THE LARGE SUBUNIT OF BASAL TRANSCRIPTION FACTOR SNAP(C) IS A MYB DOMAIN PROTEIN THAT INTERACTS WITH OCT-1

The human RNA polymerase II and III snRNA promoters have similar enhan cers, the distal sequence elements (DSEs), and similar basal promoter elements, the proximal sequence elements (PSEs), The DSE, which contai ns an octamer motif, binds broadly expressed activator Oct-1. The PSE binds a multiprotein complex referred to as SNAP(c) or PTF. On DNAs co ntaining both an octamer site and a PSE, Oct-1 and SNAP(c) bind cooper atively, SNAP(c) consists of at least four stably associated subunits, SNAP43, SNAP45, SNAP50, and SNAP190. None of the three small subunits , which have all been cloned, can bind to the PSE on their own. Here w e report the isolation of cDNAs corresponding to the largest subunit o f SNAP(c), SNAP190. SNAP190 contains an unusual Myb DNA binding domain consisting of four complete repeats (Ra to Rd) and a half repeat (Rh) , A truncated protein consisting of the last two SNAP190 Myb repeats, Re and Rd, can bind to the PSE, suggesting that the SNAP190 Myb domain contributes to recognition of the PSE by the SNAP complex, SNAP190 is required for snRNA gene transcription by both RNA polymerases II and III and interacts with SNAP45. In addition, SNAP190 interacts with Oct -1. Together, these results suggest that the largest subunit of the SN AP complex is involved in direct recognition of the PSE and is a targe t for the Oct-1 activator, They also provide an example of a basal tra nscription factor containing a Myb DNA binding domain.