A tactile sensory system of Myxococcus xanthus involves an extracellular NAD(P)(+)-containing protein

Characterization of the regulatory region of a cell interaction-dependent gene in Myxococcus xanthus

omega 4403 is the site of a Tn5 lac insertion in the Myxococcus xanthus genome that fuses lacZ expression to a developmentally regulated promoter. Cell-cell interactions that occur during development, including C-signaling, are required for expression of Tn5 lac omega 4403. We have cloned DNA upstream of the omega 4403 insertion site, localized the promoter, and identified a potential open reading frame. From the deduced amino acid sequence, the gene disrupted by Tn5 lac omega 4403 appears to encode a serine protease that is dispensable for development. The gene begins to be expressed between 6 and 12 h after starvation initiates development, as determined by measuring mRNA or beta-galactosidase accumulation in cells containing Tn5 lac omega 4403. The putative transcriptional start site was mapped, and sequences centered near -10 and -35 bp relative to this site show some similarity to the corresponding regions of promoters transcribed by Escherichia coli sigma70 RNA polymerase. However, deletions showed that an essential promoter element lies between -80 and -72 bp, suggesting the possible involvement of an upstream activator protein. DNA downstream of -80 is sufficient for C-signal-dependent activation of this promoter. The promoter is not fully expressed when fusions are integrated at the Mx8 phage attachment site in the chromosome. Titration of a limiting factor by two copies of the regulatory region (one at the attachment site and one at the native site) can, in part, explain the reduced expression. We speculate that the remaining difference may be due to an effect of chromosomal position. These results provide a basis for studies aimed at identifying regulators of C-signal-dependent gene expression.

C factor, a cell-surface-associated intercellular signaling protein, stimulates the cytoplasmic Frz signal transduction system in Myxococcus xanthus

C factor, an intercellular signaling protein, is required for aggregation and sporulation of the social bacterium, Myxococcus xanthus. We report that C factor, which normally is associated with the cell surface, provides input to the Frz signal transduction cascade. Elements of this cascade have sequence homology to bacterial chemotaxis systems and are known to control the frequency of gliding reversal. Exposure of developing cells of a C-factor-less mutant (csgA) to purified C factor increases the ratio of methylated to nonmethylated FrzCD protein, the Frz homolog of the methyl-accepting chemotaxis proteins. Methylation depends on the cognate methyltransferase FrzF, and its extent increases with the concentration of C factor. C-factor-induced methylation also depends on the product of a gene, called class II, which is necessary in vivo for all known responses to C factor. A model for aggregation is proposed in which C factor stimulates the Frz cascade and thereby decreases cell reversals in a way that preferentially leads cells into an aggregate.

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.