Recombination by sequence repeats with formation of suppressive or residual mitochondrial DNA in Neurospora

Recombination junctions of several Neurospora mitochondrial DNA (mtDNA) mutants and their revertants were identified. Their nucleotide sequences and putative secondary structures were determined in order to understand the nature of the elements involved in intramolecular recombination. Multiple deletions, involving the same portion of Neurospora mtDNA, were identified in six independently isolated mutants. A 9-nucleotide repeat element, CCCCNCCCC, was found to be involved in these and other Neurospora mitochondrial recombination events. The repeat elements were clustered as hot spots on the Neurospora mtDNA and were associated with palindromic DNA sequences. The palindromes have a potential to generate hairpin structures. A much lower free energy of the putative hairpins at the 5' end of the recombination site, and the possible formation of non-B-DNA structure by polypyrimidine tracks, may be important in the initiation of recombination. Using PCR, we found low levels of a specific mitochondrial deletion in certain Neurospora mutants. Their presence in low amounts in a population with a much larger number of normal mtDNA is unexpected. Contrary to earlier belief, this finding supports the view that deleted, smaller DNA molecules are not always suppressive relative to normal mtDNAs.