Adaptation to host temperature is a prerequisite for any pathogen capable of causing deep infection in humans. Our previous studies demonstrated that a Cryptococcus neoformans ccr4⌬ mutant lacking the major deadenylase involved in regulated mRNA decay was defective in host temperature adaptation and therefore virulence. In this study, the ccr4⌬ mutant was found to exhibit characteristics of chronic unfolded-protein response (UPR) engagement in both the gene expression profile and phenotype. We demonstrate that host temperature adaptation in C. neoformans is accompanied by transient induction of the endoplasmic reticulum (ER) stress response and that Ccr4-dependent posttranscriptional gene regulation contributes to resolution of ER stress during host temperature adaptation.The pathogenic fungus Cryptococcus neoformans is one of two species of cryptococci commonly associated with infection in humans (2, 6). Unifying characteristics of the pathogenic cryptococci include the production of a polysaccharide capsule, the ability to form melanin pigments through the activity of the multicopper oxidase laccase, and the ability to adapt to and thrive at mammalian host temperature. Adaptation of C. neoformans to the host temperature is accompanied by major changes in gene expression as measured by microarray analysis and serial analysis of gene expression (SAGE) (5,19,29). This modulation of gene expression likely requires alterations in mRNA synthesis rates through activation of transcriptional transactivators and repressors, as well as alterations in chromatin structure. In addition to mRNA synthesis, our previous studies of a C. neoformans ccr4⌬ mutant lacking the major mRNA deadenylase involved in regulated mRNA turnover suggest a role for posttranscriptional regulation of gene expression in C. neoformans host temperature adaptation (24).Destabilization of specific transcripts in response to stress is highly conserved. In the model yeast Saccharomyces cerevisiae, deletion of CCR4 results in stabilization of transcripts encoding distinct functional classes (ribosome biogenesis, translation initiation, and tRNA synthesis) in response to temperature stress (13). In mammalian cells, subsets of transcripts were destabilized in response to heat shock, and induction of endoplasmic reticulum (ER) stress by treatment with tunicamycin or potentiation of ER calcium release by thapsigargin treatment triggered destabilization of a subset of mRNAs in which are included several transcripts encoding ribosomal proteins (8). This suggests that in response to heat shock and ER stress, distinct pools of transcripts representing specific cellular processes are targeted for degradation.The conserved ER stress response involves engagement of the unfolded-protein response (UPR) and the ER-associated degradation (ERAD) pathway (18). The UPR serves to retool the ER for enhanced protein folding, and ERAD serves to remove unfolded proteins from the ER lumen and shunt them into a degradative pathway. Microarray analyses performed in S. cerevisiae ...