ctsR of Lactococcus lactis encodes a negative regulator of clp gene expression

Bacteria undergo a complex programme of differential gene expression in response to stress. In Bacillus subtilis, it was recently shown that CtsR, a negative transcriptional regulator, mediates stress-induced expression of components of the Clp protease complex. In this study, a gene was identified in the Gram-positive bacterium Lactococcus lactis that encodes a 17 kDa product with 38% identity to the CtsR protein of B. subtilis. By Northern analyses it was found that in a L. lactis strain carrying a large internal deletion of ctsR, including the region encoding a putative helix-turn-helix motif, the amounts of clpC, clpP, clpB and clpE mRNAs were increased 3-8-fold compared to those present in wild-type L. lactis MG1363. In another ctsR mutant strain in which only one-third of CtsR was deleted, leaving the putative DNA-binding domain and the C-terminal 29 amino acids intact, only minor derepression of clp gene expression was observed and, furthermore, all the clp genes were still induced by heat. These results indicate that the amino acids of CtsR involved in temperature sensing are located either close to the DNA-binding domain or in the C-terminal part of the protein. Thus, in L. lactis in addition to B. subtilis, CtsR is a key regulator of heat-shock-induced gene expression, suggesting that the presence of CtsR-homologous DNA-binding sites observed in many Gram-positive bacteria reflects functional heat-shock regulatory systems.