An AraC/XylS family member at a high level in a hierarchy of regulators for phenol-metabolizing enzymes in Comamonas testosteroni R5.

Comamonas testosteroni strain R5 expresses a higher level of phenol-oxygenating activity than any other bacterial strain so far characterized. The expression of the operon encoding multicomponent phenol hydroxylase (mPH), which is responsible for the phenol-oxygenating activity, is controlled by two transcriptional regulators, PhcS and PhcR, in strain R5. In this study, we identified a third transcriptional regulator for the mPH operon (PhcT) that belongs to the AraC/XylS family. While the disruption of phcT in strain R5 significantly reduced the expression of the mPH operon, it did not eliminate the expression. However, the disruption of phcT in strain R5 increased the expression of phcR. The phenol-oxygenating activity was abolished by the disruption of phcR, indicating that PhcT alone was not sufficient to activate the expression of the mPH operon. The disruption of phcS has been shown in our previous study to confer the ability of strain R5 to express the mPH operon in the absence of the genuine substrate for mPH. PhcT was not involved in the gratuitous expression. Strain R5 thus possesses a more elaborate mechanism for regulating the mPH operon expression than has been found in other bacteria.