Structure of a Co-Transcribed Gene Cluster, ndhl-frxB^3-ndh6-ndh4L, Cloned from the Filamentous Cyanobacterium Plectonema boryanum

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The frxB gene found in chloroplast genomes encodes a bacterial ferredoxin-like (Fd-like) FeS protein, whose function has not been established. Using an frxB gene probe from liverwort, we have isolated and determined the nucleotide sequence of a 4.6-kb fragment of DNA that contains the frxB gene from the filamentous cyanobacterium Plectonema boryanum. The frxB gene from Plectonema encodes a protein of 194 amino acids that shows 66% sequence identity to that encoded by liverwort chloroplasts. The sequence contains two clusters of cysteine residues (C-X-X-C-X-X-C-X-X-X-C-P), typical of bacterial ferredoxins that chelate two [4Fe-4S] clusters. Such sequences are found in highly conserved regions in the seven available sequences of frxB-related proteins. In addition, we identified three ORFs homologous to the chloroplast ndh1, ndh6 and ndh4L genes that encode the components of a putative NADH dehydrogenase. They show 60%, 50% and 65% amino acid sequence identity to the respective gene products from liverwort chloroplasts, but they show a lower degree of similarity to the mitochondrial homologues. The ndh1. frxB, ndh6 and ndh4L genes, occurring as single genomic copies, are tightly linked to one another, and they are cotranscribed as a 2.8-kb mRNA which was identified by Northern blot analysis with gene-specific probes. These four genes occur in the same order in the chloroplast genomes of liverwort, tobacco and rice as they do in Plectonema. Therefore, the organization of ndh1-frxB-ndh6-ndh4L may represent a case of clustering and co-transcription of functionally related genes in both cyanobacteria and chloroplasts. These findings lend a support to the hypothesis that cyanobacteria and chloroplasts contain an enzyme complex that is closely related to the mitochondrial complex I, although such a highly organized eazyme complex has not been identified to date in an extensive survey of cyanobacterial NAD(P)H dehydrogenases.
日本植物生理学会の論文

Structure of a Co-Transcribed Gene Cluster, ndhl-frxB^3-ndh6-ndh4L, Cloned from the Filamentous Cyanobacterium Plectonema boryanum

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