Thermal blockage of viruslike particle formation for the yeast retrotransposon Ty3 reveals differences in the cellular stress response

The long terminal repeat (LTR) retrotransposons of the yeast Saccharomyces cerevisiae are similar in their structures and life cycles to animal retrov iruses. The yeast LTR retrotransposon Ty3 does not transpose under conditio ns where the cellular stress response is activated. During stress, mature T y3 proteins, indicative of the formation of intracellular Ty3 viruslike par ticles (VLPs), do not accumulate. In order to examine the role of stress pr oteins in Ty3 transposition, a sensitive genetic assay was developed to mea sure VLP formation. The assay employs a Ty3 element marked with a mutant al lele of the yeast HIS3 gene (his3AI). To create a stable His(+) phenotype, Ty3 must form. VLPs, reverse transcribe Ty3 RNA into cDNA, and then insert the cDNA into either chromosomal or plasmid DNA. Using this assay, thermal inhibition of Ty3 transposition was evident at temperatures as low as 30 de greesC. The level of production of mature Ty3 proteins parallels the transp osition frequency. Although overexpression of the yeast UBP3 gene allows VL Ps to form and transposition to occur in the constitutively stressed ssa1 s sa2 strain, it does not alleviate the inhibition of these processes during stress induced by heat or ethanol. This suggests that the genetic and physi cal modes of stress response induction are not equivalent.