Chemotaxis genes, fibrils and PE taxis in Myxococcus xanthus.

Recently, many organisms including Myxococcus xanthus 1xFrizzy genes of Myxococcus xanthus are involved in control of frequency of reversal of gliding motility. Blackhart, B.D. and Zusman, D.R. Proc. Natl. Acad. Sci. U. S. A. 1985; 82: 8767–8770CrossRef | PubMedSee all References, 2xA new set of chemotaxis homologs is essential for Myxococcus xanthus social motility. Yang, Z. et al. Mol. Microbiol. 1998; 30: 1123–1130CrossRef | PubMed | Scopus (92)See all References, Pseudomonas aeruginosa 3xComplete genome sequence of Pseudomonas aeruginosa PAO1, an opportunistic pathogen. Stover, C.K. et al. Nature. 2000; 406: 959–964CrossRef | PubMed | Scopus (2239)See all References3 and Rhodobacter sphaeroides 4xEvidence for two chemosensory pathways in Rhodobacter sphaeroides. Hamblin, P.A. et al. Mol. Microbiol. 1997; 26: 1083–1096CrossRef | PubMedSee all References4 have been shown to possess more than one chemotaxis operon. However, discernible functions have yet to be assigned to many of the components of these operons. Although the two che operons in M. xanthus were known to affect aspects of the gliding motility of this organism 2xA new set of chemotaxis homologs is essential for Myxococcus xanthus social motility. Yang, Z. et al. Mol. Microbiol. 1998; 30: 1123–1130CrossRef | PubMed | Scopus (92)See all References, 5xRegulation of directed motility in Myxococcus xanthus. Ward, M.J. and Zusman, D.R. Mol. Microbiol. 1997; 24: 885–893CrossRef | PubMedSee all References, only recently have advances been made regarding the specific mechanisms involved. There is now strong evidence that the dif system regulates type IV pilus-mediated motility 6xType IV pilus of Myxococcus xanthus is a motility apparatus controlled by the frz chemosensory system. Sun, H. et al. Curr. Biol. 2000; 10: 1143–1146Abstract | Full Text | Full Text PDF | PubMed | Scopus (159)See all References6 in a manner similar to the mechanism of twitching motility seen in P. aeruginosa and Neisseria gonorrhoeae 7xStructure, function and biogenesis of the type IV pili. Strom, M.S. and Lory, S. Annu. Rev. Microbiol. 1993; 47: 565–596CrossRef | PubMedSee all References7. Additionally, recent reports by Kearns and Shimkets have led to the identification of a small-molecule chemoattractant (phosphatidylethanolamine, PE) and its putative fibril receptor 8xChemotaxis in a gliding bacterium. Kearns, D.B. and Shimkets, L.J. Proc. Natl. Acad. Sci. U. S. A. 1998; 95: 11957–11962CrossRef | PubMed | Scopus (56)See all References, 9xMyxococcus xanthus fibril appendages are essential for excitation by a phospholipid attractant. Kearns, D.B. et al. Proc. Natl. Acad. Sci. U. S. A. 2000; 97: 11505–11510CrossRef | PubMedSee all References.Kearns and Shimkets have shown that M. xanthus cells display directed motility up gradients comprising dilauroyl and dioleoyl PE compounds 8xChemotaxis in a gliding bacterium. Kearns, D.B. and Shimkets, L.J. Proc. Natl. Acad. Sci. U. S. A. 1998; 95: 11957–11962CrossRef | PubMed | Scopus (56)See all References8. Their data also strongly promote a model for fibril-mediated chemosensing of PE and suggest that this sensing could play a significant role in regulating the complex social structure of the M. xanthus multicellular lifecycle 9xMyxococcus xanthus fibril appendages are essential for excitation by a phospholipid attractant. Kearns, D.B. et al. Proc. Natl. Acad. Sci. U. S. A. 2000; 97: 11505–11510CrossRef | PubMedSee all References9. In their model 10xLipid chemotaxis and signal transduction in Myxococcus xanthus. Kearns, D.B. and Shimkets, L.J. Trends Microbiol. 2001; 9: 126–129Abstract | Full Text | Full Text PDF | PubMed | Scopus (37)See all References10, fibril-mediated sensing of specific PE compounds feeds signal information via the Dif system to the putative gliding motor. One aspect of their model incorporates the interesting finding that the Dif system is necessary for fibril production 11xMyxococcus xanthus dif genes are required for biogenesis of cell surface fibrils essential for social gliding motility. Yang, Z. et al. J. Bacteriol. 