Expression of the Azotobacter vinelandii Poly-β-Hydroxybutyrate Biosynthetic phbBAC Operon Is Driven by Two Overlapping Promoters and Is Dependent on the Transcriptional Activator PhbR

摘要

The Azotobacter vinelandii phbBAC genes encode the enzymes for poly-β-hydroxybutyrate (PHB) synthesis. The phbR gene, which is located upstream of and in the opposite direction of phbBAC, encodes PhbR, a transcriptional activator which is a member of the AraC family of activators. Here we report that a mutation in phbR reduced PHB accumulation and transcription of a phbB-lacZ fusion. We also report that phbB is transcribed from two overlapping promoters, pB1 and pB2. The region corresponding to the −35 region of pB1 overlaps the pB2 −10 region. In the phbR mutant, expression of phbB from the pB1 promoter is significantly reduced, whereas expression from the pB2 promoter is slightly increased. Two phbR promoters, pR1 and pR2, were also identified. Transcription from pR2 was shown to be dependent on σ^S. Six conserved 18-bp sites, designated R1 to R6, are present within the phbR-phbB intergenic region and are proposed to be putative binding targets for PhbR. R1 overlaps the −35 region of the pB1 promoter. A model for the regulation of phbB transcription by PhbR is proposed.