Mutant DnaK Chaperones Cause Ribosome Assembly Defects in Escherichia coli

To determine whether the biogenesis of ribosomes in Escherichia coli is the result of the self-assembly of their different constituents or involves the participation of additional factors, we have studied the influence of a chaper-one, the product of the gene dnaK, on ribosome assembly in vivo. Using three thermosensitive (ts) mutants carrying the mutations dnaK756-ts, dnaK25-ts, and dnaK103-ts, we have observed the accumulation at nonpermissive temperature (45⚬C) of ribosomal particles with different sedimentation constants-namely, 45S, 35S, and 25S along with the normal 30S and 50S ribosomal subunits. This is the result of a defect not in thermostability but in ribosome assembly at the non-permissive temperature. These abnormal ribosomal particles are rescued if the mutant cells are returned to 30⚬C. Thus, the product of the dnaK gene is implicated in ribosome biogenesis at high temperature.