Fig. 5. The mutant polymerase complexes that lack endonuclease activity possess wild-type activities for binding (A) 5′ vRNA, (B) 3′ vRNA and (C) capped RNA. Wild-type (lane 2) or mutant (508A, 519A, 522A) polymerases (lanes 3–5) were incubated with (A) 5′ vRNA (labeled with ³²P at its 5′ end), (B) labeled 3′ vRNA in the presence of unlabeled 5′ vRNA or (C) the 13-nt-long capped AlMV RNA 4 RNA fragment containing an m⁷G³²pppG_m 5′ end in the presence of unlabeled 5′ vRNA. Polymerase–RNA complexes were separated from unbound RNA by electrophoresis on 4% non-denaturing gels. Lane 1 (C), each labeled RNA incubated in the absence of a polymerase complex.