Binding of hnRNP L to the Pre-mRNA processing enhancer of the herpes simplex virus thymidine kinase gene enhances both polyadenylation and nucleocytoplasmic export of intronless mRNAs

Liu and Mertz (Genes Dev. 9:1766-1780, 1995) previously identified a 119-nt pre-mRNA processing enhancer (PPE) element within the herpes simplex virus type 1 thymidine kinase gene that enables intron-independent gene expression in higher eukaryotes by binding heterogeneous nuclear ribonucleoprotein L (hnRNP L). Here, we identify a 49-nt subelement within this PPE that enhanced stability, polyadenylation, and cytoplasmic accumulation of transcripts synthesized in CV-1 cells from an intronless variant of the human beta-globin gene when present in two or more tandem copies. This 2 x TK49 PPE also enhanced (i) the efficiency of polyadenylation of intronless beta-globin RNA in a cell-free polyadenylation system and (ii) the kinetics of nucleocytoplasmic export of an intronless variant of adenovirus major late leader region RNA in Xenopus oocytes. This 2 X TK49 PPE bound only hnRNP L. Analysis of 2 x TK49 PPE mutants showed a strong positive correlation existed between binding hnRNP L and enhancement of intronless beta-globin gene expression. hnRNP L was found to associate with both the mRNA export factor TAP and the exon-exon junction complex protein Aly/REF. Thus, we conclude that hnRNP L plays roles in enhancing stability, polyadenylation, and nucleocytoplasmic export; it does so, at least in part, by directly recruiting to intronless PPE-containing RNAs cofactors normally recruited to intron-containing RNAs.