Pitfalls of the synthetic lethality screen in Saccharomyces cerevisiae: an improved design.

The colony color assay in yeast enables the visual identification of plasmid-loss events. In combination with a plasmid-dependence assay, it is commonly used to identify synthetic interactions between functionally related genes. Frequently, the plasmid carries the ADE3 gene and mutants are recognized as red colonies that fail to produce sectors. In these assays, a high percentage of false-positives is obtained, most of which result from synthetic lethality with the ade3 mutation. Here, we study the nature of these mutants. We report that mutations in the HIP1 and SHM1 genes exhibit synthetic lethality with ade3 deletions. A similar interaction is found between the fur1 and ura3 mutations. Lethality in the absence of the mitochondrial Shm1 and the cytoplasmic Ade3 enzymes indicates that, under certain circumstances, these cellular compartments cooperate in carrying out essential metabolic processes. In addition, we report the identification of a truncated ADE3 allele with a unique coloration phenotype and show that it can be used to improve synthetic lethal screens.