Matej Vesteg, Rostislav Vacula, Jurgen M. Steiner, Bianka Mateasikova, Wolfgang Loffelhardt, Brona Brejova, Juraj Krajcovic. A possible role for short introns in the acquisition of stroma-targeting peptides in the flagellate Euglena gracilis. DNA Research, 17(4):223-231. 2010.

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The chloroplasts of Euglena gracilis bounded by three membranes arose via
secondary endosymbiosis of a green alga in a heterotrophic euglenozoan
host. Many genes were transferred from symbiont to the host nucleus. A
subset of Euglena nuclear genes of predominately symbiont, but also host,
or other origin have obtained complex presequences required for
chloroplast targeting. This study has revealed the presence of short
introns (41-93 bp) either in the second half of presequence-encoding
regions or shortly downstream of them in nine nucleus-encoded E. gracilis
genes for chloroplast proteins (Eno29, GapA, PetA, PetF, PetJ, PsaF, PsbM,
PsbO, and PsbW). In addition, the E. gracilis Pbgd gene contains two
introns in the second half of presequence-encoding region and one at the
border of presequence-mature peptide-encoding region. Ten of 12 introns
present within presequence-encoding regions or shortly downstream of them
identified in this study have typical eukaryotic GT/AG borders, are
T-rich, 45-50 bp long, and pairwise sequence identities range from 27 to
61%. Thus single recombination events might have been mediated via these
cis-spliced introns. A double crossing over between these cis-spliced
introns and trans-spliced introns present in 5'-UTRs of Euglena nuclear
genes is also likely to have occurred. Thus introns and exon-shuffling
could have had an important role in the acquisition of chloroplast
targeting signals in E. gracilis. The results are consistent with a late
origin of photosynthetic euglenids.