Primordial spliceosomal introns were probably U2-type

Abstract

The two types of eukaryotic spliceosomal introns, U2 and U12, possess different splice signals and are excised by distinct spliceosomes. The nature of the primordial introns remains uncertain. A comparison of the amino acid distributions at insertion sites of introns that retained their positions throughout eukaryotic evolution with the distributions for human and Arabidopsis thaliana U2 and U12 introns reveals close similarity with U2 but not U12. Thus, the primordial spliceosomal introns were, most likely, U2-type.

Figure 1

Protosplice sites of U2 and U12 introns. Negative numbers indicate the nucleotide positions in the exon immediately preceding the splice junction and the positive numbers indicate the nucleotide positions in the exon immediately after the splice junction. (a) Position-specific information content (sequence logo) for the complete set of human U2 splices sites. (b) Position-specific information content (sequence logo) for the complete set of Arabidopsis U2 splices sites. (c) Position-specific information content (sequence logo) for the complete set of human U12 splices sites. (d) Position-specific information content (sequence logo) for the complete set of Arabidopsis U12 splices sites.

Figure 2

Comparison of amino acid frequencies at protosplice site of primordial, U2 and U12 introns. The distributions are shown separately for each intron phase. ‘Phase aa0’ and ‘Phase 0aa’ denote phase 0 introns located immediately downstream and immediately upstream of a conserved amino acid, respectively.