2 COEXISTING MECHANISMS ACCOUNT FOR THE LARGE-SCALE DELETIONS OF MITOCHONDRIAL-DNA IN PODOSPORA-ANSERINA THAT INVOLVE THE 5'-BORDER OF A GROUP-II INTRON

A degenerative syndrome associated with the accumulation of site-speci fic deletions within mitochondrial chromosomes occurs in strains of Po dospora anserina carrying the AS1-4 nuclear mutation. The site-specifi c deletion event has been assumed to result from the transposition of a group-II intron (intron alpha) behind an IBS motif, followed by reco mbination between the two intron repeats. We show here that a number o f distinct deletions can accumulate in AS1-4 strains. Most of them are present in low amounts in wild-type cells where they are only detecta ble in PCR experiments. The deletions can be divided into two classes. In class I, intron alpha is joined to an IBS motif. In class II, the intron is not joined to an IBS site, it can be truncated or contain a few upstream exonic nucleotides; some junctions carry non-templated nu cleotides. These results indicate that at least two mechanisms are inv olved in the generation of large-scale mitochondrial deletions in Podo spora. One of them seems to be based on the transposition properties o f the group-II alpha intron, the other one on illegitimate recombinati on. We propose that these two mechanisms use DNA double-strand breaks occurring within the 5' region of intron alpha.