A Transcription Factor with a Leucine-Zipper Motif Involved in Light-Dependent Inhibition of Expression of the puf Operon in the Photosynthetic Bacterium Rhodobacter sphaeroides

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In the purple nonsulfur photosynthetic bacterium Rhodobacter sphaeroides the synthesis of components of the photosystem is regulated in response to oxygen tension and light intensity. We have purified and cloned a trans-acting protein (SPB) that binds to the promoter region of the puf operon, which encodes the apoproteins of light-harvesting complex I and the reaction center. The SPB was composed of a single polypeptide with an apparent molecular mass of 15.0 kDa. The nucleotide sequence of the spb gene was determined. The gene encoded 104 amino acid resi-dues, which correspond to a molecular mass of 11.5 kDa. SPB exhibited 53% homology to HvrA in Rhodobacter capsulatus. The deduced amino acid sequence indicated that SPB contained a region with homology to the leucine-zipper motif of C-JUN, a transcription factor in eukaryotes, and SPB also had a DNA-binding domain on the amino-terminal side of the leucine-zipper motif. The leucine-zipper motif of SPB might contribute to the formation of a dimer. Northern analysis indicated that spb was constitutively and monocistronically transcribed in R. sphaeroides, irrespective of growth conditions. Structural and functional differences between SPB and HvrA are discussed.
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