The Nla28S/Nla28 two-component signal transduction system regulates sporulation in Myxococcus xanthus

The response regulator Nla28 is a key component in a cascade of transcriptional activators that modulates expression of many important developmental genes in Myxococcus xanthus. In this study, we identified and characterized Nla28S, a histidine kinase that modulates the activity of this important regulator ofM. xanthus developmental genes. We show that the putative cytoplasmic domain of Nla28S has the in vitro biochemical properties of a histidine kinase protein: it hydrolyzes ATP and undergoes an ATP-dependent autophosphorylation that is acid labile and base stable. We also show that the putative cytoplasmic domain of Nla28S transfers a phosphoryl group to Nla28 in vitro, that the phosphotransfer is specific, and that a substitution in the predicted site of Nla28 phosphorylation (aspartate 53) abolishes the phosphotransfer reaction. In phenotypic studies, we found that a mutation in nla28S produces a developmental phenotype similar to, but weaker than, that produced by a mutation in nla28; both mutations primarily affect sporulation. Together, these data indicate that Nla28S is the in vivo histidine kinase partner of Nla28 and that the primary function of the Nla28S/Nla28 two-component signal transduction system is to regulate sporulation genes. The results of genetic studies suggest that phosphorylation of Nla28S is important for the in vivo sporulation function of the Nla28S/Nla28 two-component system. In addition, the quorum signal known as A-signal is important for full developmental expression of the nla28S-nla28 operon, suggesting that quorum signaling regulates the availability of the Nla28S/Nla28 signal transduction circuit in developing cells.