THIAMIN METABOLISM AND THIAMIN DIPHOSPHATE-DEPENDENT ENZYMES IN THE YEAST SACCHAROMYCES-CEREVISIAE - GENETIC-REGULATION

The yeast Saccharomyces cerevisiae utilises external thiamin for the p roduction of thiamin diphosphate (ThDP) or can synthesise the cofactor itself. Prior to uptake into the cell thiamin phosphates are first hy drolysed and thiamin is taken up as free vitamin which is then pyropho sphorylated by a pyrophosphokinase. Synthesis of ThDP starts with the production of hydroxyethylthiazole and hydroxymethylpyrimidine. Those are linked to yield thiamin phosphate which is hydrolysed to thiamin a nd subsequently pyrophosphorylated. The THI genes encoding the enzymes of these final steps of ThDP production and of thiamin utilisation ha ve been identified. Their expression is controlled by the level of thi amin and a number of regulatory proteins involved in regulated express ion of the THI genes are known, However, the molecular details of the regulatory circuits need to be deciphered, Since the nucleotide sequen ce of the entire yeast genome is known we can predict the number of Th DP-dependent enzymes in S. cerevisiae. Eleven such proteins have been found: pyruvate decarboxylase (Pdc, three isoforms), acetolactate synt hase, a putative alpha-ketoisocaproate decarboxylase with a regulatory role in ThDP synthesis and two proteins of unknown function form the group of Pdc related enzymes, In addition there are two isoforms for t ransketolase as well as the El subunits of pyruvate dehydrogenase and 2-oxoglutarate dehydrogenase. Expression of most of these genes is eit her induced or repressed by glucose, Surprisingly, it has been found r ecently that expression of one of the genes for Pdc is repressed by th iamin. In addition, the regulatory protein Pdc2p was shown to be requi red for high level expression of both the THI and the PDC genes. Appar ently, the production of ThDP and of the enzymes using this cofactor i s coordinately regulated. Future research will focus on the elucidatio n of the molecular mechanisms of this novel type of regulation, (C) 19 98 Elsevier Science B,V. All rights reserved.