A GROUP-I SELF-SPLICING INTRON IN THE NUCLEAR SMALL-SUBUNIT RIBOSOMAL-RNA-ENCODING GENE OF THE GREEN-ALGA, CHLORELLA-ELLIPSOIDEA C-87

We report the presence of a 442-bp group-I self-splicing intron in the nuclear small subunit (SSU) rRNA-encoding gene (rDNA) of the unicellular green alga, Chlorella ellipsoidea C-87 (C. saccharophila 211-1a). The intron was found to be inserted at a position within the highly conserved helix 48 that was close to the 3' terminus of the SSU rRNA. The position was exactly the same as previously identified for the Pneumyocystis carinii intron. A secondary structure model for the C. ellipsoidea intron contained all P1-P10 motifs of the group-I introns. Although the overall secondary structure of the C. ellipsoidea intron was substantially different from that of the intron in the nuclear large subunit rDNA of Tetrahymena thermophila, the nucleotide (nt) sequences constituting the catalytic core were strikingly conserved between the two; only three of 48 nt were different. The C. ellipsoidea intron was autocatalytically excised from the transcript in vitro via the group-I mechanism under somewhat unique conditions. No SSU rDNA intron was found in six other Chlorella species, including C. fusca var. vacuolata, C. kessleri, C. minutissima, C. protothecoides, C. sorokiniana and C. vulgaris.