Characterization of two operons encoding the cytochrome b6-f complex of the cyanobacterium Nostoc PCC 7906. Highly conserved sequences but different gene organization than in chloroplasts

Abstract

We have isolated and determined the nucleotide and derived protein sequences for the four genes, petCA and BD, which encode the cytochrome b6-f, electron-transfer complex of the filamentous cyanobacterium, Nostoc PCC 7906. The primary structure and cotranscription of the petCA genes encoding the Rieske-FeS (nuclear encoded in plants) and apocytochrome f proteins has been described previously (Kallas, T., Spiller, S., and Malkin, R. (1988) Proc. Natl. Acad. Sci. U.S.A., in press). The petBD genes (645 and 480 protein-coding nucleotides, respectively) for the apocytochrome b6 (24.3 kDa) and subunit-IV (17.5 kDa) proteins comprise a second operon located at least 12 kilobases (kb) from petCA. The Nostoc petBD genes are not closely linked to the psbB gene (encoding the 51-kDa photosystem II polypeptide) and do not contain introns as do the closely related chloroplast genes. DNA probes specific for each of the Nostoc cytochrome-complex genes hybridized to single bands in genomic DNA blots at intensities expected for single copy genes. These data suggest that a single set of cytochrome b6-f proteins function in the different types of membranes found in Nostoc vegetative and heterocyst cells. RNA blot hybridizations identified an 1.8-kb mRNA common to cytochrome b6 and subunit IV, and an intensely hybridizing 0.8-kb mRNA specific to the subunit IV gene probe. The role of the latter RNA is not clear but it may represent a transcript from the opposite strand. The deduced Rieske, apocytochrome f, apocytochrome b6, and subunit IV proteins exhibit 59, 58-63, 84-85, and 79-83% sequence identity with the proteins from chloroplast cytochrome b6-f complexes. The Nostoc proteins show lower but still significant sequences identity with the corresponding proteins of the mitochondrial-type b-c1 complexes. The four probable heme-liganding His residues, and the approximate spacings between them, have been conserved in all of the available cytochrome b6 and b sequences from divergent sources. The Nostoc apocytochrome b6 and subunit IV proteins, as well as the Rieske, appear to be translated and thus inserted into the membrane as mature forms without cleavable presequences. Hydropathy analyses revealed five potential membrane spans in cytochrome b6 and three in the subunit IV protein, consistent with the profiles observed for the chloroplast proteins and the related cytochrome b proteins of cytochrome b-c1 complexes.