Structure of a Co-Transcribed Gene Cluster,ndhl-frxB3-ndh6-ndh4L,Cloned from the Filamentous CyanobacteriumPlectonema boryanum4

摘要

ThefrxBgene found in chloroplast genomes encodes a bacterial ferredoxin-like (Fd-like) Fe-S protein, whose function has not been established. Using anfrxBgene probe from liverwort, we have isolated and determined the nucleotide sequence of a 4.6-kb fragment of DNA that contains thefrxBgene from the filamentous cyanobacteriumPlectonema boryanum.ThefrxBgene fromPlectonemaencodes a protein of 194 amino acids that shows 66sequence 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 ofrxrB-related proteins. In addition, we identified three ORFs homologous to the chloroplastndhl, ndh6andndh4Lgenes that encode the components of a putative NADH dehydrogenase. They show 60, 50and 65amino acid sequence identity to the respective gene products from liverwort chloroplasts, but they show a lower degree of similarity to the mitochondrial homologues. Thendhl, frxB, ndh6andndh4Lgenes, occurring as single genomic copies, are tightly linked to one another, and they are co-transcribed 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 inPlectonema.Therefore, the organization ofndhl-frxB-ndh6-ndfi4Lmay 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 enzyme complex has not been identified to date in an extensive survey of cyanobacterial NAD(P)H dehydrogenases.