A lariat branch point motif-interrupted spliceosomal twin intron in Aspergillus nidulans

In the primary transcript of nuclear genes, coding sequences – exons –alternate with non-coding sequences – introns. The latter are removed and former are joined to create the mRNA ORF that translates into the functional peptide product. Ubiquitous intron splicing provides a means of post-transcriptional regulation of expression by coupling alternative splicing with nonsense-mediated mRNA decay, hardily addressed in fungi. We use spliceosomal twin introns (“stwintrons”) as model systems to study spliceosomal introns and their excision. Stwintrons are unconventional intervening sequences where a standard “internal” intron interrupts one of the three canonical splicing motifs of an “external” intron, and that consequently, can only be removed by consecutive splicing reactions. Previously, we have characterised stwintrons where the internal intron interrupts either the donor- or the acceptor sequence of the external intron (**). We have demonstrated that stwintrons can emerge by the appearance of a new intron within a pre-extant intron, consistent with mechanisms of intron gain from an endogenous origin. Here we present a new type of stwintron in which the internal intron is nested in the conserved sequence element around the lariat branch point adenosine of the external intron. This particular lariat branch point motif-interrupted stwintron is a recently evolved feature in Aspergillus nidulans and we show that it emerged by an alternative mechanism which involves intronisation of exonic sequences on either side of a pre-extant standard intron.

This work was supported by the EU and co-financed by the European Regional Development Fund [GINOP-2.3.2-15-2016-00008], and the Hungarian Scientific Research Fund [OTKA NN116519 to LK]

(**) Flipphi et al. (2013) Fungal Genet. Biol. 57:48; Ág et al. (2015) Fungal Genet. Biol. 85:7; Fekete et al. (2017) Nucleic Acids Res. 45:9085; Flipphi et al. (2017) Fungal Biol. Biotechnol. 4:7