Cpr6 and Cpr7, the Saccharomyces cerevisiae homologs of cyclophilin-40 (CyP-40), were shown to form complexes with Hsp90, a protein chaperone that functions in several signal transduction pathways. Deletion of CPR7 caused severe growth defects when combined with mutations that decrease the amount of Hsp90 or Sti1, another component of the Hsp90 chaperone machinery. The activities of two heterologous Hsp90-dependent signal transducers expressed in yeast, glucocorticoid receptor and pp60(v-src) kinase, were adversely affected by cpr7 null mutations. These results suggest that CyP-40 cyclophilins play a general role in Hsp90-dependent signal transduction pathways under normal growth conditions.

The budding yeast Saccharomyces cerevisiae is a powerful model organism for studying fundamental aspects of eukaryotic cell biology. This Primer article presents a brief historical perspective on the emergence of this organism as a premier experimental system over the course of the past century. An overview of the central features of the S. cerevisiae genome, including the nature of its genetic elements and general organization, is also provided. Some of the most common experimental tools and resources available to yeast geneticists are presented in a way designed to engage and challenge undergraduate and graduate students eager to learn more about the experimental amenability of budding yeast. Finally, a discussion of several major discoveries derived from yeast studies highlights the far-reaching impact that the yeast system has had and will continue to have on our understanding of a variety of cellular processes relevant to all eukaryotes, including humans.

The heat shock response is a highly conserved mechanism that allows cells to withstand a variety of stress conditions. Activation of this response is characterized by increased synthesis of heat shock proteins (HSPs), which protect cellular proteins from stress-induced denaturation. Heat shock transcription factors (HSFs) are required for increased expression of HSPs during stress conditions and can be found in complexes containing components of the Hsp90 molecular chaperone machinery, raising the possibility that Hsp90 is involved in regulation of the heat shock response. To test this, we have assessed the effects of mutations that impair activity of the Hsp90 machinery on heat shock related events in Saccharomyces cerevisiae. Mutations that either reduce the level of Hsp90 protein or eliminate Cpr7, a CyP-40-type cyclophilin required for full Hsp90 function, resulted in increased HSF-dependent activities. Genetic tests also revealed that Hsp90 and Cpr7 function synergistically to repress gene expression from HSFdependent promoters. Conditional loss of Hsp90 activity resulted in both increased HSF-dependent gene expression and acquisition of a thermotolerant phenotype. Our results reveal that Hsp90 and Cpr7 are required for negative regulation of the heat shock response under both stress and nonstress conditions and establish a specific endogenous role for the Hsp90 machinery in S. cerevisiae.All cells possess a defense mechanism known as the heat shock response, which allows them to survive exposure to otherwise lethal doses of certain stresses (1-3). These stresses include environmental challenges, such as elevated temperatures, and pathophysiological states, such as viral infections (4). The heat shock response is characterized by increased synthesis of a set of proteins collectively referred to as heat shock proteins (HSPs) 1 whose principal role is to assist target substrates in their synthesis, transport, and proper folding (5-7). The requirement for chaperoning activities increases as cells are exposed to elevated temperatures or to other conditions that promote protein denaturation and aggregation. Because chaperoning activity is also crucial for the function of proteins not exposed to stress, HSPs play important roles for life under normal conditions as well.Expression of HSPs is under the control of heat shock transcription factors (HSFs) (4,8,9). In most eukaryotic systems, HSF is maintained as a monomer unable to bind DNA until activated by stress (4,8). Activated HSF forms homotrimers capable of binding to heat shock elements (HSEs) present at promoters of genes encoding HSPs, ultimately leading to transcriptional activation (4,8). The monomer to trimer transition is believed to be negatively regulated, at least in part, by Hsp70 (4,8). The acquisition of transcriptional activity by HSF is correlated with increased phosphorylation (10); however, the functional relationship between phosphorylation and regulation of the heat shock response is still not fully understood. In contrast to most eukaryoti...

A previous study of histone H3 in Saccharomyces cerevisiae identified a mutant with a single amino acid change, leucine 61 to tryptophan, that confers several transcriptional defects. We now present several lines of evidence that this H3 mutant, H3-L61W, is impaired at the level of transcription elongation, likely by altered interactions with the conserved factor Spt16, a subunit of the transcription elongation complex yFACT. First, a selection for suppressors of the H3-L61W cold-sensitive phenotype has identified novel mutations in the gene encoding Spt16. These genetic interactions are allele specific, suggesting a direct interaction between H3 and Spt16. Second, similar to several other elongation and chromatin mutants, including spt16 mutants, an H3-L61W mutant allows transcription from a cryptic promoter within the FLO8 coding region. Finally, chromatin-immunoprecipitation experiments show that in an H3-L61W mutant there is a dramatically altered profile of Spt16 association over transcribed regions, with reduced levels over 59-coding regions and elevated levels over the 39 regions. Taken together, these and other results provide strong evidence that the integrity of histone H3 is crucial for ensuring proper distribution of Spt16 across transcribed genes and suggest a model for the mechanism by which Spt16 normally dissociates from DNA following transcription.T HE basic unit of chromatin is the nucleosome, a structure consisting of 146 nucleotides of DNA wrapped around a protein octamer composed of pairs of each of the four core histone proteins (Luger et al. 1997). In addition to directing the condensation of DNA, histone proteins also play crucial and active roles in the regulation of cellular processes that use chromatin as their substrate (Luger 2006). Studies from many laboratories employing a variety of experimental approaches have converged into a general model for chromatin function in which dynamic alterations in chromatin structure are of central importance to processes such as gene transcription and DNA replication.The presence of nucleosomes over transcription units can pose a structural barrier that is inhibitory to transcription initiation and elongation (Sims et al. 2004;Workman 2006). Several mechanisms have been described that can overcome the repressive nature of nucleosomes in initiation, including covalent chemical modifications of specific histone residues, ATP-dependent remodeling of nucleosomes, and the removal of histones (Berger 2002;Narlikar et al. 2002;Boeger et al. 2003;Reinke and Horz 2003). Furthermore, recent studies have demonstrated that regulatory regions of genes are inherently low in nucleosome density and that histone loss at promoter regions, a process known to be mediated at some genes by the Asf1 histone chaperone, is associated with active transcription (Boeger et al. 2003;Reinke and Horz 2003;Adkins et al. 2004;Bernstein et al. 2004;Ercan and Simpson 2004;Lee et al. 2004;Sekinger et al. 2005;Yuan et al. 2005;Korber et al. 2006;Segal et al. 2006). These and ...