Crossing the line between RNA polymerases: transcription of human snRNA genes by RNA polymerases II and III

In eukaryotic organisms, transcription is carried out bythree RNA polymerases, RNA polymerases I, II, and III.In general, these RNA polymerases recognize promoterswith very different structures. For example, RNA polymerase II (pol II) promoters are mostly located upstreamof the transcription start site, whereas RNA polymeraseIII (pol III) promoters are mostly located downstreamfrom the transcription start site. The U1, U2, and U6small nuclear RNA (snRNA) genes, which encode essential RNA components of small nuclear ribonucleoproteinparticles (snRNPs) involved in mRNA splicing, serve asrepresentative models for a group of pol II and pol IIIgenes with unusually similar promoters. The pol II U1and U2 promoters contain a distal sequence element(DSE) and a proximal sequence element (PSE), whereasthe pol III U6 promoter contains a DSE, a PSE, and aTATA box, all located upstream of the transcriptionalstart sit..