THE GTPASE CENTER PROTEIN L12 IS REQUIRED FOR CORRECT RIBOSOMAL STALKASSEMBLY BUT NOT FOR SACCHAROMYCES-CEREVISIAE VIABILITY

Protein L12, together with the P0/P1/P2 protein complex, forms the pro tein moiety of the GTPase domain in the eukaryotic ribosome. In Saccha romyces cerevisiae protein L12 is encoded by a duplicated gene, rpL12A and rpL12B. Inactivation of both copies has been performed and confir med by Southern and Western analyses. The resulting strains are viable but grow very slowly. Growth rate is recovered upon transformation wi th an intact copy of the L12 gene. Ribosomes hom the disrupted strain lack protein L12 but are able to carry out translation in vitro at abo ut one fourth of the control rate. The L12-deficient ribosomes have al so a defective stalk containing standard amounts of the 12-kDa acidic proteins P1 beta and P2 alpha, but proteins P1 alpha and P2 beta are d rastically reduced. Moreover, the affinity of PO is reduced in the def ective ribosomes. Footprinting of the 26 S rRNA GTPase domain indicate s that protein L12 protects in different extent residues G1235, G1242, A1262, A1270, and A1272 from chemical modification. The results in th is report indicate that protein L12 is not essential for cell viabilit y but has a relevant role in the structure and stability of the eukary otic ribosomal stalk.