Cloning, Nucleotide Sequences and Differential Expression of thenifHandnifH-Like (frxC) Genes from the Filamentous Nitrogen-Fixing CyanobacteriumPlectonema boryanum

摘要

ThefrxCgene, found in liverwort chloroplast DNA, encodes a protein of unknown function. The deduced amino acid sequence of the protein shows significant homology to that of ni-trogenase Fe-protein encoded by thenifHgene. We have cloned thefrxCandnifHgenes from the nitrogen-fixing cyanobacteriumPlectonema boryanum,usingfrxC-andnifH-specificprobes, and have determined their nucleotide sequences. The amino acid sequence deduced from thefrxCgene of P.boryanumexhibits 83homology to that of the protein encoded by the/rxCgene from liverwort, whereas it exhibits only 34homology to that encoded by thenifHgene from the same organism, namely,P. boryanum.Northern blot analysis showed that thefrxCgene was transcribed more actively under nitrogenase-repressed conditions than under nitrogenase-induced conditions, suggesting that the FrxC protein has a function distinct from nitrogen fixation. These results, together with the phylogenetic relationship between thenifHandfrxCgenes, indicate that thefrxCandnifHgenes are derived from a common ancestral gene but have evolved independently to encode proteins with different functions.