Cloning and sequence of the Ascobolus immersus S-adenosyl-L-methionine synthetase-encoding gene

Rapid cloning of metK encoding methionine adenosyltransferase from Corynebacterium glutamicum by screening a genomic library on a high density colony-array

The genes SAM1 and SAM2 encoding the two different methionine adenosyltransferases (EC 2.5.1.6) in Saccharomyces cerevisiae were used as templates to generate specific DNA-probes. This heterologous mixture of DNA-probes was hybridized under low stringency hybridization conditions to a Corynebacterium glutamicum colony-array representing the complete genome. Subsequently, one genomic fragment was isolated which contained the C. glutamicum methionine adenosyltransferase gene metK (1.224 kb). When overproduced in Escherichia coli, MetK (44.2 kDa) of C. glutamicum had methionine adenosyltransferase activity. In addition, overexpression of metK in C. glutamicum led to an increased intracellular S-adenosylmethionine concentration. The metK transcript was detected by reverse transcription PCR in C. glutamicum cells in the exponential growth phase but not in the stationary phase.

Gene sam1 encoding adenosylmethionine synthetase: effects of its expression in the fission yeast Schizosaccharomyces pombe

By screening gene libraries of Schizosaccharomyces pombe with a DNA fragment encoding part of the Saccharomyces cerevisiae S-adenosylmethionine synthetase (SAMS), we isolated the fission yeast sam1 gene. Its sequence exhibits good homology to SAMSs of other organisms and reveals the motifs characteristic for SAMSs. SAMS activity and sam1 mRNA levels decrease when cells enter stationary phase. In haploid strains, gene sam1 is essential for growth; if weakly expressed, cells mate and sporulate at a reduced rate. Strains overexpressing sam1 exhibit methionine-sensitive growth. This methionine-induced growth inhibition is partially relieved by adenine. We assume that methionine reduces the level of one or several adenine nucleotides by a SAMS-mediated mechanism. Intracellular SAM levels increase drastically by exogenously added methionine. This increase predicts that mutants exhibiting methionine revertible phenotypes can be indicative for mutations in proteins exhibiting SAM-dependent functions. In agreement with this prediction, we show that mutant pmt2-5 has this phenotype and that gene pmt2 encodes a potential SAM-dependent enzyme.

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

eth-1r, 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 (D48-->N48), perfectly conserved in prokaryotic, fungal and higher eukaryotic AdoMet synthetases, was found responsible for both thermosensitivity and ethionine resistance conferred by eth-1r. Gene fusion constructs, designed to overexpress eth-1r in vivo, render transformant cells resistant to ethionine. Dominance of ethionine resistance was further demonstrated in eth-1+/eth-1r partial diploids carrying identical gene doses of both alleles. Heterozygous eth-1+/eth-1r cells have, at the same time, both the thermotolerance conferred by eth-1+ and the ethionine-resistant phenotype conferred by eth-1r. AdoMet levels and AdoMet synthetase activities were dramatically decreased in heterozygous eth-1+/ eth-1r cells. We propose that this negative effect exerted by eth-1r results from the in vivo formation of heteromeric eth-1+/eth-1r AdoMet synthetase molecules.