Isolation and characterization of a putative transcription factor involved in the regulation of the Rhodobacter sphaeroides pucBA operon.

The pucBA operon of the photosynthetic bacterium Rhodobacter sphaeroides encodes the light harvesting 2 (LH2) apoproteins of the photosynthetic apparatus, and transcription of this operon is markedly depressed under aerobic conditions. The region upstream of the LH2 genes has been subjected to gel retardation analysis, and two distinct bandshifts have been observed. The intensity of the upper bandshift was reduced upon lowering the oxygen tension under which the cells were grown, and this reduction preceded the appearance of LH2 in the cell membrane. Purification of the sequence-specific DNA binding protein responsible for this bandshift activity revealed a protein of M(r) 26,000, and DNase I footprint analysis revealed that the binding site was located 120 base pairs upstream of the transcription initiation point. We propose that this protein is an oxygen-regulated repressor of pucBA transcription. This work, therefore, describes the first purification of a transcription factor from any photosynthetic bacterium.