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Abstract
Although flagella are the best-understood means of locomotion in bacteria [1], other
bacterial motility mechanisms must exist as many diverse groups of bacteria move without
the aid of flagella [2-4]. One unusual structure that may contribute to motility is
the type IV pilus [5,6]. Genetic evidence indicates that type IV pili are required
for social gliding motility (S-motility) in Myxococcus, and twitching motility in
Pseudomonas and Neisseria [6,7]. It is thought that type IV pili may retract or rotate
to bring about cellular motility [6,8], but there is no direct evidence for the role
of pili in cell movements. Here, using a tethering assay, we obtained evidence that
the type IV pilus of Myxococcus xanthus functions as a motility apparatus. Pili were
required for M. xanthus cells to adhere to solid surfaces and to generate cellular
movement using S-motility. Tethered cells were released from the surface at intervals
corresponding to the reversal frequency of wild-type cells when gliding on a solid
surface. Mutants defective in the control of directional movements and cellular reversals
(frz mutants) showed altered patterns of adherence that correlate reversal frequencies
with tethering. The behavior of the tethered cells was consistent with a model in
which the pili are extruded from one cell pole, adhere to a surface, and then retract,
pulling the cell in the direction of the adhering pili. Cellular reversals would result
from the sites of pili extrusion switching from one cell pole to another and are controlled
by the frz chemosensory system.