The spliceosome as ribozyme hypothesis takes a second step

摘要

The spliceosome is a massive assembly of 5 RNAs and many proteins that, together, catalyze precursor-mRNA (pre-mRNA) splicing. The splicing mechanism involves 2 steps: (i) cleavage of the 5' ex-on-intron phosphodiester bond and formation of a new 2'-5' bond resulting in an intron "lariat," and (ii) exon ligation (Fig. 1A). This 2-step phosphoryl transfer mechanism is suspiciously identical to the reaction catalyzed by the group II self-splicing introns, which are ribo-zymes. The precedent for a ribozyme-catalyzed splicing mechanism, combined with the fact that the spliceosome has an RNA core that has been highly conserved for >1 billion years, led researchers to hypothesize that the spliceosome may use an RNA-based catalytic mechanism (1). However, the spliceosome as ribozyme hypothesis has been exceedingly difficult to prove, for 2 major reasons. First, the spliceosome contains many proteins that are essential for splicing (2). Second, the uncatalyzed rate of RNA ligation in vitro is significant when measured over many hours (3). This latter point makes it difficult to establish whether an inefficient reaction is actually caused by a catalytic rate enhancement or occurs spontaneously because of template-driven proximity.