A block to mRNA nuclear export in S. cerevisiae leads to hyperadenylation of transcripts that accumulate at the site of transcription

Several factors contribute to nuclear mRNA export in Saccharomyces cerevisi ae, including Mex67p, Mtr2p, Gle1p, Nup159p, Dbp5p, and Rip1p. Strains carr ying mutations in these factors show rapid and dramatic nuclear accumulatio n of poly(A)(+) RNA. We have characterized two heat shock mRNAs, SSA4 and H SP104, in these mutant backgrounds; each transcript concentrates in a singl e intranuclear focus. Evidence suggests that it coincides with the site of transcription. Interestingly, all detectable SSA4 transcripts have undergon e 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 th ese export mutants. Based on all of the data, we suggest that very early mR NA maturation events determine transcript export competence.