AN RIBOSOMAL-RNA VARIABLE REGION HAS AN EVOLUTIONARILY CONSERVED ESSENTIAL ROLE DESPITE SEQUENCE DIVERGENCE

Regions extremely variable in size and sequence occur at conserved loc ations in eukaryotic rRNAs. The functional importance of one such regi on,vas determined by gene reconstruction and replacement in Tetrahymen a thermophila. Deletion of the D8 region of the large-subunit rRNA ina ctivates T. thermophila rRNA genes (rDNA): transformants containing on ly this type of rDNA are unable to grow. Replacement with an unrelated sequence of similar size or a variable region from a different positi on in the rRNA also inactivated the rDNA. Mutant rRNAs resulting from such constructs were present only in precursor forms, suggesting that these rRNAs are deficient in either processing or stabilization of the mature form. Replacement with D8 regions from three other organisms r estored function, even though the sequences are very different. Thus, these D8 regions share an essential functional feature that is not ref lected in their primary sequences. Similar tertiary structures may be the quality these sequences share that allows them to function interch angeably.