2000; 182: 5793–5798CrossRef | PubMed | Scopus (82)See all References11. Kearns and Shimkets suggest that DifA acts as an intermolecular chaperone for fibril assembly; however, both the chemoreceptor DifA, and its cognate kinase, DifE, are required for fibril production 11xMyxococcus xanthus dif genes are required for biogenesis of cell surface fibrils essential for social gliding motility. Yang, Z. et al. J. Bacteriol. 2000; 182: 5793–5798CrossRef | PubMed | Scopus (82)See all References11, a point their model does not address. The observation that production of these extracellular appendages is dependent upon chemosensory components is interesting, but is not without precedent.An apparently similar phenomenon was reported over a decade ago when it was observed that Vibrio parahaemolyticus regulates swarmer cell differentiation in response to changes in viscosity in the external medium 12xSurface-induced swarmer cell differentiation of Vibrio parahaemolyticus. McCarter, L. and Silverman, M. Mol. Microbiol. 1990; 4: 1057–1062CrossRef | PubMed | Scopus (118)See all References12. In that system, the state of rotation of the polar flagellum (encoded by fla genes) senses physical characteristics of the external environment and, by an unknown mechanism, regulates expression of the lateral flagella (laf) genes. More recently, swarmer cell differentiation in Escherichia coli was shown to depend on a chemoreceptor and the CheA kinase even though the system need not be functionally chemotactic 13xThe chemotaxis system, but not chemotaxis, is essential for swarming motility in Escherichia coli. Burkart, M. et al. Proc. Natl. Acad. Sci. U. S. A. 1998; 95: 2568–2573CrossRef | PubMed | Scopus (74)See all References13. In essence, components thought to be dedicated to the control of motility are somehow necessary for the regulation of gene expression of extracellular appendages.The implication is that chemosensing via the chemotaxis machinery, usually thought to be dedicated to the control of motility, is also involved in control of cell morphological changes, which are dependent on altered gene expression. By extension, it is possible that fibril gene expression in M. xanthus is dependent upon signalling via the Dif system, which also regulates type IV pilus-based motility. The mechanism is, of course, unknown. However, this proposal would explain the data obtained by Kearns and Shimkets while retaining their most important observation that fibrils are the PE chemoreceptor 9xMyxococcus xanthus fibril appendages are essential for excitation by a phospholipid attractant. Kearns, D.B. et al. Proc. Natl. Acad. Sci. U. S. A. 2000; 97: 11505–11510CrossRef | PubMedSee all References9. Transduction of the PE signal through the fibril would, however, not necessarily proceed through the Dif system as in their model but could involve the Frz system as the authors noted 8xChemotaxis in a gliding bacterium. Kearns, D.B. and Shimkets, L.J. Proc. Natl. Acad. Sci. U. S. A. 1998; 95: 11957–11962CrossRef | PubMed | Scopus (56)See all References, 9xMyxococcus xanthus fibril appendages are essential for excitation by a phospholipid attractant. Kearns, D.B. et al. Proc. Natl. Acad. Sci. U. S. A. 2000; 97: 11505–11510CrossRef | PubMedSee all References. In addition, another putative signal would need to be transduced by the Dif system that results in fibril expression. Because M. xanthus utilizes complex intercellular signalling to coordinate its multicellular lifestyle, many signals exist that could regulate fibril gene expression. Identification of the fibril biosynthetic genes will prove invaluable in analysis of these possibilities.