Crystallization of RNA-protein complexes I. Methods for the large-scale preparation of RNA suitable for crystallographic studies

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In vitro transcription using bacteriophage RNA polymerases and linearised plasmid or oligodeoxynucleotide templates has been used extensively to produce RNA for biochemical studies. This method is, however, not ideal for generating RNA for crystallisation because efficient synthesis requires the RNA to have a purine rich sequence at the 5' terminus, also the subsequent RNA is heterogenous in length. We have developed two methods for the large scale production of homogeneous RNA of virtually any sequence for crystallization. In the first method RNA is transcribed together with two flanking intramolecularly-, (cis-), acting ribozymes which excise the desired RNA sequence from the primary transcript, eliminating the promoter sequence and heterogeneous 3' end generated by run-off transcription. We use a combination of two hammerhead ribozymes or a hammerhead and a hairpin ribozyme. The RNA-enzyme activity generates few sequence restrictions at the 3' terminus and none at the 5' terminus, a considerable improvement on current methodologies. In the second method the BsmAI restriction endonuclease is used to linearize plasmid template DNA thereby allowing the generation of RNA with any 3' end. In combination with a 5' cis-acting hammerhead ribozyme any sequence of RNA may be generated by in vitro transcription. This has proven to be extremely useful for the synthesis of short RNAs.