Accumulation of pre-apocytochrome f in a Synechocystis sp PCC 6803 mutant impaired in cytochrome c maturation

Cytochrome c maturation involves heme transport and covalent attachment of heme to the apoprotein, The 5' end of the ccsB gene, which is involved in t he maturation process and resembles the ccs1 gene from Chlamydomonas reinha rdtii, was replaced by a chloramphenicol resistance cartridge in the cyanob acterium Synechocystis sp. PCC 6803, The resulting Delta(M1-A24) mutant lac king the first 24 ccsB codons grew only under anaerobic conditions. The mut ant retained about 20% of the wild-type amount of processed cytochrome f wi th heme attached, apparently assembled in a functional cytochrome b(6)f com plex, Moreover, the mutant accumulated unprocessed apocytochrome f in its m embrane fraction. A pseudorevertant was isolated that regained the ability to grow under aerobic conditions. The locus of the second-site mutation was mapped to ccsB, and the mutation resulted in the formation of a new potent ial start codon in the intergenic region, between the chloramphenicol resis tance marker and ccsB, in frame with the remaining part of ccsB. In this ps eudorevertant the amount of holocyt f increased, whereas that of unprocesse d apocytochrome f decreased. me suggest that the original deletion mutant D elta(M1-A24) expresses an N-terminally truncated version of the protein. Th e stable accumulation of unprocessed apocytochrome f in membranes of the De lta(M1-A24) mutant may be explained by its association with truncated and o nly partially functional CcsB protein resulting in protection from degradat ion. Our attempt to delete the first 244 codons of ccsB in Synechocystis sp , PCC 6803 was not successful, suggesting that this would lead to a lack of functional cytochrome b(6)f complex, The results suggest that the CcsB pro tein is an apocytochrome chaperone, which together with CcsA may constitute part of cytochrome c lyase.