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 ANALOG ETHIONINE

eth-l(r) a thermosensitive allele of the Neurospora crassa S-adenosylm ethionine (AdoMet) synthetase gene that confers ethionine resistance, has been cloned and sequenced. Replacement of an aspartic amino acid r esidue (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 tr ansformant cells resistant to ethionine. Dominance of ethionine resist ance was further demonstrated in eth-1(+)/eth-1(r) partial diploids ca rrying 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 de creased 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.