Group I self-splicing introns in both large and small subunit rRNA genes of Chlorella

We report the presence of Group I self-splicing introns in both nuclear small subunit (SSU) and large subunit (LSU) rRNAs of the unicellular green alga Chlorella ellipsoidea. The SSU intron (442 nt) was located at a position within the highly conserved helix 48 that was close to the 3' terminus of SSU rRNA: the position was exactly the same as previously reported for the Pneumocystis carinii Group I intron. The LSU intron (445 nt) was found in a highly conserved helix region (between positions 924-925) of LSU rRNA. Both introns can be folded into the secondary structures characteristic of Group I introns. The Chlorella introns self-spliced in vitro, yielding the typical Group-I intron splicing intermediates and products. Unlike the ordinary introns, however, splicing of the SSU intron was inhibited by high concentrations of monovalent cations at the second stage of splicing reaction. The Chlorella introns offer an opportunity to study the origin and evolution, physiological roles, and relationship between structure and splicing activity of Group I self-splicing introns.