A number of translocatable drug-resistance elements have recently been described which are able to insert themselves into a large number of different sites in prokaryotic genomes. These elements cause recognizable mutations when insertion occurs within a structural gene or an operon. Drug-resistance elements are also associated with other kinds of illegitimate recombination events, notably deletions and inversions. This paper summarizes uses to which these properties of translocatable drugr-esistance elements can be put in genetic manipulations of bacteria. Translocatable drug-resistance elements are useful in isolation of mutants (even where the mutant phenotype is not easily scored), in the construction of strains and other genetic manipulations (even when selection is difficult or impossible), in localized mutagenesis, in chromosomal mapping, in construction of Hfr strains with known origin and direction of chromosome transfer, in complementation tests, in construction of new F′ plasmids, in construction of new specialized transducing phages, in isolation of deletions with one or both endpoints specified, in construction of gene and operon fusions, and in the selection and maintenance of chromosomal duplications. Experiments are described which illustrate many of these techniques.
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