Identification of a nifDK-like gene (ORF467) involved in the biosynthesis of chlorophyll in the cyanobacterium Plectonema boryanum.
The frxC gene, which is found in chloroplast DNA (ctDNA) and in cyanobacteria, encodes a protein that is required for the light-independent reduction of protochlorophyllide (Pchlide) to chlorophyllide a (Chlide). A DNA fragment downstream of frxC in the filamentous cyanobacterium Plectonema boryanum was cloned and analyzed. Sequencing of the DNA fragment revealed an open reading frame (ORF) that encoded a protein of 467 amino acid residues (designated ORF467), which showed extensive homology to the proteins encoded by genes on ctDNAs (ORF465 in liverwort, gidA in pine and chlN in Chlamydomonas reinhardtii) and to ORF469 protein of the cyanobacterium Synechocystis sp. strain PCC 6803. We isolated a targeted mutant YFM6D-3 in which ORF467 was inactivated by the insertion of a kanamycin-resistance gene into the coding region. YFM6D-3 exhibited a phenotype similar to that of YFC1004, an frxC-disrupted mutant, which did not synthesize chlorophyll (Chl) and accumulated Pchlide, a precursor to Chl, in the dark. These phenotypic characteristics of YFM6D-3 indicate that the light-independent reduction of Pchlide requires not only the FrxC protein but also the ORF467 protein. The amino acid sequences of the homologues of ORF467 exhibit low but significant similarity to those of the alpha and beta subunits of nitrogenase MoFe-protein, suggesting a phylogenetic relationship between the light-independent Pchlide reductase and nitrogenase, as is observed between the FrxC protein and the Fe-protein of nitrogenase.