Subunit of the two-component enzyme NicAB that mediates nicotinate hydroxylation, the first step in the aerobic nicotinate degradation pathway. Mediates conversion of nicotinate into 6-hydroxynicotinate (6HNA). 2 Fe(III)-[cytochrome] + H2O + nicotinate = 6-hydroxynicotinate + 2 Fe(II)-[cytochrome] + 2 H(+) Binds 2 [2Fe-2S] clusters per subunit. Cofactor degradation; nicotinate degradation. Repressed by NicS in the absence of 6-hydroxynicotinate (6HNA) or nicotinate inducers. In presence of 6HNA, repression is alleviated. Cells lacking both nicA and nicB lack the ability to hydroxylate nicotinate. Cells do not grow in a nicotinate medium but grow in a 6-hydroxynicotinate medium.