Figure 1.

Possible mechanisms for the acquisition of presequences in the ancestor of phototrophic euglenids. A single recombination event mediated via cis-spliced introns (Cis i1 and Cis i2) can result in the addition of presequence (or its part) from donor gene to acceptor gene (A). The donor gene would also acquire trans-spliced intron (Trans i) necessary for the addition of capped SL-leader ensuring translation of acceptor gene mRNA. Note that intron Cis i1 is shown to be present exactly at the presequence-mature peptide border encoding region for illustration in (A), but it can be present also within second half of presequence-encoding region or shortly downstream of it. Intron Cis i2 is also shown to be present at 5′-end of the acceptor gene for illustration in (A), but it can be also present within 5′-end of the acceptor gene. Another possible mechanism for the acquisition of chloroplast-targeting signals (B) may involve double crossing over, i.e. the first recombination event occurring between trans-spliced introns Trans i1 and Trans i2, present at the 5′-ends of donor and acceptor gene, respectively, and the second recombination event occurring between cis-spliced intron (Cis i1) present at the presequence-mature peptide border-encoding region of the donor gene and cis-spliced intron (Cis i2) present somewhere within the 5′-end of the protein-coding region of the acceptor gene.