C-factor has distinct aggregation and sporulation thresholds during Myxococcus development

C-factor, the protein product of the csgA gene, acts as a short-range morphogenetic signal. It is required for fruiting body development of the gram-negative bacterium Myxococcus xanthus. Aggregation, sporulation, and expression of a set of genes that are C-factor dependent, all of which fail in csgA mutant cells, are completely restored by addition of purified C-factor. We report here that, depending on its concentration, C-factor can elicit two distinct morphogenetic and transcriptional responses from csgA cells. Low levels of C-factor bring about aggregation and expression of an early C-dependent gene, whereas higher levels lead to the same effects plus expression of a late C-dependent gene and spore formation. C-factor positively regulates its own transcription. An approximately fourfold net increase in csgA transcription and C-factor levels during development was measured. We propose that autoregulation and the two distinct activity thresholds allow C-factor to act as a timer, first triggering aggregation, then sporulation, thereby producing the appropriate developmental order.