To the FF motif (558-571FF, CPLEMSHKKSFFDKK) did not inhibit the binding of His-Pol(1?15) to GST-REV1(826-1251). The failure of 558-571FF to bind REV1-CTD was confirmed independently by SPR assay (data not shown), thus indicating that more than 3 residues C-terminal to the FF motif are necessary for REV1 interaction. The finding that several residues C-terminal to the FF motif in the Pol RIR seque