Pre-mRNA processing factors are required for nuclear export

RNA export from the nucleus is thought to be linked to proper processing and packaging into ribonucleoprotein protein complexes. A system to observe mRNA nuclear export in living yeast cells was developed by fusing the U1A RNA-binding protein to the green fluorescent protein to follow specific mRNAs with U1A hairpins engineered into them. RNAs encoding Rp125, Pgkl, and Ssa4 were examined for the effects of 3' UTRs, introns, RNA processing factors, nucleoporins, and transport factors on their export. All accumulated in the nucleus in mutants affecting components of the nuclear export machinery and certain nucleoporins. However, under conditions of stress, PGK1 and RPL25 transcripts accumulate in the nucleus whereas SSA4 RNA is exported. Moreover, when export is blocked, only RNAs containing the ASH13'UTR accumulated in the nucleolus. Mutations in the splicing machinery selectively blocked export of only intron-containing RNAs. Mutations in RNA14 RNA15, and PAP1, which encode factors important for 3' processing, also blocked export of all RNAs, including SSA4 thereby linking export to the process of polyadenlyation. Taken together, these data graphically display the connections between mRNA processing and nuclear export.