Gene in the major cotransduction gap of the Escherichia coli K-12 linkage map required for the expression of chromosomal resistance to tetracycline and other antibiotics

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Abstract
In E. coli K-12, amplifiable resistance to tetracycline, chloramphenicol and other unrelated antibiotics was mediated by at least 4 spatially separated loci. Tetracycline-sensitive mutants were isolated by Tn5 insertional inactivation of an amplified multiply resistant strain. One of these, studied in detail, showed coordinate loss of expression of all other resistance phenotypes. The Tn5 element in this mutant mapped to 34 min on the E. coli K-12 linkage map. The locus was designated marA (multiple antibiotic resistance). Tetracycline-sensitive mutants containing marA::Tn5 regained all resistance phenotypes at frequencies of 10-8 to 10-7 upon precise excision of Tn5. A newly described tetracycline efflux system was inactivated in tetracycline-sensitive mutants, but recovered in tetracycline-resistant revertants. In merodiploids, F-prime marA+ expressed partial or complete dominance over corresponding mutant chromosomal alleles. Dominance tests also established that a previously amplified host and a mutant marA allele were preconditions for the expression of phenotypic resistances.