A 64 kd protein was shown to bind to RNAs that contain functional polyadenylation signals by a UV crosslinking procedure in which label was transferred from RNA substrate to protein in cell-free polyadenylation extracts. The 64 kd nuclear protein bound specifically to three different substrates (adenovirus type 5 L3, SV40 early, and SV40 late polyadenylation domains), as determined by competition experiments and partial protease analysis. Deleted derivatives of the SV40 late substrate that retained the sequence 5′-CUGCAAUAA-ACAAGUU-3′ were able to bind the 64 kd polypeptide. This sequence contains the canonical AAUAAA element that has been shown to be indispensable for polyadenylation. A single nucleotide change, converting AAUAAA to AAUAAA, prevented binding of the 64 kd moiety. The 64 kd protein was shown to be distinct from poly(A) polymerase by biochemical fractionation.
