Resistance ofEscherichia colito penicillins: IV. Genetic study of mutants resistant to D, L-ampicillin concentrations of 100 μg/ml.

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Abstract
The first two steps towards increasing ampicillin resistance inEscherichia coliconcern the genesampAandampBwhich are located at least 20 min from each other (Eriksson-Grennberget al.1965 and the two preceding papers). This paper describes a third class ofampA-containing mutants (designatedampAB) which are resistant to D, L-ampicillin concentrations of 100 μg/ml. When such as F-strain (D31) was crossed to different Hfr strains analysis of thetrp+andproB+recombinants indicated that resistance genes were located betweentrpandproB. Altogether five classes of recombinants were produced but only the two genesampAandampBwere recovered. One interpretation suggested is that the resistance of D31 is due to the presence ofampAandampBand a chromosomal aberration by whichampAwas moved to a position nearampB. It was possible to transduce both intermediate levels of resistance as well as the full resistance of anampABdonor strain, but the strains produced were unstable. In crosses the presence ofampABin the recipient reduced the number of recombinants by reducing the number of stable pairs. In an Hfr strainampABwas shown to give rise to additional difficulties in establishing the cell contact during mating. Some ampicillin resistant mutants also showed decreased ability to propagate the RNA phage MS2.