Variant mitochondrial transcripts of a broad bean line are associated with two point mutations located upstream of the nad5 exon c

Abstract

We have analysed the variability at the RNA level between two cytoplasms of broad bean: N (fertile) and 447 (male sterile). Northern blots were performed, using 111 cloned HindIII fragments of mitochondrial DNA as probes. A clone of 6.2 kb encoding exons d and e of the nad5 gene recognized two variant transcripts, the sizes of which were 150 nucleotides larger in the N (2450 and 1550 nucleotides) than in the 447 (2300 and 1400 nucleotides) cytoplasm. A combination of genomic and cDNA cloning showed that the nad5 gene is, like in other plant species, formed from two trans-splicing events. Genomic sequencing and transcript mapping indicated that the conversion of two thymidine in N into two adenosine nucleotides in 447 leads to altered RNA processing and shortening of the intermediate transcripts linking nad5 exons c, d and e. Moreover, a new open reading frame of 143 amino acid residues was found between this processing site and exon c in both cytoplasms.