PLASMID RECOMBINATION BY THE RECBCD PATHWAY OF ESCHERICHIA-COLI

Previously, we demonstrated that exonuclease I-deficient strains of Es cherichia coli accumulate high-molecular-weight linear plasmid concate mers when transformed with plasmids carrying the chi sequence (5'-GCTG GTGG-3') (M. M. Zaman and T. C. Boles, J. Bacteriol. 176:5093-5100, 19 94). Since high-molecular-weight linear DNA is believed to be the natu ral substrate for RecBCD-mediated recombination during conjugation (A. J. Clark and K. B. Low, p. 155-215, in K. B. Low, ed., The Recombinat ion of Genetic Material, 1988), we analyzed the recombination frequenc ies of chi(+) and chi(0) plasmids in sbcB strains. Here, we report tha t chi sites stimulate plasmid recombination frequency by 16-fold in sb cB strains, Chi-stimulated plasmid recombination is dependent on RecBC D but is independent of RecF pathway genes. The distribution of recomb ination products suggests that high-molecular-weight linear plasmid DN A is a substrate for RecBCD-mediated recombination. Surprisingly, our data also suggest that chi(+) plasmids also recombine by the RecBCD pa thway in rec(+) sbcB(+) cells.