Elements required for transcription initiation of the human U2 snRNA gene coincide with elements required for snRNA 3' end formation.

Formation of the human U1 and U2 snRNA 3' ends requires both a conserved sequence, the 3' box, located downstream of the snRNA termini and sequences within the snRNA promoter regions. Indeed, replacement of the U1 snRNA promoter by mRNA promoters inhibits U1 3' end formation. We have now mutated the 5' flanking region of the human U2 gene and assayed the effects on initiation of transcription and 3' end formation. The 5' flanking region of the U2 gene contains two major promoter elements, a previously characterized distal element that enhances the efficiency of transcription and a proximal element, which our analysis localizes between positions -59 and -43 in a segment conserved in vertebrate snRNA genes. The 5' flanking region does not contain an element required solely for 3' end formation. However, when enhancer elements from an mRNA-encoding gene are introduced into a U2 promoter lacking its distal element, 3' end formation is inhibited. Together, these results suggest that the U2 promoter elements themselves are involved in 3' end formation, presumably by directing the formation of a unique transcription complex which is compatible with 3' end formation at the 3' box. Alteration of the composition of this transcription complex results in increased read-through at the 3' box.