Transcriptional repressor for the nicAB operon, encoding the upper aerobic nicotinate degradation pathway. Acts under non-induced conditions: repression of the nicAB operon becomes alleviated in presence of either nicotinate or 6-hydroxynicotinate (6HNA). Cofactor degradation; nicotinate degradation [regulation]. Expressed at a basal level under non-inducing conditions, but expression increases when the cells are grown in nicotinate as sole carbon source. Growth in nicotinate or 6HNA is similar to that of the wild-type strain. Mutant strain shows constitutive levels of nicotinate hydroxylase activity, and these levels are similar to the induced levels detected in the wild-type strain grown in nicotinate. On the contrary, the NicX activity shows the same induction profile than that observed in the parental strain.