Different ‘Spectra’ of Mutant Types by Extracellular Treatment of Phage Kappa with Differing Mutagens

Abstract

FOR about twenty years it has been known that the yield of induced mutants has a different type composition in differing mutagenic conditions¹. This electiveness of mutagenic action can be either intragenic, intergenie or cell-type specific². Intragenic electiveness has recently been examined in phage T4 of E. coli especially by Benzer³. This work led to the discovery of sites of high mutability within a gene; the local distribution and the extent of these ‘hot spots’ varies with the mutagenic agent. Since free phage represents extracellular genetic material (DNA) a ‘hot spot’ must be due to the specific nucleotide composition at its site in the DNA which is attacked preferentially by the mutagen. In the case of treatment of cells, mutagenic electiveness could also be an effect of the intracellular environment of the genes but not, or not only, of the DNA-composition differing at different sites within a gene, in different genes or groups of genes. It has been proposed that with forward mutations (for example, auxotrophy) electiveness should not show up as likely as with back-mutations, since the former would be produced by changes at many sites along a gene, while the latter requires a rather specific change at only one or a few points⁴. No, or only very weak, electiveness should then be expected, especially in forward mutations, if groups of several genes are observed, for example, mutations leading to certain groups of phenotypes.