Euglena gracilis chloroplast small subunit rRNA. Sequence and base pairing potential of the 3' terminus, cleavage by colicin E3.
RNA sequencing methods have been used to determine sequences at the 3' ends of the small subunit RNAs from Euglena gracilis cytoplasmic and chloroplast ribosomes. The cytoplasmic rRNA sequence, AUCAU psi GOH, has features typical of eukaryotic 18 S rRNAs. The sequence at the 3' terminus of the chloroplast 17 S rRNA, AACAACUCCCOH, differs in several positions from nucleotides conserved in both prokaryotic and eukaryotic small subunit rRNAs, but it does terminate with a pyrimidine tract. Like prokaryotic ribosomes, the chloroplast ribosome displays highly specific binding to purine-rich oligonucleotides. The 30 S subunits select from a random mixture of oligonucleotides only those that contain the sequence GGGAG, which is complementary to the 3' end of 17 S rRNA. The 68S chloroplast ribosome has been shown to be a substrate in vitro for colicin E3. Colicin E3 introduces a cleavage at a position in the chloroplast 17 S rRNA that corresponds exactly to that hydrolyzed in Escherichia coli S rRNA. These results suggest that the 3'-terminal region of 17 S rRNA has an arrangement in the 68 S chloroplast ribosome that may be equivalent to that of the prokaryotic ribosome, with the 3' end exposed in the 30 S subunit for interactions with complementary RNA sequences.