Article

  • 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 inArabidopsis 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.
Nuclear eukaryotic genomes are transcribed by three related RNA polymerases (Pol), which transcribe distinct gene sets. Specific Pol recruitment is achieved through selective core promoter recognition by basal transcription factors (TFs). Transcription by an inappropriate Pol appears to be rare and to generate mostly unstable products. A collection of short noncoding RNA genes [for example, small nuclear RNA (snRNA) or 7SK RNA genes], which play essential roles in processes such as maturation of RNA molecules or control of Pol II transcription elongation, possess highly similar core promoters, and yet are transcribed for some by Pol II and for others by Pol III as a result of small promoter differences. Here we discuss the mechanisms of selective Pol recruitment to such promoters.