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 s pecific transcripts are transported by Mex67p is not known, We observed tha t thermosensitive mex67-5 cells do not produce a heat shock response at 37 degrees C but will induce heat shock proteins (Hsp) (e.g, Hsp104p and Hsp70 p) when shifted back from the restrictive to permissive temperature (30 deg rees C). This memory of a previous heat stress in mex67-5 cells could be ex plained if HSP mRNAs accumulated inside the nucleus during heat shock and m ere exported and translated in the cytoplasm on return to the permissive te mperature. To test this hypothesis, nuclear export of heat shock mRNAs was directly analyzed by in situ hybridization using fluorescent-labeled oligon ucleotide probes specific for SSA transcripts. This revealed that Mex67p is required for nuclear export of heat shock mRNAs. Furthermore, other polyme rase II transcripts encoding the transcriptional repressor ASH1 and the gly colytic enzyme PGK1 are shown to require Mex67p for their export into the c ytoplasm, Thus, Mex67p is an mRNA export factor for a broad range of polyme rase II transcripts.