Phenotypic switching in Candida albicans : a candidate gene approach
This thesis describes the cloning and characterisation of two Candida albicans genes which were candidates for having a role in the phenotypic switching phenomenon of C. albicans. Phenotypic switching in C. albicans is a spontaneously occurring event whereby the surface morphology and several physiological processes of the C. albicans colony can change. These spontaneous switching events occur at high frequency and there are a range of up to fifteen different morphological forms that have been described. Switching is reversible and interconvertible (between the different phenotypes). The candidate genes chosen to investigate phenotypic switching were RAD52, a DNA double strand break repair gene and H4, a histone. RAD52 was isolated following homologous probing of a C. albicans genomic library using a fragment of the gene sequence which was available on a public data base [http://alces.med.umn.edu/candida/html], as a probe. H4 was isolated following PCR probing of a cosmid library. The switching repertoire of the ura- CAI4 strain of C. albicans was characterised. Attempts were also made to characterise switching frequencies. This strain was then used as the host for both knockout and overexpression studies of the candidate genes. The effect of overexpression of these genes on phenotypic switching was observed by recording growth rates, phenotypes and phenotypic switching frequencies. It was found that overexpression of RAD52 affected the morphotype and growth of the yeast colonies compared the CAI4 parental strain. Overexpression of the H4 gene did not appear to affect growth, but a fourth morphological form named "root" appeared that had not arisen during characterisation of the CAI4 phenotypic switching repertoire. The degree to which the "root" phenotype was manifest appeared to correlate with the degree of overexpression of the H4 gene. The effect of knocking out a single copy of H4 was also observed. Growth was not affected. Observations of colony morphologies showed a preponderance of one particular morphology ("irregular wrinkle"). This data suggests that altering the wild type levels of expression of these genes can affect phenotypic switching in C. albicans.