| The adventurous and social motility systems control gliding motility in Myxococcus xanthus. In this work, ten contiguous genes from the Sgl I region of the chromosome are identified which appear to play roles in both type IV pilus (fimbrial) biogenesis and social motility. Seven are closely related to similarly named genes in Pseudomonas aeruginosa. These genes (and anticipated functions) are pilA (pilin), pilB and pilC (secretion or assembly), pilD (prepilin leader peptidase), pilR and pilS (a two-component regulatory system), and pilT (pilus retraction). Three additional genes, pilG, pilH and pilI, have no known homologs in pilus biogenesis systems, but appear to encode a bacterial ATP-binding cassette (ABC) exporter complex.; A counterselectable cassette containing sacB from Bacillus subtilis was developed to facilitate construction of null mutations of these genes in M. xanthus. Null mutations of pilA, pilB, pilC, pilG, pilH, pilI and pilR abolish pilus biosynthesis and social motility. A deletion of pilT abolishes social motility but not pilus biogenesis. A deletion of pilS does not abolish either phenotype. Expression of pilA is positively regulated by pilR, negatively regulated by pilS, and negatively autoregulated. It does not require pilB, pilC, pilG, pilH, pilI, pilT or tgl (involved in stimulation of pilus biosynthesis). pilA expression is induced from 6 to 15 hours during fruiting body development; it is also induced in high nutrient concentrations. Induction under either of these conditions is not dependent on pilS or pilR. In addition, PilA is shown to be required for normal fruiting body morphogenesis beyond its role in pilus biogenesis and social motility.; These findings demonstrate that M. xanthus has type IV pili, that type IV pili are required for social motility, and that social gliding and twitching motility (found in other bacteria with type IV pili) are functionally related. From these data, it is proposed that social gliding motility employs the same motor (pilus retraction) as twitching motility, but is constrained by cell surface properties to motion in the direction of the long axis of the cell. Adventurous gliding motility is expected to rely on a separate, as yet unidentified, gliding motor. |
