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

doi:2003.18.3.183

Algae > Volume 18(3); 2003 > Article

Algae 2003;18(3): 183-190. doi: https://doi.org/10.4490/ALGAE.2003.18.3.183

Jungho Lee, James R. Manhart

BK21, School of Biological sciences, Seoul National University, Biology Department, Texas A&M University, College Station

ABSTRACT

Except for a group Ⅰ introns in trnL-uaa occuring in eubacteria and plastids, group Ⅰintrons 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 LAGLI-DADG motifs. CS-cpSSU.i1orf and CS-cpSSU.i3orf occur at Loop 6 in the intron secondary structure and CS-cpSSU.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 insection 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.

Key words: 16S, Caulerpa, cpSSU, group IA3 intron, LAGLI-DADG motif containing ORF, Ulvophyceae