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Abstract:

The start-up of fermentation by Saccharomyces cerevisiae is associated with a rapid drop on the yeast stress resistance. Previously we isolated a S. cerevisiae mutant (fil1-CYR1E1682K) that does not exhibit the loss of stress resistance induced by fermentation. This strain was used as tool to identify genes responsible for stress resistance. Target genes of fil1 mutation can be regulated by the transcription factors Msn2, Msn4, Gis1 and Hsf1, components known to be under negative control of the cAMP-PKA pathway and responsible for the expression of stress responsive genes. The fil1 point mutation is localized at position 1682 of CYR1 gene that encodes adenylate cyclase. This enzyme partially controls PKA activity by the synthesis of cAMP, a messenger that activates PKA. Therefore the adenylate cyclase and PKA have a central role on the regulation of stress genes. The deletion of downstream components of the PKA pathway, stress responsive genes TPS1, MSN2, MSN4, HSP104 and GIS1, cause a decrease on heat stress resistance of S. cerevisiae W303-1A. However the deletion of these genes in the fil1 background has limited effect on stress tolerance of the mutant. These results indicated that in addition of trehalose, expression of Hsp104 and genes controlled by transcription factors Msn2/4 and Gis1, other genes are responsible for the higher stress resistance phenotype of fil1 mutant. This work aimed to identify other genes responsible for stress resistance and therefore we have investigated the role of small heat shock proteins Hsp12 and Hsp26 on stress resistance of strains W303-1A and fil1. The HSP12 and HSP26 genes were deleted in both strains using standard methods and the mutants constructed were challenge with different stress conditions. The hsp26Δ completely abolished the heat shock tolerance and decreased the oxidative stress resistance of stationary phase cells of fil1 mutant. A decrease on heat stress resistance was also observed at the start-up of fermentation. Interestingly, the deletion of HSP12 has limited effect on stress resistance of the fil1 mutant. The control of expression of small heat shock proteins is not yet completely understood. Some studies demonstrated that these proteins can be redundantly controlled by transcriptional factors Msn2/4 and Hsf1 and another study showed that Hsp12 expression is controlled by Msn2/4 whereas Hsp26 is controlled by Hsf1. Our results support that regulation of Hsp12 and Hsp26 are different at least in the fil1 mutant. The point mutation on adenylate cyclase seems to reduce the protein kinase A activity at the level that induces derepression of transcription factor Hsf1, up-regulating its target genes as Hsp26. This may explain why deletion on Hsp26 has a clear effect on the higher stress resistance phenotype of the fil1 mutant.