Methionine suppressors in Aspergillus nidulans: their genetics and behaviour in heterokaryons and diploids

Abstract

Nine methionine mutants of Aspergillus nidulans (six new mutants plus three isolated previously) were examined; five responded to homocysteine, none responded to cystathionine or cysteine.Ten revertants of one of the mutants, methB3, blocked before homocysteine, were shown to be due to suppressor mutations. The suppressors were divided into six genes on the basis of complementation tests and recombination data. Mutants of two of the genes were semi-dominant in the heterokaryon but recessive in the diploid. Experiments in which conidial ratios in the heterokaryons were determined suggested that semi-dominance of these suppressors is due to a shift in nuclear ratios in the heterokaryon in favour of the suppressor nuclei.Some of the suppressors were tested for suppression of two other methionine loci; they acted on methG1 blocked before homocysteine, but not on methH2 blocked after homocysteine, although four out of 13 crosses tested gave ambiguous results.