Cloning and characterization of the Kluyveromyces lactis homocysteine synthase gene.

The Kluyveromyces lactis homocysteine synthase gene was cloned by complementation of the Saccharomyces cerevisiae met25 mutation. The coding sequence of the K. lactis gene shows a high similarity to the S. cerevisiae gene. Very little similarity is found in the 5' and 3' untranslated regions. However, one finds short DNA stretches in the promoter of the K. lactis gene which are identical to the nucleotide sequences implicated in the regulation of the S. cerevisiae homologue. This could explain strong transcriptional inhibition of the K. lactis gene by exogenous methionine in the S. cerevisiae host, and indicates a substantial conservation of the sulphur regulatory system between both yeast species.