Two intercellular signals required for fruiting body formation in Myxococcus xanthus act sequentially but non‐hierarchically

Abstract

SummaryStarvation-induced fruiting body formation in Myxococcus xanthus depends on intercellular signalling. A-signal functions after 2 h of starvation and its synthesis depends on the asg genes. C-signal functions after 6 h of starvation and is generated by proteolytic cleavage of a precursor by the protease PopC. Previous gene expression studies suggested that the Aand C-signal lie on a hierarchical pathway. Here we explored the causal relationship between the A-and C-signal. The asgA and asgB mutants have reduced popC expression, PopC accumulation and C-signal accumulation. popC expression was shown not to depend on A-signal but on the AsgA and AsgB proteins. Restored popC expression in the two mutants rescued PopC and C-signal accumulation as well as C-signalling and the developmental defects of the two mutants without restoring A-signalling. Based on these results we suggest that A-and C-signal do not lie on a hierarchical, dependent pathway. Instead the Aand C-signal act sequentially and without a causal relationship suggesting that they are linked by a shared timing mechanism, which ensures the early and late onset of A-signalling and C-signalling, respectively, during starvation. This pathway topology represents a novel architecture for bacterial intercellular signalling systems involving more than one signal.

Genetic Redundancy, Proximity, and Functionality of lspA , the Target of Antibiotic TA, in the Myxococcus xanthus Producer Strain

“…The above-mentioned result is consistent with prior findings that the ta biosynthetic gene cluster is transcriptionally repressed in tan variants and that expression of DKxanthene (yellow pigment) and that of TA are transcriptionally coupled (36,41,42). From these findings, we hypothesized that the tan phase bias of the ⌬lspA3 mutant may explain why it produced a smaller ZOI (Fig.…”

“…These findings, in conjunction with the E. coli complementation results, suggest that the lspA genes have redundant functions in M. xanthus, although either lspA1 or lspA2 must be retained for viability. In this work, we provide evidence that the reasons for redundancy may be related to expanded functions of LspA3 and LspA4 in TA resistance and regulation, and the genes themselves can be differentially regulated (36,41,42). Other possible reasons for redundancy include alternative routes of lipoprotein secretion (11), modification, and perhaps sorting in the cell.…”

“…2B). In a regulatory context, the lspA3 locus is located near the middle of the ta gene cluster, and thus, when the expression of the TA biosynthetic pathway is induced, such as by yellow phase variants or by development (36,41,42), LspA3 will also be expressed. Based on these findings, it is plausible that the function of LspA3 is to help confer producer strain resistance, perhaps mediated by reduced affinity for TA, while still providing necessary SPaseII function.…”

Nutrient-Regulated Proteolysis of MrpC Halts Expression of Genes Important for Commitment to Sporulation during Myxococcus xanthus Development

“…On the other hand, nutrient addition at 30 or 36 h poststarvation did not stop the formation of compact, darkened fruiting bodies (Fig. 1C), indicating that a starving culture commits to fruiting body formation by 30 h. Nutrient addition at all time points tested (18,24,30) did not prevent the eventual reduction of total protein concentration, although it did not decrease as rapidly (see Fig. S2B in the supplemental material).…”

“…A-signaling has been proposed to play a quorum-sensing role that at a high enough cell density stimulates expression of certain genes and causes cells to begin building mounds (15,16). Like A-signal, production of extracellular C-signal depends on starvation-linked (p)ppGpp accumulation inside cells and on Asg proteins, but C-signal is produced more slowly (17,18), accumulates on the surface of cells (19,20), and its effects are manifest slightly later as cells align during aggregation into mounds (21)(22)(23)(24). C-signaling stimulates expression of many genes beginning at about 6 h poststarvation (24) and is required for cells to form compact mounds that mature into fruiting bodies as spores form inside (25).…”