The <italic>nifH</italic>-Like (<italic>frxC</italic>) Gene Is Involved in the Biosynthesis of Chlorophyll in the Filamentous Cyanobacterium <italic>Plectonema boryanum</italic>

Abstract

The frxC gene found in the DNA of the liverwort chloroplast encodes a protein of unknown function. The deduced amino acid sequence shows significant homology to that of the nitrogenase Fe-protein encoded by the nifH gene. We previously identified the frxC and nifH genes in the filamentous cyanobacterium Plectonema boryanum. We describe here the isolation of targeted mutants of frxC (YFC1004) and nifH (YFH201) which were generated by insertion of a kanamycin-resistance gene into the structural portion of the respective genes. As expected, YFH201 cannot grow under nitrogen-fixing conditions. However, YFC1004 grows as well as the wild type does under nitrogen-fixing, photoautotrophic and chemoheterotrophic conditions, indicating that the FrxC protein is essential neither for nitrogen fixation nor for major energytransduction systems, such as photosynthesis and respiration. YFC1004 synthesizes chlorophyll (Chi) normally in the light but not in the dark, and it accumulates a precursor to Chi, protochlorophyllide (Pchlide) in the dark. These phenotypic characteristics of YFC1004 suggest that the cyanobacterium has two pathways for the reduction of Pchlide: a light-dependent and a lightindependent system. The FrxC protein appears to be involved in the light-independent reduction of Pchlide.