Trehalose reserve in Saccharomyces cerevisiae: phenomenon of transport, accumulation and role in cell viability

Strains of Saccharomyces cerevisiae deleted for TPS1 encoding trehalose-6-p hosphate synthase still accumulate trehalose when harbouring a functional M AL locus. We demonstrate that this accumulation results from an active upta ke of trehalose present in the 'yeast extract' used to make the enriched cu lture media and that no accumulation is observed in mineral media. The upta ke of trehalose was shown to be mediated by the alpha-glucoside transporter encoded by AGT1, the expression of which is linked to the presence of a fu nctional MAL locus. Deletion of this gene in a MAL(+) tps1 mutant abolished trehalose accumulation on a maltose or galactose mineral medium. However, small amounts of disaccharide were still detected in a agt1 tps1 double mut ant when the medium was supplemented with 10 g trehalose l(-1), indicating the existence of a non-concentrative low-affinity sugar transporter. The pr esence of the high-affinity trehalose permease allowed us to investigate th e effect of increasing exogenous trehalose from 0 to 10 g l(-1) on intracel lular accumulation. A maximum of ca. 10% (wt/wt dry cells) trehalose was at tained in the presence of only 1 g l(-1) of disaccharide in the medium. The capability to monitor the intracellular content of trehalose by varying it s extracellular concentration, independent of genetic alterations of the tr ehalose metabolic machinery, allowed the remarkable contribution of this mo lecule in stress tolerance to be demonstrated, as the higher the trehalose content, the longer the cell survival to a severe heat shock and to glucose starvation. (C) 2000 Elsevier Science B.V. All rights reserved.