Three ORF-Containing Group I Introns in Chloroplast SSU of Caulerpa sertularioides (Ulvophyceae) and Their Evolutionary Implications

다국어 초록 (Multilingual Abstract)

Except for a group I intron in trnL-uaa occuring in eubacteria and plastids, group I introns are rarely documented
in plastid genomes. Here, we report that a green alga, Caulerpa sertularioides, contains three group IA3 introns in the
16S gene (cpSSU), CS-cpSSU.i1, CS-cpSSU.i2 and CS-cpSSU.i3. Each intron has an open reading frame with LAGLIDADG
motifs. CS-cpSSU.i1orf and CS-cpSSU.i3orf occur at Loop 6 in the intron secondary structure and CScpSSU.
i2orf at Loop 8. CS-cpSSU.i1orf and CS-cpSSU.i2orf contain both LAGLI-DADG motifs but CS-cpSSU.i3orf
has only one. CS-cpSSU.i1 and CS-cpSSU.i2 share the insetion sites and the ORFs at Loop 6 and 8 with CpSSU·1 and
CpSSU·2 introns of Chlamydomonas pallidostigmatica (Chlorophyceae). In contrast, CS-cpSSU.i3, containing 28 copies
of GAAATAT at Loop 6, is a novel intron found only in Caulerpa sertularioides. Possible scenarios of the evolution of
the three introns and their possible use in systematic research are discussed.