On-off regulation of polyadenylation of U1A pre-mRNA. U1A is a sequence-specific RNA-binding protein that is a component of the U1 snRNP (involved in mRNA splicing). Two U1A-binding sites are located upstream from the unusual AUUAAA polyA signal in the U1A pre-mRNA. When excess U1A protein accumulates, it binds to these sites in the nascent U1A pre-mRNA. Following assembly of the cleavage-and-polyadenylation complex on the pre-mRNA, cleavage occurs, but the bound U1A molecules interact with polyA polymerase (PAP), preventing it from adding A residues to the free 3' hydroxyl group. In the absence of polyadenylation, both cleavage products are degraded, and thus no U1A protein is produced.