Expression, inducer spectrum, domain structure, and function of MopR, the regulator of phenol degradation in Acinetobacter calcoaceticus NCIB8250.

Degradation of phenol by Acinetobacter calcoaceticus NCIB8250 involves (sigma54-dependent expression of a multicomponent phenol hydroxylase and catechol 1,2-dioxygenase encoded by the mop operon. Complementation of a new mutant deficient in phenol utilization yielded the regulatory locus mopR. It is located in divergent orientation next to the mop operon. MopR is constitutively expressed at a low level from a sigma70-type promoter and belongs to the NtrC family of regulators. The amino acid sequence is similar to that of XylR regulating xylene degradation and to that of DmpR regulating dimethylphenol degradation in Pseudomonas spp. However, it shows a different effector profile for substituted phenols than DmpR. MopR activates phenol hydroxylase expression in the presence of phenol in Escherichia coli, indicating that it binds the effector. The phenol binding A domains of MopR and DmpR have fewer identical residues than the A domains of DmpR and XylR, despite the fact that XylR recognizes different effectors. This suggests that sequence conservation in the A domain does not reflect the potential to bind the respective effectors. Overexpression of the MopR A domain in the presence of wild-type MopR causes loss of mop inducibility by phenol, establishing its negative transdominance over MopR. Deletion of 110 residues from the N terminus did not affect transdominance of the truncated domain, whereas deletion of 150 residues abolished it completely. This result establishes the distinction of two subdomains, A(N) and A(C), which together constitute the A domain. The C-terminal portion of the A domain, A(C), shows considerable affinity for the C domain, even in the presence of the trigger phenol.