The poly(A)-limiting element enhances mRNA accumulation by increasing the efficiency of pre-mRNA 3' processing.

The poly(A)-limiting element (PLE) is a conserved sequence originally found in the 3' UTR of Xenopus albumin mRNA whose presence restricts the length of the poly(A) tail on both pre-mRNA and fully processed mRNA to <20 nt. Results presented in this study show that the PLE also increases the cytoplasmic level of reporter beta-globin mRNA. Transcription run-on shows this increase was not due to increased reporter gene transcription, and experiments with tetracycline repressor-controlled reporter mRNA showed the PLE does not alter the rate of mRNA decay. Both RT-PCR and RNase protection assay showed the PLE caused a 50% increase in the 3' processing of reporter beta-globin mRNA in vivo. This was confirmed in vitro, where PLE-containing RNA was cleaved in HeLa nuclear extract at a rate 80% faster than a control RNA bearing an inactive element. These results indicate that the PLE regulates the length of the poly(A) tail and the efficiency of 3' processing. In addition, they show that PLE-containing mRNA with a <20-nt poly(A) tail is as stable as mRNA with a 100- to 200-nt poly(A) tail.