Analysis and possible role of hyperrecombination in the termination region of the Escherichia coli chromosome.

The frequency of excisive homologous recombination has been measured at various positions along the Escherichia coli chromosome. The reporter system makes use of a lambda cI857 prophage integrated by homologous recombination within Tn5 or Tn10 transposons already installed at known positions in the E. coli chromosome. The excision frequency per cell and per generation was determined by monitoring the evolution of the relative number of temperature-resistant (cured) bacteria is a function of the age of the cultures. Excisions, due to RecA-dependent homologous exchanges, appeared to occur more frequently in the preferential termination zone for chromosome replication. The highest frequency of excision observed is compatible with a recombination event at each replication cycle in this region. On the basis of these data, we propose a model involving homologous recombination in the final steps of bacterial chromosome replication and separation.