Synthesis of human U1 RNA. II. Identification of two regions of the promoter essential for transcription initiation at position +1.

We have analyzed the requirements for human U1 RNA transcription catalyzed by RNA polymerase II. In Xenopus laevis oocytes, a human U1 RNA gene with only 231 and 35 nucleotides of the 5' and 3' flanking regions, respectively (Lund, E. and Dahlberg, J. E. (1984) J. Biol. Chem. 259, 2013-2021), is able to support accumulation of human U1 RNA. We show that the point in the template corresponding to the 5' end of U1 RNA is a site of transcription initiation. That result rules out the possibility that the 5' end of U1 RNA is generated by cleavage and capping of a precursor RNA. The accumulation of correctly initiated human U1 RNA transcripts requires at least two essential upstream elements. The region between positions -231 and -203 is indispensable for transcription both in oocytes and in vitro. The other region, between positions -105 and -6, fixes the location of the 5' ends of the U1 RNA transcripts in oocytes while not altering the overall level of transcription. This latter region contains a sequence located around position -50, which we propose serves as the analog of the T-A-T-A sequence in U1 and U2 RNA genes.