Yeast mRNA splicing in vitro.

Synthetic actin and CYH2 pre-mRNAs containing a single intron are accurately spliced in a soluble whole cell extract of yeast. Splicing in vitro requires ATP. The excised intron is released as a lariat in which an RNA branch connects the 5' end of the molecule to the last A in the "intron conserved sequence" UACUAAC. Two other discrete RNA species produced during splicing in vitro may represent reaction intermediates: free, linear exon 1 and a form of the intron lariat extending beyond the 3' splice site to include exon 2. Both lariat forms correspond to molecules previously shown to be produced during yeast pre-mRNA splicing in vivo.