HCF208, a homolog of Chlamydomonas CCB2, is required for accumulation of native cytochrome b6 in Arabidopsis thaliana

The cytochrome b(6) subunit of the cytochrome b(6)f complex is a multiheme protein. Two b-type hemes are bound non-covalently to the protein, whereas the third heme (heme C-n) is covalently attached via an atypical thioether bond. To understand the maturation of cytochrome b(6) and to identify the assisting factors, we characterized the ethyl methanesulfonate-induced nuclear mutant hcf208. This Arabidopsis mutant shows a high chlorophyll fluorescence phenotype and does not accumulate the major cytochrome b(6)f complex subunits. Transcript levels and patterns of the four major polypeptides of the complex are equal to those of the wild type. The mutant cytochrome b(6) polypeptide shows a faster migration behavior in SDS-PAGE compared with the wild type and it has no peroxidase activity. The HCF208 locus was mapped and the gene was cloned. Sequence analysis revealed that HCF208 is a homolog of the Chlamydomonas reinhardtii CCB2 protein, which is a factor mediating attachment of heme Cn to the cytochrome b6 polypeptide as part of a novel heme biogenesis pathway, called system IV. Blue Native PAGE revealed residual amounts of the cytochrome b(6)f complex dimer in hcf208; however, this form is unable to participate in electron transport reactions.