Cap and polyA tail enhance translation initiation at the hepatitis C virusinternal ribosome entry site by a discontinuous scanning, or shunting, mechanism

Objectives: Hepatitis C virus (HCV) does not replicate in vitro, suggesting that cultured cells may lack factors that are essential for efficient use of HCV messenger RNAs (mRNAs). Here, we have studied the efficiency of HCV mRNA translation compared with translation of capped and polyadenylated mRN As in human cells. Study Design/Methods: We have generated noninfectious mini-virus mRNAs from an infectious HCV genome. These mRNAs were transfected into human cells, a nd the translation efficiency was determined. Results: Hepatitis C virus mRNAs under control of the HCV internal ribosome entry site (IRES) were inefficiently trans lated compared with capped and polyadenylated mRNAs. Addition of a cap and a polyA tail on the HCV mRNAs r evealed that these structures interacted with the hepatitis C IRES in a syn ergistic manner to load ribosomes onto the HCV mRNAs, thereby strongly enha ncing translation. The positive effect of the cap and the polyA rail on ini tiation of translation at the initiator AUG embedded in the HCV IRES was th e result of a discontinuous scanning, or shunting, mechanism. Conclusions: The results demonstrated that recruitment of ribosomes to the HCV mRNAs was inefficient in dividing cultured cells. Factors that are nece ssary for efficient translation of the HCV mRNAs in hepatocytes may be abse nt or inactive in cultured cells. This may be one reason for the inefficien t replication of the HCV in vitro.