DETERMINATION OF MESSENGER-RNA FATE BY DIFFERENT RNA POLYMERASE-II PROMOTERS

Translational stop mutations of the human beta-globin gene cause a red uction of cytoplasmic mRNA accumulation in thalassemia patients and in transfection models. The exact mechanism underlying this phenomenon h as remained enigmatic but is known to be post-transcriptional. We have used transfected HeLa cells to study the expression of beta-globin mR NAs with nonsense or frameshift mutations within the three exons of th is gene. Mutations in exons 1 or 2 reduce cytoplasmic mRNA accumulatio n whereas a mutation in exon 3 permits essentially normal expression. We report here that the post-transcriptional fate of mutated beta-glob in mRNAs is differentially affected by the type of RNA polymerase II p romoter driving expression. Replacement of the beta-globin promoter wi th the herpes simplex virus type 1 thymidine kinase gene promoter but not the cytomegalovirus immediate early promoter rescues the cytoplasm ic accumulation of mutated mRNA to wild-type levels. This effect is sh own to be independent of the absolute quantity and the kinetics of acc umulation of mutated mRNA synthesized, and primer-extension analyses c onfirm that both viral promoters accurately utilize identical transcri ption start sites. These data thus reveal an unexpected property of RN A polymerase II promoters: determination of the post-transcriptional f ate of the maturing mRNA, presumably by influencing alternative chokes between as yet undefined processing and/or transport pathways.