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Bacteria are often highly polarized, exhibiting specialized
structures at or near the ends of the cell. Among such structuresare
actin-organizing centers, which mediate the movement of certainpathogenic bacteria within the cytoplasm of an animal host cell;organized arrays of membrane receptors, which govern chemosensorybehavior in swimming bacteria; and asymmetrically positioned septa,which generate specialized progeny in differentiating bacteria.This
polarization is orchestrated by complex and dynamic changesin the
subcellular localization of signal transduction and cytoskeletonproteins as well as of specific regions of the chromosome. Recentwork
has provided information on how dynamic subcellular localizationoccurs
and how it is exploited by the bacterial cell.
The main task of a bacterial cell is
to survive and duplicate itself. The bacterium must replicate its
genetic material anddivide at the correct site in the cell and at the
correct timein the cell cycle with high precision. Each kind of
bacteriumalso executes its own strategy to find nutrients in its
habitatand to cope with conditions of stress from its environment.
Thisinvolves moving toward food, adapting to environmental extremes,and, in many cases, entering and exploiting a eukaryotic host.These
activities often involve processes that take place at ornear the poles
of the cell. Here we explore some of the schemesbacteria use to
orchestrate dynamic changes at their poles andhow these polar events
execute cellular functions.
In spite of their small size, bacteria have a remarkably complex
internal organization and external architecture. Bacterialcells are
inherently asymmetric, some more obviously so than others.The most
easily recognized asymmetries involve surface structures,e.g.,
flagella, pili, and stalks that are preferentially assembledat one
pole by many bacteria. "New" poles generated at the celldivision
plane differ from old poles from the previous round ofcell division.
Even in Escherichia coli, which is generally thoughtto be
symmetrical, old poles are more static than new poles withrespect to
cell wall assembly (1), and they differ inthe deposition of
phospholipid domains (2). There aremany instances of
differential polar functions; among these isthe preferential use of
old poles when attaching to host cellsas in the interaction of
Bradyrhizobium with plant root hairs(3) or the
polar pili-mediated attachment of the Pseudomonasaeruginosa
pathogen to tracheal epithelia (4). An unusualpolar
organelle that mediates directed motility on solid surfacesis found in
the nonpathogenic bacterium Myxococcus xanthus. Thegliding
motility of this bacterium is propelled by a nozzle-likestructure that
squirts a polysaccharide-containing slime fromthe pole of the cell
(5). Interestingly, M. xanthus,which has nozzles
at both poles, can reverse direction by closingone nozzle and opening
the other in response to end-to-end interactionsbetween cells.
1 Department of Developmental Biology, Stanford
University School of Medicine, B300 Beckman Center, Stanford, CA 94305, USA.
2 Department of Molecular and Cellular Biology,
Harvard University, Cambridge, MA 02138, USA.
*
To whom correspondence should be addressed. E-mail:
shapiro{at}cmgm.stanford.edu
Elisabeth Knust and Olaf Bossinger (6 December 2002) Science298 (5600), 1955.
[DOI: 10.1126/science.1072161] |Abstract »|Full Text »|PDF »
REVIEW
Barry J. Dickson (6 December 2002) Science298 (5600), 1959.
[DOI: 10.1126/science.1072165] |Abstract »|Full Text »|PDF »
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