|Ph.D Student||Ofir Ayala|
|Subject||Role of Cell Cycle Regulation in Candida Albicans|
|Department||Department of Medicine||Supervisor||Professor Daniel Kornitzer|
|Full Thesis text|
Candida albicans is a fungal pathogen that causes superficial infections in human but also life threatening systemic infection in immunocompromised patients. C. albicans cells can reversibly switch between several growth forms, this ability contributes to its virulence. Cyclin-dependent kinases (CDKs) are key regulators of eukaryotic cell-cycle progression. The cyclin subunit activates the CDK and also imparts to the complex, at least in some cases, substrate specificity. Analysis of the genome of the pathogenic yeast C. albicans revealed only two sequences corresponding to B-type cyclins, CaClb2 and CaClb4. Notably, no homolog of the S. cerevisiae S-phase-specific cyclins, Clb5/-6, could be detected.
Sol1 was identified in our lab as a functional homologue of S. cerevisiae CDK inhibitor (CDKI) Sic1, however, the consequences of overexpression of the two proteins differ: Sic1 causes cell cycle arrest at G1/S in both C. albicans and S. cerevisiae, whereas Sol1 does not.
In order to investigate the differential function of these two CDKIs at the molecular level it was important to identify the S-phase-specific cyclin in C. albicans. We performed an in vitro analysis of the activity of all C. albicans cyclins, as well as an analysis of the phenotype of S. cerevisiae cells expressing CaClb2 or CaClb4 instead of Clb5. Both in vivo and in vitro analysis indicate that CaClb4 is the functional homolog of Clb5/-6.
In order to better understand the link between cell cycle and hyphal morphogenesis, we next identified and characterized CaNrm1, a C. albicans homolog of the S. cerevisiae Whi5 and Nrm1 transcription inhibitors that regulate the cell cycle transcription program in the G1 phase and at the G1/S transition. CaNRM1 is able to complement the phenotypes of both whi5 and nrm1 mutants in S. cerevisiae. Deletion of CaNRM1 causes a reduction in cell size and results in increased resistance to hydroxyurea. Analysis of the expression of ribonucleotide reductase, the target of hydroxyurea, suggests that its transcriptional induction in response to hydroxyurea is mainly dependent upon CaNrm1. Genetic epistasis analysis suggests that CaNrm1 interacts with the SBF and MBF transcription factors in S. cerevisiae and with their homolog in C. albicans. Morphologically, the Canrm1-/- strain is mildly defective in filamentation. At the transcription level, deletion of CaNRM1 causes an induction of many G1 and G1/S-specific genes, and a repression of HSGs, placing CaNrm1 at the crossroads of the coordination between the cell cycle and morphogenetic transcription programs of C. albicans.