A dominant negative effect of eth-1(r), a mutant allele of the Neurospora crassa S-adenosylmethionine synthetase-encoding gene conferring resistance to the methionine toxic analogue ethionine

eth-l(r) a thermosensitive allele of the Neurospora crassa S-adenosylmethionine (AdoMet) synthetase gene that confers ethionine resistance, has been cloned and sequenced. Replacement of an aspartic amino acid residue (D-48 --> N-48), perfectly conserved in prokaryotic, fungal and higher eukaryotic AdoMet synthetases, was found responsible for both thermosensitivity and ethionine resistance conferred by eth1(r). Gene fusion constructs, designed to overexpress eth-1(r) in vivo, render transformant cells resistant to ethionine. Dominance of ethionine resistance was further demonstrated in eth-1(+)/eth-1(r) partial diploids carrying identical gene doses of both alleles. Heterozygous eth-1(+)/eth-1(r) cells have, at the same time, both the thermotolerance conferred by eth-1(+) and the ethionine-resistant phenotype conferred by eth1(r). AdoMet levels and AdoMet synthetase activities were dramatically decreased in heterozygous eth-1(+)/eth-1(r) cells. We propose that this negative effect exerted by eth-1(r) results from the in vivo formation of heteromeric eth-1(+)/eth-1(r) AdoMet synthetase molecules.