RNA polymerase I, bending the rules?

Transcription initiation is one of the key regulatory steps in expressing the genetic information encoded in the DNA. Mechanisms of RNA Pol II transcription have been extensively studied, whereas the structural basis of RNA Pol I and III transcription is still poorly defined. Three recent studies discussed here give a first glimpse into the molecular mechanisms underlying the process of RNA Pol I transcriptional initiation and reveal unexpected structural differences compared to the previously described homologous Pol II structures.