Pre-mRNA with a trimethylguanosine cap structure can be spliced efficiently in vitro.

A chemically synthesized 2,2,7-trimethylguanosine cap (TMG-cap) analogue was added to an in vitro pre-mRNA splicing system. A slight inhibition of the splicing was observed, but it was much less remarkable than that by a 7-methylguanosine cap analogue. This result does not favor a direct role of the TMG-cap structure of small nuclear RNAs in pre-mRNA splicing, and suggests that the TMG-cap does not effectively interact with the 7-methylguanosine cap binding factors. Using the SP6 in vitro transcription system and a TMG-cap analogue, we prepared a beta-globin transcript which has a TMG-cap at the 5' end. This TMG-capped transcript was spliced accurately and efficiently in vitro.