Direction of deoxyribonucleic acid transfer and replication in a derivative of plasmid R100-1

A system for in vivo selection of genomic rearrangements with predetermined endpoints in Escherichia coli using modified Tn10 transposons

Using recombinant DNA techniques, the Tn10-specific tetA gene (coding for tetracycline resistance) has been mutagenized by insertion of a streptomycin-resistance or a kanamycin-resistance gene. The insertions occurred at loci separated by 920 bp. The mutated tetA fragments, respectively designated as Tes (for tetracycline-streptomycin) and Tek (for tetracycline-kanamycin), were subsequently cloned into a phage lambda cIII+cIts857cII+ in replacement of the att lambda region. The two recombinant phages are convenient delivery vehicles which permit the in vivo substitution of the tetA locus of any Tn10 insertion with the Tes or the Tek fragment. The procedure involves two selectable steps: (i) integration of a lambda-Tes (or lambda-Tek) prophage into the Tn10 of interest; (ii) excision of the prophage by a second exchange which leaves the extra resistance gene installed within the Tn10. A major interest of the system is that, once a bacterium carries both Tn10-Tes and Tn10-Tek insertions, a recombination event between the two Tn10 sequences can reconstitute an active tetA gene. This selectable event may be associated with the rearrangement of the sequences surrounding the transposons. This unique property of the "Tes and Tek" system makes it very useful for selection of genomic rearrangements using the Tn10-Tes and Tn10-Tek as predetermined endpoints. The successful isolation of a chromosomal inversion is reported.

Suppression of Escherichia coli dnaA46 mutations by integration of plasmid R100.1. derivatives: constraints imposed by the replication terminus

We have studies the phenotypic suppression of a dnaA46 mutation by plasmid integration at preselected chromosomal sites after introducing homologous sequences (Mu prophages) onto both the chromosomes and the suppressive plasmid. The plasmids used were all derived from plasmid R100.1. We found that the conditions required to get viable suppressive integration varied as the plasmid integration site moved from the origin to the terminus of chromosome replication. Two constraints were observed. Both appeared to be linked to the new characteristics acquired by chromosome replication from the integrated plasmid. One constraint was that strains with integrative suppression near the terminus terC were viable only in minimal medium. The rich medium sensitivity of these strains was correlated with a loss of regulation of initiation. The other constraint was a requirement for a specific orientation in certain regions of the chromosome. The two branches defined by normally initiated replication, between oriC and terC, were also symmetrical with respect to these plasmid orientation constraints. In studying the possible reasons for a plasmid orientation constraint, we found that, of the two forks initiated in bidirectional replication from the integrated plasmid, one was capable of moving across the terC region with a higher movability than the other.