SPONTANEOUS MUTATION AT THE MTR LOCUS IN NEUROSPORA - THE MOLECULAR-SPECTRUM IN WILD-TYPE AND A MUTATOR STRAIN

Sequence analysis of 34 mtr mutations has yielded the first molecular spectrum of spontaneous mutants in Neurospora crassa. The great majori ty of the mutations are base substitutions (48%) or deletions (35%). I n addition, sequence analysis of the entire mtr region, including the 1472-base pair open reading frame and 1205 base pairs of flanking DNA, was performed in both the Oak Ridge and Mauriceville strains of Neuro spora, which are known to be divergent at the DNA level. Sixteen seque nce differences between these two strains have been found in the mtr r egion, with 13 of these in DNA flanking the open reading frame. The di fferences consisted of base substitutions and small frameshifts at mon otonic runs. This set of sequence differences has allowed a comparison of mutations in unselected DNA to those mutations that produce a phen otypic signal. We have isolated a mutator strain (mut-1) of Neurospora in which the spontaneous mutation rate at various loci is as much as SO-fold higher than in die non-mutator (wild type). Twenty-one mtr mut ations in the mutator background have been sequenced and compared to t he non-mutator spectrum, revealing a striking increase in -1 frameshif t mutations. These frameshifts occur exclusively within or adjacent to monotonic runs and can be explained by small slippage events during D NA replication. This argues for a role of the mut-1 gene in this proce ss.