A mutation in the ciaH gene of Streptococcus pneumoniae induces cefotaxime resistance and transformation deficiency. ciaH encodes a putative sensor protein that belongs to the family of signal-transducing histidine kinases. This gene is adjacent to ciaR, which encodes a DNA-binding protein involved in the regulation of genes responding to environmental signals sensed by the histidine kinase. The authors have characterized a mutation that induces reversion of both cefotaxime resistance and transformation deficiency. It is a T/A deletion in the ciaR gene resulting in the synthesis of a truncated protein containing only 125 amino acids instead of 224. The ciaH mutation requires a functional CiaR protein for expression. Northern blot analysis, using ciaR-ciaH as a probe, revealed one mRNA from the wild-type strain, indicating that the two genes constitute an operon. Comparisons of Northern blots show that the operon is constitutively activated in the strain carrying only the ciaH mutation. In the wild-type strain the activation occurs when the Ca2+ concentration is very low, demonstrating that Ca2+ is the environmental signal. The pleiotropic effects caused by the ciaH mutation include sensitivity to antibiotics and toxins, the ability to form protoplasts and the susceptibility to lysis with deoxycholate. Null-mutants were constructed in both genes and the particular features of the ciaR null mutant determined. It is able to grow in choline-deprived medium, and competence development occurs in a phosphate-deprived competence medium (CH-maleate), suggesting that the CiaH-CiaR system regulates several pathways, including teiochoic acid synthesis.