QSR1, an essential yeast gene with a genetic relationship to a subunit of the mitochondrial cytochrome bc(1) complex, codes for a 60 S ribosomal subunit protein

QSR1 (quinol-cytochrome c reductase subunit-requiring) is a highly conserved, essential gene in Saccharomyces cerevisiae that was identified through a synthetic lethal screen by its genetic relationship to QCR6, the gene for subunit 6 (Qcr6p) of the mitochondrial cytochrome be, complex. The function of the QSR1-encoded protein (Qsr1p) and its relationship to the QCR6 encoded protein are unknown. When yeast cell lysates are fractionated by density gradient centrifugation, Qsr1p separates from organelles and sediments with a uniformly sized population of particles that are similar to eukaryotic ribosomes upon velocity gradient centrifugation. When 40 S and 60 S ribosomal subunits are separated on velocity gradients, Qsr1p is found exclusively with the 60 S subunits, where it is a stoichiometric component, Extracts pre pared from qsr1-1 cells are defective in in vitro translation assays relative to the wild type. In yeast cell. lysates in which QCR6 rescues an otherwise lethal qsr1-1 mutation, Qcr6p is found only in mitochondria, both in respiratory-competent cells and in rho(0) cells in which the be, complex is no longer present, These results suggest that suppression of the qsr1-1 mutation by QCR6 occurs by a trans-relationship across the outer mitochondrial membrane.