VARIANT MITOCHONDRIAL TRANSCRIPTS OF A BROAD BEAN LINE ARE ASSOCIATEDWITH 2 POINT MUTATIONS LOCATED UPSTREAM OF THE NAD5 EXON-C

We have analysed the variability at the RNA level between two cytoplas ms of broad bean: N (fertile) and 447 (male sterile). Northern blots w ere 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 nucleo tides larger in the N (2450 and 1550 nucleotides) than in the 447 (230 0 and 1400 nucleotides) cytoplasm. A combination of genomic and cDNA c loning showed that the nad5 gene is, like in other plant species, form ed from two trans-splicing events. Genomic sequencing and transcript m apping indicated that the conversion of two thymidine in N into two ad enosine nucleotides in 447 leads to altered RNA processing and shorten ing of the intermediate transcripts linking nad5 exons c, d and e. Mor eover, a new open reading frame of 143 amino acid residues was found b etween this processing site and exon c in both cytoplasms. (C) 1997 El sevier Science Ireland Ltd.