Isolation of hem3 mutants from Candida albicans by sequential gene disruption.

Molecular methods for directed mutagenesis in Candida albicans have relied on a combination of gene disruption by transformation to inactivate one allele and UV-induced mitotic recombination or point mutation to produce lesions in the second allele. An alternate method which uses two sequential gene disruptions was developed and used to construct a C. albicans mutant defective in a gene essential for synthesizing tetrapyrrole (uroporphyrinogen I synthase). The Candida gene was cloned from a random library by complementation of the hem3 mutation in Saccharomyces cerevisiae. The complementing region was limited to a approximately 2.0 kb fragment by subcloning and a Bg/II site was determined to be within an essential region. Linear fragments containing either the Candida URA3 or LEU2 gene inserted into the Bg/II site were used to disrupt both alleles of a leu2, ura3 mutant by sequential transformation. Ura+, Leu+ heme-requiring strains were recovered and identified as hem3 mutants by Southern hybridization, transformation to heme independence by the cloned gene, and enzyme assays.