TRANSLATIONAL CONTROL OF POLY(A)-BINDING PROTEIN EXPRESSION

Poly(A)-binding protein (PARR) is important for translation of eukaryo tic mRNA and may be involved in shortening of its poly(A) tract. In ma ny eukaryotic cells, this mRNA is inefficiently translated. The 5' unt ranslated region (UTR) of PABP mRNA has several adenine-rich regions w hich may serve as the PABP-binding sites to control its translation by a feed-back mechanism. This postulate was tested by using in vitro tr anscribed PABP mRNA and a rabbit reticulocyte lysate cell-free system. Results of our studies show that removal of the putative PABP-binding sites from the 5' UTR of this mRNA enhances its translation in the ra bbit reticulocyte cell-free system. Furthermore, in vitro translation of the full-length PABP mRNA was inhibited by addition of purified PAB P to the cell-free system. In contrast, translation of truncated mRNA lacking the putative PABP-binding sites at the 5' UTR was not inhibite d by exogenous PABP. We have also tested the ability of purified PABP to bind to the 5' UTR of PABP mRNA using ultraviolet-mediated covalent cross-linking of RNA and proteins in vitro. Our results show that exo genous PARR binds to the 5' UTR of its full-length mRNA. Furthermore, incubation of PABP mRNA in rabbit reticulocyte lysate also led to bind ing of the endogenous PABP within the first 223 nucleotides of the 5' UTR. The adenine-rich regions are located within this segment of PABP mRNA. Following incubation of PABP mRNA in the reticulocyte lysate cel l-free system under conditions of mRNA translation, the polysomal and non-translated free mRNA fractions were separated by centrifugation. A nalysis of free and polysomal mRNA-protein (mRNP) complexes following ultraviolet-induced cross-linking showed that the free mRNP population was preferentially enriched in PABP. Results of our studies, therefor e, suggest that PABP mRNA translation may be repressed by a unique fee d-back mechanism.