Proper expression of the dev operon is important for normal development of Myxococcus xanthus. When starved, these bacteria coordinate their gliding movements to build mounds that become fruiting bodies as some cells differentiate into spores. Mutations in the devTRS genes impair sporulation. Expression of the operon occurs within nascent fruiting bodies and depends in part on C signaling. Here, we report that expression of the dev operon, like that of several other C-signal-dependent genes, is subject to combinatorial control by the transcription factors MrpC2 and FruA. A DNA fragment upstream of the dev promoter was bound by a protein in an extract containing MrpC2, protecting the region spanning positions ؊77 to ؊54. Mutations in this region impaired binding of purified MrpC2 and abolished developmental expression of reporter fusions. The association of MrpC2 and/or its longer form, MrpC, with the dev promoter region depended on FruA in vivo, based on chromatin immunoprecipitation analysis, and purified FruA appeared to bind cooperatively with MrpC2 to DNA just upstream of the dev promoter in vitro. We conclude that cooperative binding of the two proteins to this promoter-proximal site is crucial for dev expression. 5= deletion analysis implied a second upstream positive regulatory site, which corresponded to a site of weak cooperative binding of MrpC2 and FruA and boosted dev expression 24 h into development. This site is unique among the C-signal-dependent genes studied so far. Deletion of this site in the M. xanthus chromosome did not impair sporulation under laboratory conditions. Myxococcus xanthus is a Gram-negative bacterium that undergoes multicellular development (1). Upon starvation on a solid surface, cells coordinate their movements to build mounds that contain thousands of cells. Within these nascent fruiting bodies, some cells differentiate from rods to ovoid spores. Other cells lyse during the developmental process or remain outside fruiting bodies as peripheral rods. The spores remain viable during prolonged starvation and resist environmental insults. Under favorable conditions, spores germinate, producing rod-shaped cells capable of growth and division.The developmental process of M. xanthus provides an attractive model to study signaling and gene regulatory mechanisms (1). Here, we focus on regulation of the dev operon in response to extracellular C signaling. The dev locus was identified by two transposon insertions that created reporter fusions induced during development (2, 3). Expression from the fusions was reduced in a csgA mutant incapable of C signaling (4). The transposon insertions in the dev locus prevented darkening of nascent fruiting bodies and reduced sporulation Ͼ100-fold (3, 5). The sporulation defect can be accounted for by the observation that a dev mutant fails to express the ⍀7536 locus (6), the site of the exo operon, whose products are necessary for spore formation (7). How the products of the dev operon regulate the expression of the exo operon is unknown. The dev op...
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