THE CONSERVED GTPASE CENTER AND VARIABLE REGION V9 FROM SACCHAROMYCES-CEREVISIAE 26S RIBOSOMAL-RNA CAN BE REPLACED BY THEIR EQUIVALENTS FROM OTHER PROKARYOTES OR EUKARYOTES WITHOUT DETECTABLE LOSS OF RIBOSOMAL FUNCTION

Using the "tagged" rRNA gene system, which allows in vivo mutational analysis of Saccharomyces cerevisiae rRNA, we studied the role of two distinct structural elements of 26S rRNA in ribosome biogenesis and function-namely, the evolutionarily highly conserved "GTPase center" located in domain II and the eukaryote-specific variable region V9 in domain III. Replacement of the S. cerevisiae GTPase center with its counterpart from Escherichia coli did not affect the assembly of the mutant 26S rRNA into functional (as judged by their polysomal distribution) 60S subunits, indicating that the E. coli GTPase center functions efficiently in the context of the heterologous rRNA. Removal of most of the S. cerevisiae V9 region or replacement of this segment by the equivalent segment from mouse 28S rRNA also did not affect the formation of functional 60S subunits carrying the mutant 26S rRNA. Therefore, the V9 region does not seem to play a role in the biological functioning of the yeast 60S subunits, and these subunits appear to be able to accommodate V9 regions of various size and secondary structure without apparent loss of function.