Role of σDin Regulating Genes and Signals duringMyxococcus xanthusDevelopment

Abstract

Starvation-induced development ofMyxococcus xanthusis an excellent model for biofilm formation because it involves cell-cell signaling to coordinate formation of multicellular mounds, gene expression, and cellular differentiation into spores. The role of σ^D, an alternative σ factor important for viability in stationary phase and for stress responses, was investigated during development by measuring signal production, gene expression, and sporulation of asigDnull mutant alone and upon codevelopment with wild-type cells or signaling mutants. ThesigDmutant responded to starvation by inducing (p)ppGpp synthesis normally but was impaired for production of A-signal, an early cell density signal, and for production of the morphogenetic C-signal. Induction of early developmental genes was greatly reduced, and expression of those that depend on A-signal was not restored by codevelopment with wild-type cells, indicating that σ^Dis needed for cellular responses to A-signal. Despite these early developmental defects, thesigDmutant responded to C-signal supplied by codeveloping wild-type cells by inducing a subset of late developmental genes. σ^DRNA polymerase is dispensable for transcription of this subset, but a distinct regulatory class, which includes genes essential for sporulation, requires σ^DRNA polymerase or a gene under its control, cell autonomously. The level ofsigDtranscript in arelAmutant during growth is much lower than in wild-type cells, suggesting that (p)ppGpp positively regulatessigDtranscription in growing cells. ThesigDtranscript level drops in wild-type cells after 20 min of starvation and remains low after 40 min but rises in arelAmutant after 40 min, suggesting that (p)ppGpp negatively regulatessigDtranscription early in development. We conclude that σ^Dsynthesized during growth occupies a position near the top of a regulatory hierarchy governingM. xanthusdevelopment, analogous to σ factors that control biofilm formation of other bacteria.