EVIDENCE FOR INDEPENDENT ACQUISITION OF GROUP-I INTRONS IN GREEN-ALGAE

We report the occurrence of a group I intron, 452 nucleotides in lengt h, in the nuclear small-subunit ribosomal gene of the benthic seaweed Urospora penicilliformis, a member of the green algal class Ulvophycea e. Group I introns have been reported in fungi, myxomycetes, the cilia te genus Tetrahymena, and recently in five unicellular chlorophycean a lgae. The sequence of the conserved core of the U. penicilliformis gro up I intron was aligned with that of 15 other introns of this type, an d the evolutionary relationships among these introns were examined. Co mparison of the ''intron'' tree with phylogenetic hypotheses of the in tron-harboring organisms shows that nuclear group I introns in green a lgae have arisen several times and are not lineage specific.