In Rhizobium meliloti, Tn 5 conferred resistance not only to kanamycin but to streptomycin, as well, in Escherichia coli, however only to kanamycin. Using in vitro recombinant DNA techniques, it was shown that the streptomycin resistance determinant was located downstream from the kanamycin resistance gene in the unique central region of Tn 5. Expression of various cloned fragments of Tn 5 suggested that both kanamycin and streptomycin resistance genes were transcribed from the same promoter. E. coli mutants allowing the expression of streptomycin resistance from Tn 5 were isolated. The differential expression of the streptomycin resistance gene provides a simple selection/counterselection criterion, using only streptomycin in transfer expriments of Tn 5 between E. coli and R. meliloti.
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