Control of asgE expression during growth and development of Myxococcus xanthus

One of the earliest events in the Myxococcus xanthus developmental cycle is production of an extracellular cell density signal called A-signal (or A-f actor). Previously, we showed that cells carrying an insertion in the asgE gene fail to produce normal levels of this cell-cell signal. In this study we found that expression of asgE is growth phase regulated and developmenta lly regulated. Several lines of evidence indicate that asgE is cotranscribe d with an upstream gene during development. Using primer extension analyses , we identified two 5' ends for this developmental transcript. The DNA sequ ence upstream of one 5' end has similarity to the promoter regions of sever al genes that are A-signal dependent, whereas sequences located upstream of the second 5' end show similarity to promoter elements identified for gene s that are C-signal dependent. Consistent with this result is our finding t hat mutants failing to produce A-signal or C-signal are defective for devel opmental expression of asgE. In contrast to developing cells, the large maj ority of the asgE transcript found in vegetative cells appears to be monoci stronic. This finding suggests that asgE uses different promoters for expre ssion during vegetative growth and development. Growth phase regulation of asgE is abolished in a relA mutant, indicating that this vegetative promote r is induced by starvation. The data presented here, in combination with ou r previous results, indicate that the level of AsgE in vegetative cells is sufficient for this protein to carry out its function during development.