DRS1 TO DRS7, NOVEL GENES REQUIRED FOR RIBOSOME ASSEMBLY AND FUNCTIONIN SACCHAROMYCES-CEREVISIAE

To identify Saccharomyces cerevisiae mutants defective in assembly or function of ribosomes, a collection Of cold-sensitive strains generate d by treatment with ethyl methanesulfonate was screened by sucrose gra dient analysis for altered ratios of free 40S to 60S ribosomal subunit s or qualitative changes in polyribosome profiles. Mutations defining seven complementation groups deficient in ribosomal subunits, drs1 to drs7, were identified. We have previously shown that DRS1 encodes a pu tative ATP-dependent RNA helicase necessary for assembly of 60S riboso mal subunits (T. L. Ripmaster, G. P. Vaughn, and J. L. Woolford, Jr., Proc. Natl. Acad. Sci. USA 89:11131-11135, 1992). Strains bearing the drs2 mutation process the 20S precursor of the mature 18S rRNA slowly and are deficient in 40S ribosomal subunits. Cloning and sequencing of the DRS2 gene revealed that it encodes a protein similar to membrane- spanning Ca2+ ATPases. The predicted amino acid sequence encoded by DR S2 contains seven transmembrane domains, a phosphate-binding loop foun d in ATP- or GTP-binding proteins, and a seven-amino-acid sequence det ected in all classes of P-type ATPases. The cold-sensitive phenotype o f drs2 is suppressed by extra copies of the TEF3 gene, which encodes a yeast homolog of eukaryotic translation elongation factor EF-1gamma. Identification of gene products affecting ribosome assembly and functi on among the DNAs complementing the drs mutations validates the feasib ility of this approach.