The Myxococcus xanthus Red two-component signal transduction system: a novel “four component” signaling mechanism

Two-component systems are widely used by bacteria as signaling modules to sense, response and adapt to environmental changes. In Myxococcus xanthus, two-component systems play an essential role during the complex starvation induced developmental program. During development, cells first migrate into mounds and then, within these mounds differentiate into spores, forming multicellular structures termed fruiting bodies. It has been previously demonstrated that progression through the developmental program is modulated by the RedCDEF proteins which are postulated to form an unusual two-component signal transduction system consisting of two histidine kinase homologs (RedC and RedE) and two response regulator homologs (RedD and RedF) (Higgs et al, 2005). To determine how the signals flow between these unusual two-component signaling proteins, both genetic and biochemical approaches were employed. Analysis of in-frame deletion and non-functional point mutants in each gene determined that RedF in its phosphorylated state and the histidine kinase activity of RedC are necessary to repress progression through the developmental program, while RedE and RedD are necessary to induce developmental progression. Genetic epistasis experiments indicated that RedE specifically antagonizes function of RedF, and RedD acts upstream to RedE. Our biochemical analyses demonstrate that RedC readily autophosphorylates and the phosphoryl group can be transferred to the RedD. Interestingly, RedE does not appear to autophosphorylate, but instead receives a phosphoryl group from RedD. Furthermore, RedE also acts as phosphatase on RedF. Taken together, these data suggest a model for a sophisticated signaling system in which RedC is likely to act as kinase on RedF to repress developmental progression. Developmental repression is relieved when RedC is induced, by an unknown mechanism, to transfer its phosphoryl group to RedD, which then passes the phosphoryl group to RedE. The phosphorylation of RedE allows RedE to de-phosphorylate RedF. Thus, this work defines a novel “four component” signal transduction mechanism within the two-component signal transduction family.