Summary: Stable small-colony variants of K-12 and several of its auxotrophs were isolated by treatment of cultures with copper sulphate solutions. These variants did not show any associated changes in nutritional, fermentative or serological characters. The small-colony forms showed normal recombining character in two instances, but in one variant the F - form showed a lower recombination rate and atypical transfer of markers. However, when rendered F + this variant behaved in a manner analogous to the normal F + parent strain. The results have been interpreted as evidence that the small-colony character can arise by a nuclear mutation at one of several possible genetic loci.


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