Analysis of rpsD mutations in Escherichia coli. II. Physiology of some representative mutants

The effects of ribosomal ambiguity mutations (ram A-) on the assembly of ribosomal 30S subunits in Escherichia coli were studied in some representative mutant strains. It was found that the inability of these strains to produce active 30S subunits at nonpermissive temperatures is correlated with a halt in the accumulation of protein S4. It is demonstrated that 30S-precursor particles lacking this protein accumulate and break down at nonpermissive temperatures and that most of the 30S proteins as well as the 17S RNA constituting these particles are similarly unstable. These findings are discussed and related to the finding that merodiploid strains containing genes for both mutant and wild type protein S4 do not accumulate the mutant form of the protein. Experiments indicating that ribosomal precursor particles are associated with polysomes are presented. The implications of these findings are discussed and it is suggested that the assembly of ribosomes is tightly coupled to the synthesis of ribosomal proteins.