Trehalose in Y. lipolytica in different conditions was below 1 nmol/mg dry weight. Disruption of a gene encoding a putative neutral trehalase or overexpression of YlTPS1 increased trehalose content. A similar situation occurred in vascular plants in which trehalose was thought to be absent; incubation with validamycin A, an inhibitor of trehalase, showed the existence of the disaccharide. Hydrolysis of trehalose by trehalase and a low level of Tps1 activity may be responsible for the low levels of the sugar in Y. lipolytica. It could be speculated that the main role of Tps1 is to provide T6P as intermediate for pathways different from trehalose synthesis. Some bacteria produce biosurfactants or glycolipids that require T6P for their synthesis. Y. lipolytica also produces biosurfactants even growing in aqueous media but their detailed structure is not known. Heat shock increased the levels of trehalose and changed the levels of mRNA corresponding to YlTPS2 and YlTPS3 but not those of YlTPS1. A similar lack of response of A. nidulans tpsA has been described. The increase of mRNA corresponding to YlTPS3 as well as the absence of trehalose in the heat shocked Yltps3 mutant indicate an important role for the protein in the stability of the Y. lipolytica trehalose biosynthetic complex. While in S. cerevisiae the complex consists of four proteins, Tps1,Tps2, Tsl1 and Tps3, only one sequence similar to that of Tps3/Tsl1 was found in the Y. lipolytica database. Decrease of trehalose levels during heat shock in S. cerevisiae requires the disruption of both Tsl1 and Tps3. In S. cerevisiae different mechanisms such as transcriptional activation of some genes, stabilization of certain RNAs and activation of the trehalose synthase complex contribute to trehalose accumulation by heat shock. Such detailed studies are not yet BIBW2992 manufacturer available for Y. lipolytica. Transcriptional response to heat shock in the case of the genes of the trehalose biosynthetic pathway in S. cerevisiae depends on repetitions of a CCCCT stretch (STRE sequence) in their promoters. Function of STRE sequences in S. cerevisiae requires the Msn2/ Msn4 proteins. The corresponding gene(s) is not known in Y. lipolytica. Hurtado and Rachubinsky observed the high sequence homology of the Zn finger domain of Mhy1 with that of Msn2/4 and showed that this protein was able to bind to STRE sequences in vitro. These authors reported that the levels of MHY1 mRNA were not increased after a heat shock at 35uC.