Suppressors that permit A-signal-independent developmental gene expression in Myxococcus xanthus

Progression through the early stages of Myxococcus xanthus fruiting body development requires the cell-to-cell transmission of soluble material called A signal. During these early stages, expression from the gene identified by Tn5 lac insertion omega 4521 increases. A DNA probe of the omega 4521 gene was constructed. Use of this probe showed that accumulation of mRNA corresponding to the omega 4521 gene depends upon A signal. A-signal-deficient (asg) mutants fail to accumulate this RNA, and the external addition of A signal restores accumulation. To identify links between A signal and its responsive gene, omega 4521, suppressors of an asg mutation were generated. All of the suppressor alleles restored lacZ expression from omega 4521 in the absence of A signal, and they were demonstrated to be neither reversions of the asgB mutation nor mutations in the promoter of omega 4521. Fifteen suppressor mutations map to two loci, sasA and sasB (for suppressor of asg). sasA and sasB mutants differ phenotypically during growth and development. Mid-logarithmic-phase sasA asgB double mutants, like sas+ asg+ strains, express low levels of lacZ, whereas sasB asgB double mutants express high levels. sasA asg+ mutants form abnormal colonies, are less cohesive than wild type, and are defective in fruiting body formation and sporulation. In contrast, sasB asg+ mutants form normal colonies, are as cohesive as wild type, and appear to develop normally. The characteristics of sasA suppressors implicate the sasA+ product as a negative regulator in the A-signal-dependent regulation of omega 4521.