Characterization of a trimeric complex containing Oct-1, SNAPc, and DNA.

The human small nuclear (sn) RNA promoters contain a proximal sequence element (PSE), which recruits the basal transcription factor SNAPc, and a distal sequence element characterized by an octamer sequence, which recruits the POU domain transcription factor Oct-1. The Oct-1 POU domain and SNAPc bind cooperatively to probes containing a PSE and an octamer sequence, and this effect contributes to efficient transcription in vitro. In vivo, however, Oct-1 regions outside of the POU domain can activate snRNA gene transcription. Here, we have examined whether the role of these regions is to contribute to cooperative binding with SNAPc. We find that they indeed improve cooperative binding, but most of the effect is nevertheless mediated by just the POU domain. This suggests that Oct-1 activates transcription of snRNA genes in at least two steps, recruitment of SNAPc mediated primarily by the POU domain, and a later step mediated by regions outside of the POU domain. We also show that a PSE-binding complex observed in nuclear extracts consists of Oct-1 and SNAPc. Although Oct-1 cannot bind effectively to the PSE probe on its own, in the complex it contacts DNA. Thus, in a nuclear extract, SNAPc can recruit Oct-1 to a probe to which Oct-1 cannot bind on its own.