The act operon controls the level and time of C-signal production for Myxococcus xanthus development

Abstract

The C-signal is a morphogen that controls the assembly of fruiting bodies and the differentiation of myxospores. Production of this signal, which is encoded by the csgA gene, is regulated by the act operon of four genes that are co-transcribed from the same start site. The acta and actB genes regulate the maximum level of the C-signal, which never rises above one-quarter of the maximum wild-type level of CsgA protein in null mutants of either gene. The actA and actB mutants have the same developmental phenotype: both aggregate, neither sporulates, both prolong rippling. By sequence homology, actA encodes a response regulator, and actB encodes a sigma-54 activator protein of the NTRC class. The similar phenotypes of actA and actB deletion mutants suggest that the two gene products are part of the same signal transduction pathway. That pathway responds to C-signal and also regulates the production of CsgA protein, thus creating a positive feedback loop. The actC and actD genes regulate the time pattern of CsgA production, while achieving the same maximum level. An actC null mutant raises CsgA production 15 h earlier than the wild type, whereas an actD null mutant does so 6 h later than wild type. The loop explains how the C-signal rises continuously from early development to a peak at the time of sporulation, and the act genes govern the time course of that rise.