The human U1A snRNP protein regulates polyadenylation via a direct interaction with poly(A) polymerase.

The human U1 snRNP-specific U1A protein autoregulates its production by binding its own pre-mRNA and inhibiting polyadenylation. The mechanism of this regulation has been elucidated by in vitro studies. U1A protein is shown not to prevent either binding of cleavage and polyadenylation specificity factor (CPSF) to its recognition sequence (AUUAAA) or to prevent cleavage of U1A pre-mRNA. Instead, U1A protein bound to U1A pre-mRNA inhibits both specific and nonspecific polyadenylation by mammalian, but not by yeast, poly(A) polymerase (PAP). Domains are identified in both proteins whose removal uncouples the polyadenylation activity of mammalian PAP from its inhibition via RNA-bound U1A protein. Finally, U1A protein is shown to specifically interact with mammalian PAP in vitro. The possibility that this interaction may reflect a broader role of the U1A protein in polyadenylation is discussed.

A common octamer motif binding protein is involved in the transcription of U6 snRNA by RNA polymerase III and U2 snRNA by RNA polymerase II.

The structure of a Xenopus U6 gene promoter has been investigated. Three regions in the 5'-flanking sequences of the gene that are important for U6 expression are defined. Deletion of the first, between positions -156 and -280 relative to the site of transcription initiation, reduces transcription to roughly 5% of its original level. Deletion of the second, between -60 and -77, abolishes transcription. These regions contain not only functional but also sequence homology to the previously defined distal and proximal sequence elements (DSE and PSE) of the Xenopus U2 promoter, although U2 is transcribed by RNA polymerase II and U6 by RNA polymerase III. Competition experiments show that at least the distal sequence elements of the two promoters bind to a common factor both in vivo and in vitro. Part of the sequence recognized by this factor is the octamer motif (ATG-CAAAT). A sequence similar to the common RNA polymerase II TATA box is also shown to have an effect, albeit minor, on U6 transcription. The U6 coding region contains a good match to the A box, part of all previously characterized RNA polymerase III promoters. Deletion of this region has no apparent effect on the efficiency or accuracy of U6 transcription.