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

In the purple nonsulfur photosynthetic bacterium Rhodobacter sphaeroid es the synthesis of components of the photosystem is regulated in resp onse to oxygen tension acid light intensity. We have purified and clon ed a trans-acting protein (SPB) that binds to the promoter region of t he puf operon, which encodes the apoproteins of light-harvesting compl ex I and the reaction center. The SPB was composed of a single polypep tide with an apparent molecular mass of 15.0 kDa. The nucleotide seque nce of the spb gene was determined. The gene encoded 104 amino acid re sidues, which correspond to a molecular mass of 11.5 kDa. SPB exhibite d 53% homology to HvrA in Rhodobacter capsulatus. The deduced amino ac id 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 th e leucine-zipper motif. The leucine-zipper motif of SPB might contribu te to the formation of a dimer. Northern analysis indicated that spb w as constitutively and monocistronically transcribed in R. sphaeroides, irrespective of growth conditions. Structural and functional differen ces between SPB and HvrA are discussed.