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

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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

J. L. Barra, M. R. Mautino and A. L. Rosa
Present address: Institute de Genetique et Microbiologie, C.N.R.S., Universite de Paris-Sud, Bat. 400, 91405 Orsay Cedex, France.

eth-1(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 eth-1(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 eth-1(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.