Characterization of a Drosophila proximal-sequence-element-binding protein involved in transcription of small nuclear RNA genes.

In a wide variety of eukaryotic organisms, transcription of small nuclear RNA (snRNA) genes is dependent upon a proximal sequence element (PSE) located upstream of position -40 relative to the transcription start site. There is little or no existent knowledge concerning the PSE-binding proteins of organisms other than human. Here, we report the purification of a fraction enriched in the Drosophila melanogaster PSE-binding protein (DmPBP). DmPBP forms a highly specific complex with the PSE. The protein stimulates transcription from the U1 gene promoter by RNA polymerase II and from the U6 gene promoter by RNA polymerase III in Drosophila nuclear extracts, and activation is dependent upon the presence of a PSE. The molecular mass of native DmPBP as measured by gel-filtration chromatography is 375 kDa. Two polypeptides (apparent molecular masses 59 kDa and 61 kDa) appear to be in close contact with the DNA in that they can be very efficiently and specifically crosslinked to the PSE sequence by ultraviolet irradiation.