Qsr1p, a 60S ribosomal subunit protein, is required for joining of 40S and 60S subunits

QSR1 is a recently discovered, essential Saccharomyces cerevisiae gene, which encodes a 60S ribosomal subunit protein. Thirty-one unique temperature-sensitive alleles of QSR1 were generated by regional codon randomization within a conserved 20-amino-acid sequence of the QSR1-encoded protein. The temperature-sensitive mutants arrest as viable, large, unbudded cells 24 to 48 h after a shift to 37 degrees C. Polysome and ribosomal subunit analysis by velocity gradient centrifugation of lysates from temperature-sensitive qsr1 mutants and from cells in which Qsr1p was depleted by down regulation of an inducible promoter revealed the presence of half-mer polysomes and a large pool of free 60S subunits that lack Qsr1p. In vitro subunit-joining assays and analysis of a mutant conditional for the synthesis of Qsr1p demonstrate that 60S subunits devoid of Qsr1p are unable to join with 40S subunits whereas 60S subunits that contain either wild-type or mutant forms of the protein are capable of subunit joining. The defective 60S subunits result from a reduced association of mutant Qsr1p with 60S subunits. These results indicate that Qsr1p is required for ribosomal subunit joining.