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

Abstract

ABSTRACT 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, p B 1 and p B 2. The region corresponding to the −35 region of p B 1 overlaps the p B 2 −10 region. In the phbR mutant, expression of phbB from the p B 1 promoter is significantly reduced, whereas expression from the p B 2 promoter is slightly increased. Two phbR promoters, p R 1 and p R 2, were also identified. Transcription from p R 2 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 p B 1 promoter. A model for the regulation of phbB transcription by PhbR is proposed.