The long-term goal is to understand how peptidyl-prolyl isomerases (PPIases) control important cellular processes such as transcription and mitosis. PPIases catalyze the cis/trans isomerization of the peptide bond that precedes the cyclic amino acid proline. Prolyl-isomerization results in conformational changes that affect the folding of newly synthesized proteins and regulates the activity of mature proteins. PPIases are found in all organisms, and are best known because they are the targets of immunosuppressive drugs. However, their normal function in cells is poorly understood, in part, because most can be removed by gene deletion in their respective organism without observable consequences. One exception is a PPIase called Ess1, which is essential for growth in the yeast, Saccharomyces cerevisiae. Ess1 and its human homolog, Pin1, are implicated in transcription regulation and cell cycle control. Ess1 interacts physically and genetically with the carboxy-terminal domain (CTD) of the large subunit of RNA polymerase II. Ess1 is proposed to isomerize the CTD and thereby control the transition between multiple, discrete stages of transcription and mRNA processing. Loss of Ess1 causes cells to undergo mitotic arrest, perhaps due to defects in transcription of key cell cycle genes. This study will focus on determining the mechanism by which Ess1 controls transcription and its importance for global gene expression. It will also explore the structural basis for targeting of Ess1 in pathogenic yeast for clinical applications. Specifically, the aims are to: (1) Use biochemical and molecular approaches to determine the mechanism by which Ess1 regulates RNA polymerase II activity, (2) Use genetic and genomic approaches to identify genes that require Ess1, (3) Use structure and function analysis to examine enzyme-substrate interactions by Candida albicans Ess1, with the long-term goal of exploiting key differences between the fungal and human enzymes for development of antifungal drugs. Public Health Relevance: The study of Ess1 in fungi will be important for our understanding of the human counterpart, Pin1, which has been associated with cancers and neurodegenerative disorders. The results may also establish Ess1 as a new drug target for treatment of patients with life- threatening fungal infections.