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The RNA polymerase (pol) II and III human small nuclear RNA (snRNA) genes have very similar promoters and recruit a number of common factors. In particular, both types of promoters utilize the small nuclear RNA activating protein complex (SNAP_c) and the TATA box binding protein (TBP) for basal transcription, and are activated by Oct-1. We find that SNAP_c purified from cell lines expressing tagged SNAP_c subunits is associated with Yin Yang-1 (YY1), a factor implicated in both activation and repression of transcription. Recombinant YY1 accelerates the binding of SNAP_c to the proximal sequence element, its target within snRNA promoters. Moreover, it enhances the formation of a complex on the pol III U6 snRNA promoter containing all the factors (SNAP_c, TBP, TFIIB-related factor 2 (Brf2), and B double prime 1 (Bdp1)) that are sufficient to direct in vitro U6 transcription when complemented with purified pol III …