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 mutation 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.