Molecular evolution of the Escherichia coli chromosome. V. Recombination patterns among strains of diverse origin

Incorporation patterns of donor DNA into recipient chromosomes following tr ansduction or conjugation have been studied in the progeny of a variety of Escherichia coli crosses in which donor and recipient nucleotide sequences differ by 1-3%. Series of contiguous or variously spaced PCR fragments have been amplified from each recombinant chromosome and digested with a commer cial restriction endonuclease previously shown to distinguish the respectiv e parents in a given fragment. We conclude that entering donor DNA fragment s are frequently abridged (cut and shortened) before incorporation, the cut ting being due to restriction systems, and the shortening presumably due to exonuclease activity. Analysis of several backcrosses confirms, and extend s to conjugation, the importance of restriction in E, coli recombination in nature. The transmission patterns in conjugation are similar to those of t ransduction, but (as expected) on a much larger scale. Asymmetric results o f reciprocal crosses imply that mismatch frequency is not a major factor. M arked differences among the results of simple crosses according to parental strain combinations are consistent with observations that E. coli strains in nature vary dramatically in their restriction-modification systems.