Fig. 9. A putative model for regulating splicing of the 3′ splice site in tat pre-mRNA. Binding of hnRNP A1 (A1) to the ISS may physically block the association of factors with the branch point sequences, e.g. SF1/mBBP and/or U2 snRNP, but without affecting U2AF65 binding to the polypyrimidine tract. SF2/ASF binds the ESE3 and stimulates binding of U2AF65 while the ESS3 has a negative effect on U2AF65 binding (Tange and Kjems, 2001). In addition, our data imply that ESS3 also affects spliceosome assembly after U2AF binding via an hnRNP A1-dependent mechanism.