaments in a helical
form by preventing torsion-induced transformations. It was also argued
that the torque produced in E. coli would allow the
formation of straight filaments but that this process is prevented by
HAP3, which allows the hook subunits to move but restricts the movement
of the flagellin subunits. Nothing is known about the HAP proteins from
R. sphaeroides, but these data suggest that there may be
less control over conformational changes in R. sphaeroides
flagellar filaments than in those of enteric species. A change in
rotation rate or transient stops may cause a switch in filament
conformation in free-swimming cells, and this switch would cause
changes in swimming speed. It may be worth noting that R. sphaeroides does not swarm quite as well as E. coli in
soft agar, but it does swarm much better than sag mutants.
R. sphaeroides has four different CheY proteins. In E. coli, the single CheY protein is responsible for flagellar
switching. The reason for multiple copies in a monoflagellate species
with a unidirectional motor is unknown, but it is tempting to speculate that one or more of these proteins may have a role in transient changes
in torque and thus flagellar transformation and swimming speed.
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ACKNOWLEDGMENTS |
J.P.A. thanks the Rowland Institute of Science and Howard Berg
for use of their facilities and Howard Berg and Richard Berry for
useful discussion of the data.
J.P.A. is supported by the BBSRC, and H.L.P. is supported by NERC.
T.P.P. is supported by the Rowland Institute of Science, and M.A.-S.V.
is supported by IBM and NSF through the PIRCH program sponsorship.
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FOOTNOTES |
*
Corresponding author. Mailing address: Microbiology
Unit, Department of Biochemistry, University of Oxford, Oxford OX1 3QU, United Kingdom. Phone: 44 1865 275299. Fax: 44 1865 275297. E-mail: armitage{at}bioch.ox.ac.uk.
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Abstract
Introduction
