Ribosomal protein modification in Escherichia coli. I. A mutant lacking the N-terminal acetylation of protein S5 exhibits thermosensitivity.

A thermosensitive mutant (JE386) of Escherichia coli which harbours an alteration in protein S5 of the smaller ribosomal subunit has been isolated. Genetic studies have shown that the lesion causing thermosensitivity also causes the alteration in protein S5, and that this mutation is not in the structural gene for S5 (rpsE). Hence the mutation has been termed rimJ (ribosomal modification). Protein-chemical studies of protein S5 purified from JE386 and its wild-type parent indicated an alteration in the N-terminal tryptic peptide. Amino acid sequence analysis of the N-terminal peptides showed complete homology between wild-type and mutant, suggesting that the N-terminal modification (acetylation) of the parent was absent in the mutant. Gradient transmission mapping has located the rimJ mutation at 31 minutes on the current E. coli genetic map. By constructing a derivative of the mutant heterozygous for rimJ, it has been found that the wild-type allele is dominant over the mutant one. Ts⁺ revertants of JE386 have been isolated which show either a wild-type ribosomal protein electrophoresis pattern, or an additional alteration in either protein S4 or S5. The mutations in S4 and S5 may compensate the lesion caused by the rimJ mutation of JE386, that is even though the N-terminus of S5 remains unacetylated, bacteria can grow at 42 °C. Furthermore, a mutation near or at strA carried by JE386 has been found to be involved in the phenotypic expression of the rimJ mutation. This mutation was also found to be present in four other strA mutants. Possible implications of the modification of ribosomal proteins in vivo are discussed.