Genetic Properties of a T Factor and T‐r Recombinants

Genetic studies have shown that one of the transfer (T) factors, T95, carries the following genetic properties: autonomous replication (rep+), conjugal transmission (tra+), transmission of the host chromosome (mat+), inhibition of F‐mating (ifm+) or fi+), inhibition of superinfection with R factor (irs−), and suppression of phage proliferation (spp−). An Escherichia coli K12 strain carrying T95factor is insensitive to male phages (f1 and f2). These facts indicated that the T95 factor is a sex factor but different from the F factors. By the interaction between T95 factor and the non‐transferable resistance determinants; r31(tet) for tetracycline resistance and r3 (kan) for kanamycin resistance, recombinants, T‐tet, T‐kan, and T‐tet‐kan, factors were obtained. These are capable of conferring drug‐resistance and are conjugally transmissible as a single genetic unit. E. coli K12 strains carrying T95 or T‐tet factor were insensitive to male phages and T‐tet (probably T95) factor was transmitted sexually at about 10−5 transmission frequency after 1 hr of mixed culture. On the contrary, E. coli K12 strains carrying T‐kan or T‐tet‐kan factor were found to become sensitive to male phages and their transmission frequency was 102 to 103‐fold higher than that of T‐tet factor. As a result of these findings, we concluded that the pili formation of T95 or T‐tet factor is in a repressed state but the formation of a recombinant between T95 (or T‐tet) and r3(kan) determinants results in the derepression of pili formation.