Some bacteria produce biosurfactants or glycolipids that require T6P for their synthesis. Y. lipolytica also AbMole Scopoletin 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 AbMole Clofentezine 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 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 in their promoters. Function of STRE sequences in S. cerevisiae requires the Msn2/Msn4 proteins. The corresponding gene 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. Our results show that upon a heat shock at 40uC the levels of MHY1 mRNA increase suggesting that MHY1 may play a role in the regulatory response to this stress. It should be noticed that the high GC content in the Y. lipolytica DNA may cause the presence of CCCCT sequences in the promoters of several genes that have not been related with responses to stress. Y. lipolytica does not grow at temperatures over 35uC. The finding that disruption of YlTPS1 impairs growth at this limit temperature suggests that trehalose plays a protective role against the changes produced under this condition. Many evidences show that in different organisms the trehalose biosynthetic pathway, in addition to its primary role, has an influence in a variety of processes that range from growth on certain substrates or temperatures, to differences in virulence in pathogens. The targets of the pathway are different depending on the organism and even closely related yeast species like C. neoformans and C. gattii show important variations in the effects caused by perturbations of that pathway.