Picornavirus IRESes and the poly(A) tail jointly promote cap-independent translation in a mammalian cell-free system

In eukaryotic cells, efficient translation of most cellular mRNAs requires the synergistic interplay between the m(7)GpppN cap structure and the poly( A) tail during initiation. We have developed and characterized a cell-free system from human HeLa cells that recapitulates this important feature, dis playing more than one order of magnitude of translational synergism between the cap structure and the poly(A) tail. The stimulation of cap-dependent t ranslation by the poly(A) tail is length-dependent, but not mediated by cha nges in mRNA stability. Using this system, we investigated the effect of th e poly(A) tail on the translation of picornaviral RNAs, which are naturally polyadenylated but initiate translation via internal ribosome entry sites (IRESs). We show that translation driven by the IRESs of poliovirus (PV), e ncephalomyocarditis virus (EMCV), and hepatitis A virus is also significant ly augmented by a poly(A) tail, ranging from an approximately 3-fold stimul ation for the EMCV-IRES to a more than 10-fold effect for the PV IRES. Thes e results raise interesting questions concerning the underlying molecular m echanism(s). The cell-free system described here should prove useful in stu dying these questions as well as providing a general biochemical tool to ex amine the translation initiation pathway in a more physiological setting.