The PhaD regulator controls the simultaneous expression of the pha genes involved in polyhydroxyalkanoate metabolism and turnover in Pseudomonas putida KT2442

Abstract

The promoters of the pha gene cluster encoding the enzymes involved in the metabolism of polyhydroxyalkanoates (PHAs) in the model strain Pseudomonas putida KT2442 have been identified and compared. The pha locus is composed by five functional promoters upstream the phaC1, phaZ, phaC2, phaF and phaI genes (PC1, PZ, PC2, PF and PI respectively). PC1 and PI are the most active promoters of the pha cluster allowing the transcription of phaC1ZC2D and phaIF operons. All promoters with the sole exception of PF are carbon source-dependent. Their transcription profiles explain the simultaneous production of PHA depolymerase and synthases to maintain the metabolic balance and PHA turnover. Mutagenesis analyses demonstrated that PhaD, a TetR-like transcriptional regulator, behaves as a carbon source-dependent activator of the pha cluster. The phaD gene is mainly transcribed as part of the phaC1ZC2D transcription unit and controls its own transcription and that of phaIF operon. The ability of PhaD to bind the PC1 and PI promoters was analysed by gel retardation and DNase I footprinting assays, demonstrating that PhaD interacts with a region of 25 bp at PC1 promoter (named OPRc1) and a 29 bp region at PI promoter (named OPRi). These operators contain a single binding site formed by two inverted half sites of 6 bp separated by 8 bp which overlap the corresponding promoter boxes. The 3D model structure of PhaD activator predicts that the true effector might be a CoA-intermediate of fatty acid β-oxidation.