Messenger RNAs must assemble into mRNP complexes (protein-mRNA complexes) in order to be exported from the nucleus. The goal of this proposal is to characterize the mRNP complexes that form in S. cerevisiae. When mRNA export or processing is blocked, mRNAs are retained in foci or "dots" in the nucleus. These "dots" contain both mRNA and mRNA binding proteins. The experiments outlined in this proposal will assist in determining whether these mRNP complexes are associated with the site of transcription and in identifying the proteins that comprise these mRNP complexes. The proposal also focuses on a potential model for mRNA retention in the nucleus. Experiments to test this model will determine whether export factors are properly recruited to mRNPs that are retained in the nucleus and may identify an important factor in the release of mRNAs for export. The results of the proposed experiments will help to provide a clearer picture of the components of mRNP complexes and the means by which the cell retains mRNPs in the nucleus when mRNA processing or export is blocked.