ABSTRACT
The two-component regulatory system CiaRH of Streptococcus pneumoniae affects a variety of processes such as competence development, autolysis, bacteriocin production, host colonization, and virulence. While the targets of the regulator CiaR are known, the role of phosphorylation in CiaR regulation has not been defined. To address this issue, the presumed phosphorylation site of CiaR, aspartic acid at position 51, was replaced by alanine. The mutant CiaRD51A protein was no longer able to activate CiaR-dependent promoters, strongly suggesting that the phosphorylated form of CiaR is active in regulation. However, depending on the growth medium, inactivation of the kinase gene ciaH resulted in a subtle increase of CiaR-dependent promoter activities or in a strong reduction. Therefore, CiaH may act as a kinase or phosphatase and CiaR is apparently able to obtain its phosphate independently of CiaH. On the other hand, promoter measurements in cells with an intact CiaRH system demonstrated a high, nearly constitutive, expression level of the CiaR regulon independent from the growth medium. Thus, in contrast to many other two-component regulatory systems, CiaRH has apparently evolved to maintain high levels of gene expression under a variety of conditions rather than responding strongly to a signal.
