NUCLEOTIDE-SEQUENCE OF THE PETB GENE ENCODING CYTOCHROME B(6) FROM THE MESOPHILIC CYANOBACTERIUM SYNECHOCYSTIS PCC-6803 - IMPLICATIONS FOR EVOLUTION AND FUNCTION

The gene encoding the cytochrome b(6) subunit (petB) of the cytochrome b(6)f complex has been isolated, cloned and sequenced by nonradioacti ve methods from genomic DNA of the unicellular cyanobacterium Syunecho cystis sp. PCC 6803. The coding region consists of 666 nucleotides, co ding for a polypeptide with a molecular mass of 25.02 kDa. In contrast to higher plant petB sequences an aminoterminal extension of seven am ino acids occurs. Aminoterminal sequencing of the isolated protein exc ludes - different from higher plants;- the existence of an intron afte r the first amino acids but indicate the posttranslational removal of three amino acids from the amino terminus. The aminoterminal extension - found only in non-nitrogen-fixing, unicellular cyanobacteria - show s a high degree of homology between different species.