INTRON LOSS FROM THE NADH DEHYDROGENASE SUBUNIT-4 GENE OF LETTUCE MITOCHONDRIAL-DNA - EVIDENCE FOR HOMOLOGOUS RECOMBINATION OF A CDNA INTERMEDIATE

The mitochondrial gene coding for subunit 4 of the NADH dehydrogenase complex I (nad4) has been isolated and characterized from lettuce, Lac tuca sativa. Analysis of nad4: genes in a number of plants by Southern hybridization had previously suggested that the intron content varied between species. Characterization of the lettuce gene confirms this o bservation. Lettuce nad4 contains two exons and one group IIA intron, whereas previously sequenced nad4 genes from turnip and wheat contain three group IIA introns. Northern analysis identified a transcript of 1600 nucleotides, which represents the mature nad4 mRNA and a primary transcript of 3200 nucleotides. Sequence analysis of lettuce and turni p nad4 cDNAs was used to confirm the intron/exon border sequences and to examine RNA editing patterns. Editing is observed at the 5' and 3' ends of the lettuce transcript, but is absent from sequences that corr espond to exons two, three and the 5' end of exon four in turnip and w heat. In contrast, turnip transcripts are highly edited in this region , suggesting that homologous recombination of an edited and spliced cD NA intermediate was involved in the loss of introns two and three from an ancestral lettuce nad4 gene.