Characterization of the nuclear large-subunit rRNA-encoding gene and the group-I self-splicing intron from Chlorella ellipsoidea C-87

We found two group-I self-splicing introns in both the large subunit (LSU) and small subunit (SSU) of the nuclear rRNA-encoding genes (rDNA) of the unicellular green alga, Chlorella ellipsoidea C-87 (Ce). The primary and secondary structures of the LSU rRNA (3350 nt) and its intron (445 nt) were characterized. The intron was inserted in the conserved stem 32 of the LSU rRNA and contained all P1-P10 motifs of the group-IB intron. In vitro transcripts of the LSU rDNA containing the intron sequence displayed a strong self-splicing activity at high salt concentrations. The overall structure and splicing conditions of the LSU rRNA intron were, however, considerably different from those of the SSU rRNA intron of the same organism. These results suggest different origins and/or different evolutionary courses of these Ce introns.