We have determined from nucleotide sequence analysis that the subterminal and terminal exons of a respiratory-chain NADH dehydrogenase subunit I gene in broad bean mitochondrial DNA (mtDNA) are separated by a group II intron. Within this intron is a 687-codon open reading frame that, from considerations of similarity between amino acid sequences predicted from this open reading frame and maturase-coding sequences in group II introns ofcertain fungal mitochondrial genes, appears to encode a maturase-related protein. Transcripts complementary to this broad bean sequence (designated a mat-r gene) were detected among RNAs isolated from broad bean mitochondria. Data obtained from DNADNA hybridizations indicated that soybean and corn mtDNAs also contain a mat-r gene and suggested that only one copy of this gene occurs in each plant mtDNA. The putative protein specified by the broad bean mat-r gene contains amino acid sequences characteristic of reverse transcriptases. Because of this, consideration is given to the possibility that the maturase-related protein may be functional in the mechanisms by which plant mtDNA sequences are rearranged and foreign sequences are incorporated into plant mtDNAs.Plant mitochondrial DNAs (mtDNAs) contain the genes for a number of subunits of proteins involved in oxidative phosphorylation and genes for three ribosomal proteins (rpS12, rpS13, and rpS14) (1-3). Introns have been reported only in some plant coxIf genes, in the corn and watermelon nadl genes, and in the nadS gene of Oenothera (4-7). Each of these introns is of the group II type found only in organelle genes (8, 9). Unlike the situation in yeast and other fungi, where some group II (and group I) introns encode enzymes (maturases) necessary for excision of the maturase-encoding intron from RNA transcripts of the gene (10-12), none of the aforementioned plant mitochondrial (mt) introns appears to encode a protein.In this paper, we report the occurrence within a group II
RESULTSThe nucleotide sequence of a 4481-nucleotide-pair (ntp) segment of broad bean mtDNA is given in Fig. 1 and our analysis of this sequence is summarized in Fig. 2. Three regions of the amino acid sequence predicted from a 2061-ntp (687 codons) open reading frame within this sequence show considerable similarity to corresponding consecutive regions ofamino acid sequences of the maturase encoded by the all intron of the cytochrome c oxidase subunit I (coxI) gene of Saccharomyces cerevisiae and by the following fungal putative maturase gene-containing mt intron sequences (Fig. 3): aI2 of the coxI gene of S. cerevisiae (14), I and IA of the coxI gene of Podospora anserina (15,16), and bl of the cytochrome b (cob) gene of Schizosaccharomyces pombe (17). Because of these amino acid sequence similarities, we have designated the putative broad bean protein a maturase-related protein (MAT-R) and the sequence that encodes it a mat-r gene.The broad bean mat-r gene, like the S. cerevisiae, P. anserina, and Sc. pombe mat genes, is contained in a group II intro...