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

The C-signal is a morphogen that controls the assembly of fruiting bodies a nd the differentiation of myxospores. Production of this signal, which is e ncoded by the csgA gene, is regulated by the act operon of four genes that are co-transcribed from the same start site. The act A and act B genes regu late 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 g ene. The act A and act B mutants have the same developmental phenotype: bot h aggregate, neither sporulates, both prolong rippling. By sequence homolog y, act A encodes a response regulator, and act B encodes a sigma-54 activat or protein of the NTRC class. The similar phenotypes of act A and act B del etion mutants suggest that the two gene products are part of the same signa l transduction pathway. That pathway responds to C-signal and also regulate s the production of CsgA protein, thus creating a positive feedback loop. T he act C and act D genes regulate the time pattern of CsgA production, whil e achieving the same maximum level. An act C null mutant raises CsgA produc tion 15 h earlier than the wild type, whereas an act D null mutant does so 6 h later than wild type. The loop explains how the C-signal rises continuo usly from early development to a peak at the time of sporulation, and the a ct genes govern the time course of that rise.