The roles of the Dif chemosensory system and the extracellular proteins, FibA and PilA, in lipid chemotaxis and development of Myxococcus xanthus
Bonner, Pamela Jane
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The soil bacterium Myxococcus xanthus is a model system for the study of multicellular behavior and development. Motility is essential for the unique life cycle of M. xanthus, allowing cells to locate and feed upon prey bacteria or form fruiting bodies during nutrient deprivation. Social (S-) motility is dependent on cell-to-cell contact, type IV pili (TFP), and an extracellular matrix (ECM) composed of exopolysaccharide and protein. The Dif chemosensory pathway is required for S-motility, ECM production, and development. In this work, the Dif chemosensory pathway is shown to employ unique features that directly mediate chemotaxis to phosphatidylethanolamine (PE), a slowly diffusible molecule. First, adaptation to one PE species is slowed by a novel protein, DifB. Second, the stimulus is sensed by the cytoplasmic portion of the methyl accepting chemotaxis protein, DifA, rather than the periplasmic portion. Input into DifA through FibA, an ECM-bound zinc metalloprotease, or PilA, the functional unit of TFP, results in signaling by the Dif chemosensory pathway leading to aggregation, fruiting body formation, and sporulation. Mutants in either fibA or pilA are capable of forming spore-filled fruiting bodies, although atypical fruiting body morphology is observed. A fibA pilA mutant fails to aggregate and has a phenotype similar to difA. Loss of S-motility is not responsible for the phenotype of a fibA pilA mutant as a fibA pilT mutant forms fruiting bodies similar to a pilT mutant, which produces pili that do not retract. These results suggest that FibA and PilA are involved in the sensing or generation of different signals that are recognized by the Dif chemosensory pathway. Sensing of multiple environmental stimuli is likely to be an important part of the M. xanthus life-cycle, allowing directed motility during both vegetative growth and development.