Heat shock activates splicing at latent alternative 5′ splice sites in nematodes

Abstract

Pre-mRNA splicing is essential for the regulation of gene expression in eukaryotes and is fundamental in development and cancer, and involves the selection of a consensus sequence that defines the 5ʹ splice site (50SS). Human introns harbor multiple sequences that conform to the 5ʹSS consensus, which are not used under normal growth conditions. Under heat shock conditions, splicing at such intronic latent 5ʹSSs occurred in thousands of human transcripts, resulting in pre-maturely terminated aberrant proteins. Here we performed a survey of the C. elegans genome, showing that worm’s introns contain latent 5ʹSSs, whose use for splicing would have resulted in pre-maturely terminated mRNAs. Splicing at these latent 50SSs could not be detected under normal growth conditions, while heat shock activated latent splicing in a number of tested C. elegans transcripts. Two scenarios could account for the lack of latent splicing under normal growth conditions (i) Splicing at latent 5ʹSSs do occur, but the nonsense mRNAs thus formed are rapidly and efficiently degraded (e.g. by NMD); and (ii) Splicing events at intronic latent 5ʹSSs are suppressed. Here we support the second scenario, because, nematode smg mutants that are devoid of NMD-essential factors, did not show latent splicing under normal growth conditions. Hence, these experiments together with our previous experiments in mammalian cells, indicate the existence of a nuclear quality control mechanism, termed Suppression Of Splicing (SOS), which discriminates between latent and authentic 5ʹSSs in an open reading frame dependent manner, and allows splicing only at the latter. Our results show that SOS is an evolutionary conserved mechanism, probably shared by most eukaryotes.