SUMMARY A mutant induced by nitrosoguanidine in the Challis (group H) strain of Streptococcus contains a number of separable mutations linked to each other and to the locus governing sensitivity to streptomycin. One suppresses the level of resistance conferred by mutations in the streptomycin-sensitivity locus, two confer sensitivity to mitomycin and one confers sensitivity to ultraviolet radiation. None of these mutations affects transformability with either homologous or heterologous DNA. However, with the suppressor in the recipient strain there is a loss of streptomycin-resistant transformants, dependent upon the level of resistance conferred by the donor streptomycin-resistance marker and the degree of linkage between the suppressor and that marker.
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