Studying the deleterious effect of the QoI resistance mutation G143A in the intron-containing cytochrome b gene and the by-pass mechanisms

Abstract

The mutation G143A, in the inhibitor binding site of cytochrome b, confers a high level of resistance towards fungicides targeting the bc1 complex. The mutation, reported in many plant pathogenic fungi, has not evolved in fungi that harbour an intron immediately after the codon for G143 in the cytochrome b gene, the intron bi2. Using Saccharomyces cerevisiae as a model organism, we show here that the codon change GGT in GCT, that replaces glycine 143 by alanine, hinders the splicing of bi2 by altering the exon/intron structure needed for efficient intron excision. This lowers the level of cytochrome b and the respiratory growth. We then investigated possible by-pass mechanisms that would restore the respiratory fitness of a resistant mutant. Secondary mutations in the mitochondrial genome were found: a point mutation in bi2 restoring the correct exon/intron structure and the deletion of the intron bi2. We also found that the overexpression of the nuclear genes MRS2 and MRS3 encoding mitochondrial metal ion carriers partially restores the respiratory growth of the G143A mutant. Interestingly, the MRS3 gene from the plant pathogenic fungus Botrytis cinerea, overexpressed in an S. cerevisae G143A mutant, had a similar compensatory effect. These by-pass mechanisms identified in yeast could potentially arise in pathogenic fungi.