RNA splicing

In the 20 years since the discovery that many eukaryotic genes are interrupted by non-coding intervening sequences, or introns, biologists have learnt a tremendous amount about the nuclear machinery that removes these introns from precursor gene transcripts before they are translated into proteins. This nuclear machinery must recognize the introns in the mRNA transcripts and then excise them by means of mRNA splicing. Short conserved sequences at the ends of introns - splice sites - are crucial for intron recognition and for the accuracy of the splicing reactions. In the 1980s the development of biochemical systems for splicing mRNA precursors allowed the splicing machinery to be dissected and analysed. Splicing occurs in spliceosomes, large particles which are built up stepwise on the mRNA precursor from smaller RNA-protein sub-assemblies called snRNPs (small nuclear ribonucleoprotein particles). Each of the snRNPs that makes up the spliceosome contains a small nuclear RNA (snRNA) and a common set of snRNP proteins, plus up to ten additional snRNP-specific proteins. A spliceosome is therefore composed of dozens of proteins in addition to the five spliceosomal snRNAs (the U1, U2, U4, U5 and U6 snRNAs), so that spliceosomes rival ribosomes in size and complexity. Energy from the hydrolysis of ATP is required at several points during the formation and operation of the spliceosome. But although the spliceosome uses ATP as fuel, the actual chemical mechanism of mRNA splicing consists of two successive trans-esterification reactions, which do not themselves involve ATP hydrolysis. Interestingly, a similar reaction scheme is used by a widespread class of highly structured so-called 'autocatalytic introns' - group II introns - which can self-splice without ATP hydrolysis or any trans-acting factors. This begs the questions: what are all those spliceosomal RNA and protein components doing; and why do spliceosomes require energy input from ATP? There is still much detail to unravel but the answers to these questions are beginning to emerge.