Abstract
The intronic sequence intervening the small subunit ribosomal DNA (SSU rDNA) of Chlorella sp. T-24-5, an atypical photobiont of Paramecium bursaria was examined. The position of the insertion was found to be 10 nucleotides upstream from that of a major P. bursaria photobiont, Micractinium reisseri (S651), and was found to be a novel insertion site (S641; the numbering reflects the homologous position in the rRNA gene of Escherichia coli: S = SSU rRNA). A secondary structure diagram showed that the intron is classified as a group I intron (subgroup IC), characterized by an extended P5 helix. Phylogenetic analyses could not reveal its evolutionary relationships with other introns, but were suggestive of a monophyletic relationship with introns of some trebouxiophytes. These introns all share the insertion position S641, and their sequences are extremely conserved and are likely to have spread recently. The intron of Chlorella sp. T-24-5 had twelve-nucleotide sequence repeats lying at the head of the intron and after the insertion, which may play a role in intron invasion.
