In the primary transcript of nuclear genes, coding sequences – exons – usually alternate with noncoding
sequences – introns. The latter are removed and former are joined by means of splicing to create
the mRNA ORF that translates into the functional peptide product. In the study of the control of fungal
gene expression, the ubiquitous splicing process is largely ignored. In the evolution of genic intron-exon
structure, introns are gained at new positions or lost from extant positions or they move to a new position.
The latter “intron drift” or “intron slide” leads to discordant introns in ortholog genes.
Previously we described stwintrons (spliceosomal twin introns) in filamentous ascomycota (*). These are
complex intervening sequences in which a canonical “internal” intron interrupts one of the three
conserved domains of a canonical “external” intron and consequently, they can only be removed with two
subsequent splicing reactions. A stwintron in which the donor of the external intron is interrupted between
the first and second bp – [D1,2] – can also be removed alternatively if the acceptor of the other external
intron is interrupted between the penultimate and ultimate bp – [A2,3] – where the resultant mRNA is
identical for both splicing routes.
Here we present a new stwintron, the first uncovered in Aspergillus nidulans. Ortholog genes occur in 4
Pezizomycotina classes. Most encompass either a [D1,2] or an alternatively spliced [D1,2] /[A2,3]
stwintron at the same position. However, 9 species harbor there a discordant, canonical intron that
conforms a one-bp intron drift: 7 have a phase-2 intron and appear to miss the [D1,2] internal intron while
two carry a phase-0 intron and they appear to lack the [A2,3] internal intron instead. This is one of the few
instances of intron drift where its mechanism could be elucidated.
(*) Ág et al. (2015) Fungal Genet Biol 85:7 & Flipphi et al. (2013) Fungal Genet Biol 57:48.
This research was supported by the EU and co-financed by the European Regional Development
Fund under the project GINOP-2.3.2-15-2016-00008.
Full conference title:
- Asperfest 14 (2017)