SUMMARY: In a culture of K12 ), cells which form large colonies on agar plates containing galactose and thiomethyl β-D-galactoside (TMG) appear at high frequency. These clones are resistant to growth inhibition by TMG on galactose minimal medium. Biochemical studies of the steady-state levels of galactokinase and UDPgalactose 4-epimerase suggest that the resistant clones have extra copies of the genes for the galactose-metabolizing enzymes. The mutation for TMG resistance is not located in either the bacterial or the bacteriophage genome, but is probably due to an aberrant association between cell and prophage DNA.

Mapping the TMG-resistant characteristic by phage P1 indicates that TMG-resistant bacteria possess at least two operons, one of which is cotransducible with . In addition, TMG-resistant bacteria behave like Δ polylysogens when challenged with the phage Δ From these genetic experiments we conclude that TMG-resistant bacteria arise by duplication of the Δ prophage. Finally, bacteria which carry a single, additional, Δ prophage are TMG-resistant. TMG resistance is probably a gene dosage effect.


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