Capping of U6 small nuclear RNA in vitro can be uncoupled from transcription

U6 small nuclear RNA (snRNA) is a required component in the splicing of eukaryotic pre-mRNAs. Mammalian U6 snRNA was synthesized in vitro by T7 RNA polymerase and purified on polyacrylamide gels. This U6 RNA, with pppG on its 5' end, was accurately capped to CH3-O-pppG, when incubated with HeLa cell extract and this capping was dependent on the capping signal present within the U6 snRNA. When gamma-32P-labeled U6 RNA was used as a substrate, the U6 cap formed in vitro retained this labeled gamma-phosphate, indicating that the cap formation involves the methylation of the gamma-phosphate incorporated during transcription. U6 snRNAs with ppG or pG as their 5' ends, were not capped in this in vitro capping system. Capping of U6 snRNA in vitro requires at least two components, a heat-labile component and S-adenosylmethionine as a methyl group donor. The data presented here show that capping of U6 snRNA can be uncoupled from transcription and that the mechanism of U6 snRNA cap formation differs markedly from the capping mechanism of mRNAs and other U snRNAs where capping is cotranscriptional. While many methyltransferases have been characterized earlier, this is the first report of a methyltransferase that is specific to phosphate residues. This in vitro capping system will be useful for purification and studies on the U6 snRNA sequence-dependent methyltransferase activity.