Following Temperature Stress, Export of Heat Shock mRNA Occurs Efficiently in Cells with Mutations in Genes Normally Important for mRNA Export

Heat shock leads to accumulation of polyadenylated RNA in nuclei of Saccharomyces cerevisiae cells, transcriptional induction of heat shock genes, and efficient export of polyadenylated heat shock mRNAs. These studies were conducted to examine the requirements for export of mRNA following heat shock. We used in situ hybridization to detect SSA4 mRNA (encoding Hsp70) and flow cytometry to measure the amount of Ssa4p-green fluorescent protein (GFP) produced following heat shock. Npl3p and Yra1p are mRNA-binding proteins recruited to nascent mRNAs and are essential for proper mRNA biogenesis and export. Heat shock mRNA was exported efficiently in temperature-sensitive npl3, yra1, and npl3 yra1 mutant strains. Nevertheless, Yra1p was recruited to heat shock mRNA, as were Nab2p and Npl3p. Interestingly, Yra1p was not recruited to heat shock mRNA in yra1-1 cells, suggesting that Npl3p is required for recruitment of Yra1p. The THO complex, which functions in transcription elongation and in recruitment of Yra1p, was not required for heat shock mRNA export, although normal mRNA export is impaired in growing cells lacking THO complex proteins. Taken together, these studies indicate that export following heat shock depends upon fewer factors than does mRNA export in growing cells. Furthermore, even though some mRNA-binding proteins are dispensable for efficient export of heat shock mRNA, those that are present in nuclei of heat shocked cells were recruited to heat shock mRNA