The noc and occ regions of nopaline and octopine Ti plasmids in Agrobacterium tumefaciens contain genes for the catabolism of nopaline and octopine, respectively. We investigated the transcriptional organization and regulation of both regions. The noc region of pTiC58 contains two nopaline-inducible promoters, and one octopine-inducible promoter was identified in the occ region of pTiAch5. All three promoters are positively regulated in trans by constitutively expressed genes localized at the right end of the regions. The DNA sequence analysis of these parts revealed genes coding for related proteins (35.6% identity). The two polypeptides share significant similarity with a family of other positive gene regulators, and both contain a protein motif ("LysR" signature) that is characteristic for the DNA binding domain in these polypeptides. These proteins are the only Ti plasmid functions necessary for the activation of the opine-induced promoters. We propose the names nocR and occR for the regulator genes in the noc and in the occ regions.