VARIABLE REGIONS V13 AND V3 OF SACCHAROMYCES-CEREVISIAE CONTAIN STRUCTURAL FEATURES ESSENTIAL FOR NORMAL BIOGENESIS AND STABILITY OF 5.8S AND 25S RIBOSOMAL-RNA

The homologous ribosomal RNA species of all organisms can be folded in to a common ''core'' secondary structure. In addition, eukaryotic rRNA s contain a large number of segments, located at fixed positions, that are highly variable in size and sequence from one organism to another . We have investigated the role of the two largest of these variable r egions in Saccharomyces cerevisiae 25S rRNA, V13, and V3, by mutationa l analysis in a yeast strain that can be rendered completely dependent on the synthesis of mutant (pre-)rRNA. We found that approximately ha lf of variable region V13 can be deleted without any phenotypic effect . The remaining portion, however, contains multiple structural feature s whose disturbance causes serious growth defects or lethality. Accumu lation of 25S rRNA is strongly reduced by these mutations, at least in part because they inhibit processing of ITS2. Removal of even a relat ively small portion of V3 also strongly reduces the cellular growth ra te and larger deletions are lethal. Interestingly, some of the deletio ns in V3 cause accumulation of 27S(A) pre-rRNA and, moreover, appear t o interfere with the close coupling between the processing cleavages a t sites A3 and B1(S). These results demonstrate that both variable reg ions play an important role in 60S subunit formation.