Phosphorylation of the RitR DNA-binding domain by a Ser-Thr phosphokinase: implications for global gene regulation in the streptococci

We report selective phosphorylation of the DNA-binding domain of the Streptococcus pneumoniae transcriptional regulator RitR. RitR is annotated as a two-component response regulator, but lacks a cognate His kinase as a neighbouring locus in the genome. In addition, Asn replaces Asp at the expected acceptor site. By the use of combinatorial phage display, we identified PhpP, a S. pneumoniae Ser-Thr eukaryotic-like PP2C phosphatase as an interacting partner of RitR. RitR interacts with the phage-displayed peptide VADGMGGR which forms a part of the active-site sequence of PhpP. RitR is phosphorylated in vitro by StkP, the presumed cognate kinase of PhpP, and the site on RitR that is phosphorylated has been localized to the RitR DNA-binding domain. PhpP together with its cognate kinase StkP appear to be necessary for Piu haem transporter expression. In vitro studies suggest that PhpP and StkP interact competitively with RitR in that RitR-PhpP-piu promoter ternary complexes are disrupted by StkP. Our findings indicate a regulatory link between RitR and Ser-Thr kinase-phosphatase-based bacterial signal transduction.