To verify that the observed differences in firefly luciferase production depended on the studied CfMV proteins, we co-expressed the dual reporter vectors with plasmids having the first translation initiation codons of P27 and replicase deleted (pYES-P27DAUG and pYES-RepDAUG). Western blot analysis with antisera against ORF2A or 2B did not detect any proteins produced from these vectors (data not shown). The obtained LUC/LacZ ratios were compared to those measured from cells expressing the CfMV proteins (pYES-P27 or pYES-Rep). LUC/LacZ ratios measured from cells expressing replicase were slightly lower than the ratios calculated from cells harbouring pYES-RepDAUG plasmids, being $90% when co-expressed with pAC-A and $84% when co-expressed with pAC-Am. In the presence of P27, LUC/ LacZ ratio of pAC-A reached $81% of expression measured from cells transformed with pYES-P27DAUG. Again the effect of P27 expression was more evident with pAC-Am inframe control as P27 expression reduced LUC/LacZ ratio to half ($48%) when compared to the corresponding value measured from the cells harbouring pYES-P27DAUG. This verified that CfMV P27 was able to reduce the downstream reporter expression from dual reporter mRNAs. Since CfMV P27 had a proportionally stronger effect to firefly luciferase production from the inframe control mRNAs in comparison to the test mRNAs (Table 2) , the calculated À1 PRF efficiency increased from 14.7 to 22.4%.