CLONING AND CHARACTERIZATION OF THE SOCA LOCUS WHICH RESTORES DEVELOPMENT TO MYXOCOCCUS-XANTHUS C-SIGNALING MUTANTS

The csgA gene produces an intercellular signal during fruiting body fo rmation of the myxobacterium Myxococcus xanthus. Sporulating pseudorev ertants were isolated to allow us to understand the mechanism by which CsgA is perceived by cells and used to regulate developmental gene ex pression. Two strains, LS559 and LS560, which have closely linked tran sposon insertions, soc-559 (formerly csp-559) and soc-560 (formerly cs p-560), respectively, regained all the developmental behaviors lost by the csgA mutation including the ability to ripple, form fruiting bodi es, and sporulate. The sequence analysis of the socA locus revealed th at there are three putative protein-coding regions, designated socA1, socA2, and socA3. The deduced amino acid sequence of socA1 exhibits ch aracteristics of the short-chain alcohol dehydrogenase family. The ded uced amino acid sequence of socA2 shares 48% identity with the frdD ge ne product of the frd operon in Proteus vulgaris which anchors fumarat e reductase to the membrane. The deduced amino acid sequence of socA3 does not show homology to any known proteins. Genotypic complementatio n, Northern (RNA) blotting, DNA sequence analysis, and the pattern of gene expression all suggest that these three genes are polycistronic. Since the socA mutations effectively bypass CsgA, the question of why csgA is maintained in M. xanthus was examined by studying the long-ter m stability of socA spores. Unlike the wild type, socA mutant spores g erminated on starvation agar. Transmission electron micrographs of spo re thin sections revealed that germination is not due to an obvious st ructural deficiency of the socA spores. These results suggest that the ability of socA myxospores to survive long periods under unfavorable environmental conditions is severely compromised. Therefore, socA appe ars to be essential for the development of M. xanthus.