Genetic and Segregation Analysis of Escherichia coli Strains Containing a Tandem Duplication of the trpD-purB Region of the Chromosome

Genetic and segregation analysis ofEscherichia coli strains containing a partial duplication of thetrp operon reveal that the 2.5-min-long regiontrpD-purB is duplicated in tandem in the chromosome. The adjacent locicysB andfabD are not duplicated. Although one copy of the duplicated region is longer than the maximum size of bacteriophage P1kc transducing fragments, the frequency at which the duplicated segmenttrpDCBA is transferred by transduction totonB-trp deletion strains is equal to that observed for transfer of the normaltrp operon. This suggests that three-point recombination events believed to account for transduction of long duplications occur as frequently as two-point recombination events believed to account for normal transduction. Cotransduction frequencies oftrpDCBA with the duplicated locitonB, galU, tyrT , andhemA are very similar to those for thetrp operon with the same loci. This indicates that normal genetic linkage is maintained during the three-point recombination event. However,purB , which is normally unlinked totrp by transduction, is closely linked totrpDCBA and thus must be near the repeat point of the duplication. Transduction tests with point mutations in thetrp operon indicated that the repeat point occurs near the normal boundary betweentrpE andtrpD . Segregation analysis of heterogenotes constructed fromtonB-trp deletion strains shows that the frequency at which a marker is lost is approximately proportional to its distance from the repeat point. This finding is consistent with a random, singlesite crossover event during segregation. Several observations indicate that non-reciprocal genetic exchange also occurs between copies of the duplication. Analysis of heterogenotes containingdadR1 anddadR + demonstrate that the mutant allele is transdominant.