Site-specific recombination following conjugation in Escherichia coli K-12

In accord with the observations of other workers, unselected marker analysis of Escherichia coli K-12 transconjugants isolated from matings involving several different Hfr strains as donors has shown that most genetic exchanges are clustered either near the selected marker or near the origin of the transferred Hfr DNA. The present work increases the number of Hfr strains tested and shows that the clustering of the recombinational events near the origin of transfer is statistically significant. It is proposed that this clustering of genetic exchanges is due to the action of a unique recombination system (site-specific conjugal recombination or ssr) which recognizes the early transferred portion of the F plasmid and catalyzes a genetic exchange in or near the adjacent bacterial DNA. Twelve Hfr strains representing eleven different points of origin were tested, and only KL16 and Ra-1 did not demonstrate the typical clustering of genetic exchanges. Since these strains share a common transfer origin, they may represent spontaneous mutations affecting the ssr system.