Development of Autonomously Replicating Plasmids for Candida albicans

A pool of Candida albicans RsaI fragments cloned onto a vector containing pBR322 sequences and the Candida ADE2 gene was used to transform a Candida ade2 mutant to adenine protrophy. A potential autonomously replicating sequence (ARS) in Candida DNA was identified by two criteria: instability of the selectable marker in the absence of selection and the presence of free plasmid in total DNA preparations. Plasmids carrying the ARS transformed C. albicans at a high frequency (200 to 1,000 ADE+ transformants per microgram of DNA), and Southern hybridization analysis of these transformants indicated that multiple copies of the plasmid sequences were present and that, although they were present in high-molecular-weight molecules, these sequences had not undergone rearrangement. Orthogonal field alternation gel electrophoresis indicated that the high-molecular-weight transforming sequences were not associated with any chromosome. The simplest interpretation to account for these data is that the transforming sequences are present as oligomers consisting of head-to-tail tandem repeats. The transformed strains occasionally yield stable segregants in which the transforming sequences are integrated into the chromosome as repeats. The Candida sequence responsible for the ARS phenotype was limited to a single 0.35-kilobase RsaI fragment which is present in one copy per haploid genome.