| Liu and Mertz (1995) previously identified a 119-nt pre-mRNA processing enhancer (PPE) element within the herpes simplex virus type 1 thymidine kinase (HSV-TK) gene that enables intron-independent gene expression in higher eukaryotes by binding heterogeneous nuclear ribonucleoprotein L (hnRNP L). Here, I identified a 49-nt subelement within this PPE that enhanced expression of an intronless variant of the human beta-globin gene when inserted in two or more tandem copies. The presence of this 2xTK49 PPE enhanced stability, polyadenylation, and nucleocytoplasmic export of the globin transcripts in transient transfection, cell-free polyadenylation, and Xenopus oocyte assay systems. This 2xTK49 PPE bound only one nuclear protein, hnRNP L. Binding correlated with enhancement of intronless beta-globin gene expression. HnRNP L associated with both the mRNA export factor TAP and the exon-exon junction complex protein Aly/REF. Thus, hnRNP L plays roles in enhancing stability, polyadenylation, and nucleocytoplasmic export by directly recruiting to intronless PPE-containing RNAs cofactors normally recruited to intron-containing RNAs.; Most mRNA-encoding genes require introns for efficient expression in high eukaryotes. However, mRNAs can efficiently accumulate in the cytoplasm without intron excision if they contain cis-acting elements such as the post-transcriptional regulatory element (PRE) of hepatitis B virus (HBV), the constitutive transport element (CTE) of Mason-Pfizer monkey virus (MPMV), or HSV-TK's PPE. 1 compared the activities of these viral elements, the Rev-responsive element (RRE) of the human immunodeficiency virus (HIV), and an element, CJE, of the human c-Jun gene, newly identified here, to enable expression of an intronless variant of the beta-globin gene. The PRE, PPE, and CJE from naturally intronless genes, but not the CTE or RRE from intron-containing genes, could significantly enhance stability, 3 '-end processing, and cytoplasmic accumulation. When the transcripts included the beta-globin gene's first intron, the PRE, PPE, and CJE still enhanced mRNA biogenesis, in some cases without intron excision. Thus, stability, 3'-end formation, and nucleocytoplasmic export, not the presence of introns or their excision per se, are necessary for mRNA biogenesis. While CTE-like elements mainly enhance nucleocytoplasmic export, PPE-like elements from naturally intronless genes facilitate polyadenylation as well. |
