Molecular biology: Splicing does the two-step

Abstract

An intricate recursive RNA splicing mechanism that removes especially long introns (non-coding sequences) from genes has been found to be evolutionarily conserved and more prevalent than previously thought.

Figure 1. Mechanisms of recursive splicing

a, In recursive splicing, long intron sequences of precursor RNA are removed in a stepwise process mediated by juxtaposed internal 3′ and 5′ splice sites. In the first step, the 3′ splice site is used to remove the upstream intronic sequences. The second step uses the 5′ splice site to remove the downstream intron sequences, forming a mature messenger RNA. Duff et al. report that this recursive splicing process occurs in the fruit fly Drosophila melanogaster much more commonly than was previously thought. b, Sibley et al. find that some recursively spliced messenger RNAs — including all those known in humans — contain a recursive splicing (RS) exon. The RS exon can be either completely removed or retained in the mature mRNA, depending on which of two competing 5′ splice sites is used in the second step. Most mRNAs that harbour RS exons are degraded by nonsense-mediated RNA decay (NMD).