A low-efficiency downstream uridine-rich element and a noncanonical USE balance 3′end formation of the human prothrombin gene. (A) Comparison of the DNA encoding the 3′end processing signals of SV40 late, HBB, and F2. The CS in the pre-RNAs are underlined, and the respective distance to the polyA signals is indicated by brackets. The thymidine residues of the downstream U-rich element (ds U-rich) or the upstream U-rich element (USE) are highlighted. Positions 20210 and 20221 are indicated by ^* and ▵, respectively. (B) Northern blot of cytoplasmic RNA of cells transfected with tandem constructs with modifications of the ds U-rich element (lanes 2-5) or the USE (lanes 7-8). In construct HBB-F2/F2 + USE (lane 11), the F2 USE was inserted in the HBB 3′UTR of construct HBB-F2/F2 (lane 10). The numbers shown below the panel represent the ratio of mRNA processed at the 5′site relative to that processed at the 3′ site. The signals were quantified by phosphoimaging.