Tissue culture RT assays RT assays were performed using the dual luciferase reporter constructs, as previously described (62, 63) . To control for possible differences in stability of specific mRNA sequences, each RT construct was compared with a control construct that was identical except that the TGA stop codon was replaced with a TGG codon. RT efficiencies were calculated as (firefly activity/renilla activity) for the RT sequence normalized by (firefly activity/renilla activity) for the corresponding TGG control sequence. Standard deviations were calculated based on six independent transfections.