3' end formation of U1 snRNA precursors is coupled to transcription from snRNA promoters.

Promoters of small nuclear RNA (snRNA) genes are partly responsible for 3' end formation of snRNA precursors. In injected X. laevis oocytes, substitution of an mRNA promoter (HSV tk) for the snRNA promoter significantly reduces the utilization of a conserved snRNA 3' end signal and permits recognition of a downstream polyadenylation site. Neither the U1 enhancer nor the U1 coding region is essential for recognition of the snRNA 3' end signal. Deletion of the U1 3' end signal from genes with a U1 promoter results in utilization of "cryptic" signals resembling the consensus sequence. However, these snRNA gene-promoted transcripts are not polyadenylated, in spite of the functional polyadenylation signal they contain. Thus, the ability to recognize 3' end signals is determined during initiation, presumably by interaction of transcription complexes with specific processing or termination factors.