Transcriptional activator of the hca operon for 3-phenylpropionic acid catabolism. Belongs to the LysR transcriptional regulatory family. The 21 C-terminal amino acids have been proposed to be expressed from an independent promoter within this gene and called IroK. Overexpression of IroK has been suggested to be responsible for 3-hydroxypropionic acid (3-HP) resistance. However the IroK peptide's existence has not been proven, nor has mutagenesis of HcaR itself rather than overexpression of IroK been ruled out as the cause of 3-HP resistance (PubMed:22161628).