Figure 4.

Intron complementarities and splicing. (A) Intron complementarities (inverted arrows) coordinate splice site pairing in yeast YL8A. Open arrows represent one complementarity and filled arrows represent a distinct complementarity. Pairing between complementarities defines introns and enforces exon inclusion. (B) New complementarity within a splicing substrate induces a new pattern of splicing. In YL8A a new engineered complementarity (arrows) causes exon skipping as well as allowing residual exon inclusion (Fig. 3). In higher organisms, the introduction of new complementarity by the transposition of a mobile repeat element (arrows) is proposed to induce an alternative splicing pathway and increase the coding potential of the genome (see text).