Mex67p Mediates Nuclear Export of a Variety of RNA Polymerase II Transcripts*

Mex67p is essential for nuclear poly(A)+ RNA export in yeast, but which specific transcripts are transported by Mex67p is not known. We observed that thermosensitive mex67-5 cells do not produce a heat shock response at 37 °C but will induce heat shock proteins (Hsp) (e.g. Hsp104p and Hsp70p) when shifted back from the restrictive to permissive temperature (30 °C). This memory of a previous heat stress in mex67-5 cells could be explained ifHSP mRNAs accumulated inside the nucleus during heat shock and were exported and translated in the cytoplasm on return to the permissive temperature. To test this hypothesis, nuclear export of heat shock mRNAs was directly analyzed by in situhybridization using fluorescent-labeled oligonucleotide probes specific for SSA transcripts. This revealed that Mex67p is required for nuclear export of heat shock mRNAs. Furthermore, other polymerase II transcripts encoding the transcriptional repressorASH1 and the glycolytic enzyme PGK1 are shown to require Mex67p for their export into the cytoplasm. Thus, Mex67p is an mRNA export factor for a broad range of polymerase II transcripts.