Abstract
Several factors contribute to nuclear mRNA export in
Saccharomyces cerevisiae, including Mex67p, Mtr2p, Gle1p, Nup159p, Dbp5p, and Rip1p. Strains carrying mutations in these factors show rapid and dramatic nuclear accumulation of poly(A)
+ RNA. We have characterized two heat shock mRNAs,
SSA4 and
HSP104, in these mutant backgrounds; each transcript concentrates in a single intranuclear focus. Evidence suggests that it coincides with the site of transcription. Interestingly, all detectable
SSA4 transcripts have undergone 3′-end formation, indicating that RNAs in the foci are no longer nascent. Poly(A) tails of the transcripts are also dramatically longer in all of these export mutants. Based on all of the data, we suggest that very early mRNA maturation events determine transcript export competence.