Suppression of a signaling defect during Myxococcus xanthus development

The csgA gene encodes an extracellular protein that is essential for cell-cell communication (C-signaling) during fruiting body development of Myxococcus xanthus. Two transposon insertions in the socABC operon, soc-560 and socC559, restore development to csgA null mutants. Mixing soc-560 csgA cells or socC559 csgA cells with csgA cells at a ratio of 1:1 stimulated the development of csgA cells, suggesting that soc mutations allow cells to produce the C-signal or a similar molecule via a csgA-independent mechanism. The socABC operon contains the following three genes: socA, a member of the short-chain alcohol dehydrogenase gene family; socB, a gene encoding a putative membrane anchoring protein; and socC, a negative autoregulator of socABC operon expression. Both suppressor mutations inactivate socC, leading to a 30- to 100-fold increase in socA transcription; socA expression in suppressor strains is at least 100-fold higher than csgA expression during all stages of development. The amino acid sequence of SocA has 28% identity and 51% similarity with that of CsgA. We suggest that CsgA suppression is due to overproduction of SocA, which can substitute for CsgA. These results raise the possibility that a cell surface dehydrogenase plays a role in C-signaling.