How to Recruit the Correct RNA Polymerase? Lessons from snRNA Genes

With a few exceptions, Pol II and Pol III transcribe distinct sets of genes. Pol II and Pol III promoters are generally very different in structure. The Pol II and III snRNA promoters are the exception; they are very similar to each other, a feature conserved in several species. As a result, some of these genes, notably the RPPH1 promoter, can recruit Pol II or Pol III in vivo. The TATA box and PSE-type sequences (PSE in mammals and sea urchin, PSEA in Drosophila melanogaster, and USE in Arabidopsis thaliana) are the only well-defined core promoter elements that are shared by some Pol II and III promoters. Selective Pol II or III recruitment by snRNA promoters is specified by promoter features that shift the equilibrium of competitive interactions towards recruitment of TFIIB or BRF2, and Pol II or III transcription machineries