AN AROMATIC EFFECTOR SPECIFICITY MUTANT OF THE TRANSCRIPTIONAL REGULATOR DMPR OVERCOMES THE GROWTH CONSTRAINTS OF PSEUDOMONAS SP STRAIN CF600 ON PARASUBSTITUTED METHYLPHENOLS

The pV1150 catabolic plasmid of Pseudomonas sp. strain CF600 carries t he dmp system, which comprises the divergently transcribed dmpR gene a nd the dmp operon coding for the catabolic enzymes required for growth on (methyl)phenols. The constitutively expressed DmpR transcriptional activator positively controls the expression of the RpoN-dependent dm p operon promoter in the presence of the aromatic effector in the grow th medium. However, the magnitude of the transcriptional response diff ers depending on the position of the methyl substituent on the aromati c ring. Experiments involving an elevated copy number of the dmp syste m demonstrate that growth on para-substituted methylphenols is limited by the level of the catabolic enzymes. An effector specificity mutant of DmpR, DmpR-E135K, that responded to the presence of 4-ethylphenol, a noneffector of the wild type protein, was isolated by genetic selec tion. The single point mutation in DmpR-E135K, which results in a Glu- to-Lys change in residue 135, also results in a regulator with enhance d recognition of para-substituted methylphenols. The DmpR-E135K mutati on, when introduced into the wild-type strain, confers enhanced utiliz ation of the para-substituted methylphenols. These experiments demonst rate that the aromatic effector activation of wild-type DmpR by the pa ra-substituted methylphenols is a major factor limiting the catabolism of these compounds.