A complete inventory of all enzymes in the eukaryotic methionine salvage pathway

The methionine salvage pathway is universally used to regenerate methionine from 5’-methylthioadenosine (MTA), a by-product of some reactions involving S-adenosylmethionine. We have identified and verified genes encoding the enzymes of all steps in this cycle in a commonly used eukaryotic model system, the yeast Saccharomyces cerevisiae. The genes encoding 5’-methylthioribose-1-phosphate isomerase and 5’-methylthioribulose-1-phosphate dehydratase were hereby named MRI1 and MDE1, respectively. The MTA phosphorylase was verified as Meu1p, the 2,3-dioxomethiopentane-1-phosphate enolase/phosphatase as, Utr4p and the aci-reductone dioxygenase as Adi1p. The homolog of the enolase/phosphatase gene, YNL010w, could be excluded from its candidate role in the cycle. The methodology used was auxotrophic growth tests and analysis of intracellular MTA in deletion mutants. The last step, a transamination of 4-methylthio-2-oxobutyrate (MOB) to yield methionine, was found to be a highly redundant step. It was catalysed by amino acid transaminases mainly coupled with aromatic and branched chain amino acids as amino donors but also with proline, lysine and glutamate/glutamine. The aromatic amino acid transaminases, Aro8p and Aro9p and the branched chain amino acid transaminases, Bat1p and Bat2p, seemed to be the main enzymes exhibiting the MOB transaminase activity. Bat2p was found to be less specific and used proline, lysine, tyrosine, and glutamate as amino donors beside the branched chain amino acids. Thus, for the first time, all enzymes of the methionine salvage pathway were identified in a eukaryote