A Naturally Thermolabile Activity Compromises Genetic Analysis of Telomere Function in Saccharomyces cerevisiae

The core assumption driving the use of conditional loss-of-function reagents such as temperature-sensitive mutations is that the resulting phenotype(s) are solely due to depletion of the mutant protein under nonpermissive conditions. However, prior published data, combined with observations presented here, challenge the generality of this assumption at least for telomere biology: for both wild-type yeast and strains bearing null mutations in telomere protein complexes, there is an additional phenotypic consequence when cells are grown above 34 degrees. We propose that this synthetic phenotype is due to a naturally thermolabile activity that confers a telomere-specific defect, which we call the Tmp(-) phenotype. This prompted a re-examination of commonly used cdc13-ts and stn1-ts mutations, which indicates that these alleles are instead hypomorphic mutations that behave as apparent temperature-sensitive mutations due to the additive effects of the Tmp(-) phenotype. We therefore generated new cdc13-ts reagents, which are nonpermissive below 34 degrees, to allow examination of cdc13-depleted phenotypes in the absence of this temperature-dependent defect. A return-to-viability experiment following prolonged incubation at 32 degrees, 34 degrees, and 36 degrees with one of these new cdc13-ts alleles argues that the accelerated inviability previously observed at 36 degrees in cdc13-1 rad9-Delta mutant strains is a consequence of the Tmp(-) phenotype. Although this study focused on telomere biology, viable null mutations that confer inviability at 36 degrees have been identified for multiple cellular pathways. Thus, phenotypic analysis of other aspects of yeast biology may similarly be compromised at high temperatures by pathway-specific versions of the Tmp(-) phenotype.