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| Modern Genetic Analysis Chapter 13. Transposable Genetic Elements Transposons Figure 13-10. The role of transposable elements in the evolution of antibiotic-resistance plasmids is illustrated by a schematic map of a plasmid carrying many resistance genes. The plasmid appears to have been formed by the joining of a resistance-determinant segment and a resistance-transfer segment; there are insertion elements (IS1) at the junctions, where the two segments sometimes dissociate reversibly. Genes encoding resistance to the antibiotics chloramphenicol (cmR), kanamycin (kanR), streptomycin (smR), sulfonamide (suR), and ampicillin (ampR) and to mercury (HgR) are clustered on the resistance-determinant segment, which consists of multiple transposable elements; inverted repeat termini are designated by arrows pointing outward from the element. A transposon encoding resistance to tetracycline (tetR) is on the resistance-transfer segment. Transposon Tn3 is within Tn4. Each transposon can be transferred independently. (From S. N. Cohen and J. A. Shapiro, "Transposable Genetic Elements." Copyright © 1980 by Scientific American, Inc. All rights reserved.)

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