Genetic studies of class 2 nonsense suppressors inEscherichia coli

Abstract

Three genetically distinct ochre suppressors have been identified in a strain of Escherichia coli B/r, all of which suppress a tyrosine auxotrophy and classify as class 2 by phage suppression pattern. One ochre suppressor, which was obtained by conversion from a class 2 amber suppressor, and a second ochre suppressor obtained directly from the non-suppressing parent, were found to have separate map locations, though a peculiar phenotype with regard to a leucine auxotrophy is exhibited by strains carrying either suppressor. We suggest that both suppressors correspond to separate genes for glutamine-inserting tRNA. A Leu+ mutant of a strain carrying one of these suppressors was studied and was found to contain a further nonsense suppressor having amber-suppressing activity at a reduced level. We suggest that this suppressor might result from a mutation in another part of the translational machinery concerned with glutamine insertion. The third ochre suppressor has no effect on the leucine auxotrophy and mapping data suggest that it may be supL, an ochre suppressor probably inserting a different amino acid, from glutamine.