A Change from Nonsense to Sense in the Genetic Code

Abstract

For the genetic information in a cistron to be translated into a polypeptide chain each coding unit in the nucleotide sequence must correspond to one of the twenty or so amino acids. If not every possible sequence corresponds to an amino acid, mutations that substitute one base for another could, in certain cases, cause the continuity of the information to be interrupted, and such "nonsense" mutations would block completion of the polypeptide chain. By virtue of a mutant with special properties, it is possible to identify nonsense mutations within the A cistron of the rII region of phage T4. In this paper the criterion for nonsense is applied to certain "ambivalent" rII mutants [1], i.e., ones whose phenotypes can be reversed by suppressor mutations in the bacterial host, Escherichia coli. The results show that an ambivalent mutation that behaves like nonsense in one bacterial host may nevertheless make sense in a second (suppressor containing) host. This suggests that a suppressor mutation in the bacterium can result in addition to the cell's dictionary of a new sensible coding unit, constituting a change in the genetic code of the bacterial cell.