Alteration of ribosomal proteins in revertants of a valyl-tRNA synthetase mutant of Escherichia coli.

The work described in this paper was done to see whether the partial suppression of temperature-sensitive aminoacyl-tRNA synthetase mutations by ribosomal mutations is restricted to the aminoacyl-tRNA synthetase mutation which was used for the selection of the suppressor strains or whether the ribosomal mutations can also suppress mutations of other aminoacyl-tRNA synthetases. It is shown that a mutation in ribosomal protein S5 which was selected for suppression of an alanyl-tRNA synthetase mutation (alaS-3) can also partially compensate the temperature-sensitivity of two valyl-tRNA synthetase mutants and of another alanyl-tRNA synthetase mutant. Furthermore, revertants of a temperature-sensitive valyl-tRNA synthetase mutant were isolated and screened for alterations in ribosomal proteins by electrophoretic and immunochemical methods. Alterations in at least two proteins, S8 and S20, were clearly observed among the mutants. The alteration in protein S8 renders the growth of this strain severely cold-sensitive. Presence of the mutation in protein S8 is strictly correlated with suppression of temperature-sensitivity. The S8 mutation maps between strA and spc on the Escherichia coli chromosome. Five suppressor strains have quantitatively or qualitatively altered ribosomal proteins S20. In one strain no S20 protein could be detected at all, employing different electrophoretic and immunological methods. All five suppressor mutations map in the thr-leu region of the E. coli chromosome, i.e. in an area where the alteration of protein S20 in two alaS suppressor strains has been localized previously.