Dominance of streptomycin sensitivity over dependence in Escherichia coli K12 merodiploids.

WO kinds of mutations are known which affect bacterial sensitivity to Tstreptomycin: those conferring resistance to and others which confer dependence upon the antibiotic. Genetic analysis of such mutants from Escherichia coli has been confined to recombinational analysis by conjugation ( NEWCOMBE and NYHOLM 1950) and by transduction (HASHIMOTO 1960). By means of both types of tests, the existence of multiple alleles at a single str A locus has been inferred, but remains unconfirmed by complementation tests. Studies of the dominance-recessiveness relationships among str A alleles as well as complementation tests have been hampered due to difficulties found in constructing stable merodiploids carrying pairs of str A alleles. Stable merodiploids remain the best system amenable for this kind of study. Several workers have shown dominance of the wild-type, streptomycin-sensitive allele (str A + ) , over resistant ones (LEDERBERG 1951; APIRION and SCHLESSINGER 196810; SPARLING et al. 1968). Recent work of this type has been performed with the aid of stable F-merogenotes carrying the str A gene (APIRION and SCHLESSINGER 1968b; SPARLING et al. 1968). We have continued the genetic analysis of the str A mutations of E. coli. With the aid of KLF41, an F-merogenote constructed by Low (1968), and of mutant episomes derived from it, we have been able to construct merodiploids homozygous and heterozygous for str A alleles. Here we describe results which show that alleles conferring streptomycin sensitivity and dependence can coexist inside the same cell and that sensitivity is dominant over dependence.