3CDpro (a protease) cleaves VPg from membrane-bound 3AB. 3Dpol (picornavirusRNA polymerase) then synthesizes VPg-pUpU using the sequence AAACA from the cre (cis-acting replication element) of the template strand. The 3Dpol-VPg-pUpU complex then moves to the 3' poly(A) tail of the template strand and commences RNA synthesis using the pUpU sequence as a primer.
Sources
Principles of Virology by S.J. Flint, L.W. Enquist, V.R. Racaniello, A.M. Skalka (ISBN 1-55581-259-7)