A plasmid vector (denoted pRC2312) was constructed, which replicates autonomously in Escherichia coli, Saccharomyces cerevisiae and Candida albicans. It contains LEU2, URA3 and an autonomously replicating sequence (ARS) from C. albicans for selection and replication in yeasts, and bla (ampicillin resistance) and ori for selection and replication in E. coli. S. cerevisiae AH22 (Leu-) was transformed by pRC2312 to Leu+ at a frequency of 1.41 x 10(5) colonies per microgram DNA. Transformation of C. albicans SGY-243 (Ura-) to Ura+ with pRC2312 resulted in smaller transformant colonies at a frequency of 5.42 x 10(3) per microgram DNA where the plasmid replicated autonomously in transformed cells, and larger transformant colonies at a frequency of 32 per microgram DNA, in which plasmid integrated into the genome. Plasmid copy number in yeasts was determined by a DNA hybridization method and was estimated to be 15 +/- 3 per haploid genome in S. cerevisiae and 2-3 per genome in C. albicans replicative transformants. Multiple tandem integration occurred in integrative transformants and copy number of the integrated sequence was estimated to be 7-12 per diploid genome. The C. albicans ADE2 gene was ligated into plasmid pRC2312 and the construct transformed Ade- strains of both C. albicans and S. cerevisiae to Ade+. The vector pRC2312 was also used to clone a fragment of C. albicans genomic DNA containing an aspartic proteinase gene. C. albicans transformants harboring this plasmid showed a two-fold increase in aspartic proteinase activity. However S. cerevisiae transformants showed no such increase in proteinase activity, suggesting the gene was not expressed in S. cerevisiae.