Cofactor requirements of splicing of purified messenger RNA precursors.

The origin and functions of introns in protein coding genes is one of the enigmas of molecular biology. Splicing processes that remove intervening sequences from precursor RNAs must have either predated or co-evolved with introns. Inferences about the origin of introns and the possible modes of regulation of splicing should emerge from an understanding of the biochemical mechanisms of splicing. The biochemistry of splicing of tRNA and rRNA precursors has rapidly advanced with the development of in vitro reactions containing soluble components that duplicate in vivo reactions. We have recently shown that accurate splicing of an adenovirus mRNA precursor occurs during a coupled transcription/splicing reaction in a soluble whole cell extract. We now report that an exogenous RNA substrate containing the first and second leaders of adenovirus 2 is accurately spliced when added to an extract of HeLa cells. ATP and Mg2+ are essential cofactors for the reaction. The time course of splicing is unusual; a lag of 45 min is observed before the appearance of splicing product.