Plasmids pYES-P27, pYES-Rep, pYES-P27DAUG, pYES-RepDAUG, or empty pYES2 were co-expressed with pAC-A or with the corresponding pAC-Am inframe control in S.cerevisiae EGY48 strain (MATa, ura3, trp1, his3, 6lexAop-LEU2) (Invitrogen). Transformants were grown overnight in SC-Leu-Ura media in non-inducing conditions, and used to inoculate induction medium. Cells were harvested at late logarithmic phase. Expression of the CfMV proteins was confirmed by western blotting using polyclonal antisera against the CfMV ORF 2a and 2b proteins (28) . Determining the enzymatic activities as described below monitored the effect of CfMV P27 and replicase on À1 PRF.