rylated and activated by its protein kinase, leading to the induction of heat Autoregulation of Thermal Adaptation shock protein genes including HSP90. Fourth, we predicted that this protein kinase is down-regulated by an unknown inhibitor. Fifth, on the basis that Hsp90 negatively regulates Hsf1, we predicted that the subsequent increase in Hsp90 levels would then lead to the down-regulation of Hsf1. Our goal was to keep the mathematical model as simple as possible, reducing the complexity of the system to include the following key components: the inactive and active forms of Hsf1; the interaction of Hsf1 with Hsp90; free Hsp90; the Hsp90 complex with unfolded proteins; and HSP90 mRNA production. Therefore, we considered three main forms of Hsp90: the free form, the complex with unfolded proteins and the complex with Hsf1. We made this assumption on the basis that: molecular chaperones participate in the folding of many proteins ” in eukaryotic cells; in mammalian cells, unfolded proteins accumulate during heat shock; and these unfolded proteins are thought to compete with HSF1 for binding to Hsp90, leading to the release of free HSF1. Therefore, we proposed that Hsf1 is present in an equilibrium with Hsp90, constantly associating with and dissociating from Hsp90. At elevated temperatures the protein kinase that phosphorylates Hsf1 becomes activated , and this leads to the subsequent activation of an inhibitor I which inactivates K. The identities of the Hsf1 kinase and Hsf1 ” phosphatase are currently unknown. The active K binds free Hsf1, forming the complex Hsf1K, mediating Hsf1 Peretinoin site phosphorylation to form Hsf1P. Activated Hsf1 induces the transcription of HSP90 mRNA via heat shock elements within promoter regions, and subsequently induces the synthesis of new Hsp90. The model also accounts for the degradation of HSP90 mRNA. The transcriptional activity of Hsf1P can be repressed through the binding of Hsp90 and the formation of the complex Hsf1Hsp90. Thus Hsf1 is assumed to be negatively regulated by Hsp90 in the model. During heat shock, Hsp90 binds unfolded and/or damaged proteins, preventing their aggregation and helping them to refold . This is considered a reversible process. In addition, both the Hsp90Complex and Hsp90 can be degraded. The degradation of Hsp90 protein and HSP90 mRNA are not explicitly regulated by heat shock in the model. However, the increased formation of Hsp90Complex due to a temperature up-shift indirectly promotes Hsp90 degradation by affecting the equilibrium between free and Hsf1-bound Hsp90. The initial conditions, the ODEs that define this model, and the parameter values are presented in Dynamics of heat shock adaptation in C. albicans Having constructed the model, it was parameterised to fit the experimentally determined dynamics of thermal adaptation in C. albicans. These included the kinetics of Hsf1 phosphorylation, and the temporal induction of HSP90 mRNA levels during 30uC37uC and 30uC42uC heat shocks. Replicate time series measurements of Hsf1 phosphorylation were completed for both 30uC37uC and 30uC42uC heat shocks. Protein extracts were prepared, subjected to western blotting, 
and Hsf1 phosphorylation levels quantified. Lambda phosphatase controls were run routinely to confirm band-shifts representing Hsf1 phosphorylation. Low levels of Hsf1 phosphorylation were reproducibly detected during a 30uC37uC heat shock. These subtle band-shifts were resolvable by lambda phosphatase at 2, 5 and 10 minutes po
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