A sequence encoding a maturase-related protein in a group II intron of a plant mitochondrial nadl gene (broad bean/reverse transcriptase/evolutionary conservation/sequence rearrangements)

We have determined from nucleotide se- quence 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 of certain fungal mitochondrial genes, appears to encode a maturase-related protein. Transcripts complementary to this broad bean se- quence (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 matu- rase-related protein may be functional in the mechanisms by which plant mtDNA sequences are rearranged and foreign sequences are incorporated into plant mtDNAs.