R factor compatibility groups

Summary Eight R factors are described which fall into four compatibility groups, distinct from previously described F-like and I-like groups. E. coli K12 carrying one of these factors, TP114, supports multiplication of the I-specific phage If1. However, TP114 is fully compatible with the I-like factor T- and with all the other R factors described in this paper. Interactions between TP114 and T- are described. Another R factor, TP122, inhibits F-fertility and is therefore fi ⁺, although strains carrying it do not support multiplication of the F-specific phage 2. TP122 is compatible with the F-like R factor R1, but incompatible with three other factors, all of which fall into the N group. Two further factors, TP116 and TP117, are incompatible with each other and constitute a new group, designated Group H. The final factor, TP113, is compatible with all the R factors with which it has been tested, so that it represents yet a further group. A second member of this group has recently been identified.