Low-salt crystallization of T7 RNA polymerase: a first step towards the transcription bubble complex

Preparation and crystallization of RNA

The field of RNA structure has exploded in recent years, in part owing to advances in crystallography of RNA molecules. This phenomenon can largely be attributed to the development of three modern methods: (1) large-scale in vitro RNA synthesis, (2) cryocrystallography, and (3) high-intensity synchrotron beamlines. Milligram quantities of RNA can be routinely synthesized using either chemical or enzymatic syntheses, making it feasible to carry out routine crystallization experiments on RNA. This has allowed crystals of RNA to be readily obtained. Generally, RNA crystals tend to be susceptible to radiation damage and to diffract X-rays more weakly than their protein counterparts. However, cryocrystallography and the high-intensity X-ray sources have overcome many of the difficulties involved in solving crystal structures of RNA. As a result of these advances, we now have a database of RNA structures that span from simple duplexes and hairpins to complex ribozymes and ribosomes. The protocols presented here describe methods to synthesize, purify, crystallize, and derivatize RNA for use in crystallographic studies.

The structural mechanism of translocation and helicase activity in T7 RNA polymerase

RNA polymerase functions like a molecular motor that can convert chemical energy into the work of strand separation and translocation along the DNA during transcription. The structures of phage T7 RNA polymerase in an elongation phase substrate complex that includes the incoming nucleoside triphosphate and a pretranslocation product complex that includes the product pyrophosphate (PPi) are described here. These structures and the previously determined posttranslocation elongation complex demonstrate that two enzyme conformations exist during a cycle of single nucleotide addition. One orientation of a five-helix subdomain is stabilized by the phosphates of either the incoming NTP or by the product PPi. A second orientation of this subdomain is stable in their absence and is associated with translocation of the heteroduplex product as well as strand separation of the downstream DNA. We propose that the dissociation of the product PPi after nucleotide addition produces the protein conformational change resulting in translocation and strand separation.