Rhodococcus jostii RHA1 degrades polychlorinated biphenyls (PCBs) by cometabolism with biphenyl. The bphS1T1-coding two-component system, which is composed of a sensor kinase, BphS1, and a response regulator, BphT1, activates the transcription of biphenyl/PCB degradation genes from the five promoters of bphAa, etbAa1, etbAa2, etbAd, and etbD1 in the presence of aromatics, such as biphenyl and ethylbenzene. The transcription start sites of etbAd and etbD1 were determined and the results indicated that the 18-bp consensus sequence is shared by all five promoters at the equivalent position from their transcriptional start sites. To investigate the involvement of the 18-bp consensus sequence in the regulation of BphS1T1, a hybrid promoter was constructed by connecting the 18-bp consensus sequence of bphAa promoter to a portion of the benzoate dioxygenase gene promoter, which is not under the control of BphS1T1. The ethylbenzene-dependent induction of the hybrid promoter by BphS1T1 was not observed. Recently, a 24-bp consensus sequence that included the 18-bp consensus sequence of the bphAa promoter was identified in the regions conserved among RHA1 and other rhodococcal degraders. When the 24-bp consensus sequence was employed instead, both BphS1T1-dependent basal activation and ethylbenzene-dependent induction of the hybrid promoter were observed. Mutations in the six extra residues outside the 18-bp sequence in the 24-bp consensus sequence, affected not only ethylbenzene-dependent induction but also BphS1T1-dependent basal activation. The outstanding conservation of the 24-bp consensus sequence was confirmed by multiple sequence alignment. These results indicate that the 24-bp consensus sequence is really responsible for the regulation of BphS1T1. (C) 2011. The Society for Biotechnology, Japan. All rights reserved.