Characterization of genetic coding changes in bacteria produced by ionizing radiation and by the radioactive decay of incorporated ³h-labelled compounds

The mutagen specificity of ionizing radiation from ⁶⁰Co γrays and radioactive decay was examined, using tritium-labelled nucleic acid and protein precursors (³H-methyl-thymidine; ³H-6-uracil and ³H-histidine). Specificity was determined through an examination of bacterial revertants, using a bacterium containing a nonsense mutation in a gene for arginine biosynthesis. By using nonsense mutants of T4 as indicators, arginine revertants may be divided into classes. The presumed amino-acid insertions for classes were assigned earlier. The base changes leading to the production of revertants in particular classes were derived from amino-acid insertions, using the genetic code and knowledge that nonsense suppressors arise by single base alterations in the DNA information specifying certain tRNA anticodons. The mutagenic agents studied were not specific; rather a broad spectrum of base changes was observed. For the tritium-labelled compounds used, percentage distributions of base changes are similar to those for external ionizing radiation. For all mutagens studied approximately 75 per cent of the revertants originate as alterations in GC pairs, and of these GC→TA base changes predominate.