Type IV pilus of Myxococcus xanthus is a motility apparatus controlled by the frz chemosensory system

Although flagella are the best-understood means of locomotion in bacteria [ 1], other bacterial motility mechanisms must exist as many diverse groups o f bacteria move without the aid of flagella [2-4], One unusual structure th at may contribute to motility is the type IV pilus [5,6]. Genetic evidence indicates that type IV pill are required for social gliding motility (S-mot ility) in Myxococcus, and twitching motility in Pseudomonas and Neisseria [ 6,7]. It is thought that type IV pill may retract or rotate to bring about cellular motility [6,8], but there is no direct evidence for the role of pi li in cell movements. Here, using a tethering assay, we obtained evidence t hat the type IV pilus of Myxococcus xanthus functions as a motility apparat us. pill were required for M. xanthus cells to adhere to solid surfaces and to generate cellular movement using S-motility. Tethered cells were releas ed 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) showe d altered patterns of adherence that correlate reversal frequencies with te thering. The behavior of the tethered cells was consistent with a model in which the pill are extruded from one cell pole, adhere to a surface, and th en retract, pulling the cell in the direction of the adhering pill. Cellula r reversals would result from the sites of pill extrusion switching from on e cell pole to another and are controlled by the frz chemosensory system.