Self-splicing of the transcripts of the Tetrahymena large rRNA gene in vitro. (a) A DNA fragment including the intron (red) and flanking sequences (blue) was cloned into a plasmid, which was then transcribed in a test tube by purified RNA polymerase. (b) Samples of pre-rRNA purified from this transcription reaction were incubated in buffer solutions containing sodium and magnesium salts. The pre-rRNA was capable of self-splicing to release the intron, as shown in this agarose gel. The (shorter) intron RNA moves through the gel matrix faster than the (longer) starting transcript. Increasing salt concentration (shown at the top) facilitates the splicing reaction because salts stabilize the folded, catalytically active form of the RNA.