ROLE OF THE TRANSCRIPTIONAL ACTIVATOR ROCR IN THE ARGININE-DEGRADATION PATHWAY OF BACILLUS-SUBTILIS

In Bacillus subtilis, genes involved in arginine and ornithine catabol ism constitute two operons, rocABC and rocDEF. Inducible expression of these two operons is SigL-dependent and requires the transcriptional activator RocR. RocR is a member of the NtrC/ NifA family of regulator s. To study the molecular mechanisms leading to the activation of RocR , we constructed a series of mutants affected in various steps of argi nine catabolism. Results obtained using these mutants strongly suggest that the true inducer is ornithine or citrulline. Constitutive mutant s of rocR containing either missense mutations, frameshift mutations r esulting from deletions, or in-frame deletions leading to the synthesi s of N-terminal truncated RocR polypeptides were obtained. Analysis of these mutants indicates that the N-terminal part of RocR is an intram olecular repressor domain. AhrC is a second positive regulatory protei n of the rocABC and rocDEF operons. Two missense mutations modifying t he N-terminal domain of RocR led to high constitutive expression of th e Roc regulon in the absence of AhrC. Constitutive RocR proteins still require the presence of UAS1 and therefore probably bending of the DN A region located between the UAS1 and the promoter, suggesting that Ah rC is not involved in DNA bending which facilitates interaction betwee n RocR and sigma(54)-RNA polymerase. We suggest that the positive role of AhrC involves protein-protein interaction with RocR.