Cap and polyA tail enhance translation initiation at the hepatitis C virus internal 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 mRNAs in human cells. STUDY DESIGN/
METHODS: We have generated noninfectious minivirus mRNAs from an infectious HCV genome. These mRNAs were transfected into human cells, and the translation efficiency was determined.
RESULTS: Hepatitis C virus mRNAs under control of the HCV internal ribosome entry site (IRES) were inefficiently translated compared with capped and polyadenylated mRNAs. Addition of a cap and a polyA tail on the HCV mRNAs revealed that these structures interacted with the hepatitis C IRES in a synergistic manner to load ribosomes onto the HCV mRNAs, thereby strongly enhancing translation. The positive effect of the cap and the polyA tail on initiation of translation at the initiator AUG embedded in the HCV IRES was the 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 necessary for efficient translation of the HCV mRNAs in hepatocytes may be absent or inactive in cultured cells. This may be one reason for the inefficient replication of the HCV in vitro.