miR-122 does not modulate the elongation phase of hepatitis C virus RNA synthesis in isolated replicase complexes.

miR-122 is an abundant, liver-specific microRNA that is required for efficient amplification of hepatitis C virus (HCV) RNA. Recent studies with a miR-122-specific locked nucleic acid antagomir have shown it to be an important host target for therapeutic intervention. However, considerable controversy exists concerning the mechanisms underlying the dependence of HCV replication on miR-122. We studied the impact of miR-122 on the rate of [(32)P]-incorporation into positive-strand viral RNA by membrane-bound replicase complexes isolated from cells containing HCV RNA replicons. [(32)P]-incorporation in this cell-free system represents primarily the elongation phase of RNA synthesis, with little or no de novo initiation, and was not affected by the addition of either excess miR-122 or a miR-122-specific antisense oligonucleotide that suppresses replication in vivo. We also found no evidence that detectable quantities of miR-122 are specifically associated with replicase complexes in vivo. These results are consistent with miR-122 acting at an alternative step in the viral life cycle, promoting cap-independent viral translation, enhancing viral RNA stability, or facilitating de novo initiation of viral RNA synthesis.