Myxococcus xanthusChemotaxis Homologs DifD and DifG Negatively Regulate Fibril Polysaccharide Production

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Abstract
The extracellular matrix fibrils ofMyxococcus xanthusare essential for the social lifestyle of this unusual bacterium. These fibrils form networks linking or encasing cells and are tightly correlated with cellular cohesion, development, and social (S) gliding motility. Previous studies identified a set of bacterial chemotaxis homologs encoded by thediflocus. It was determined thatdifA,difC, anddifE, encoding respective homologs of a methyl-accepting chemotaxis protein, CheW, and CheA, are required for fibril production and therefore S motility and development. Here we report the studies of three additional genes residing at thediflocus,difB,difD, anddifG. difDanddifGencode homologs of chemotaxis proteins CheY and CheC, respectively.difBencodes a positively charged protein with limited homology at its N terminus to conserved bacterial proteins with unknown functions. Unlike the previously characterizeddifgenes, none of these three newly studieddifgenes are essential for fibril production, S motility, or development. ThedifBmutant showed no obvious defects in any of the processes examined. In contrast, thedifDand thedifGmutants were observed to overproduce fibril polysaccharides in comparison with production by the wild type. The observation that DifD and DifG negatively regulate fibril polysaccharide production strengthens our hypothesis that theM. xanthus difgenes define a chemotaxis-like signal transduction pathway which regulates fibril biogenesis. To our knowledge, this is the first report of functional studies of a CheC homolog in proteobacteria. In addition, during this study, we slightly modified previously developed assays to easily quantify fibril polysaccharide production inM. xanthus.