Boyer, Herbert (Yale University, New Haven, Conn.). Conjugation in Escherichia coli. J. Bacteriol. 91:1767-1772. 1966.-The sex factor of Escherichia coli K-12 was introduced into an E. coli B/r strain by circumventing the host-controlled modification and restriction incompatibilities known to exist between these closely related strains. The sexual properties of the constructed F(+) B strain and its Hfr derivatives were examined. These studies showed that the E. coli strain B/r F(+) and Hfr derivatives are similar to the E. coli strain K-12 F(+) and Hfr derivatives. However, the site of sex factor integration was found to be dependent on the host genome.