Molecular studies of R factor compatibility groups.

Molecular studies of R factors of six groups, F(II), I(1), I(2), N, B, and H, defined on the basis of compatibility, support the conclusions drawn from genetic studies. In general, R factors of a given compatibility group are similar in size. Deoxyribonucleic acid (DNA) reassociation occurs freely between members of the same group but is minimal between heterologous groups. An exception to this was found in group H, of which one factor showed minimal homology with the remaining plasmids of the group. A further exception was found with groups I(1) and B, which, although genetically distinct, show between 18 and 28% of DNA homology. Groups I(1) and I(2) are molecularly distinct, despite the fact that they both stimulate the synthesis of I-fimbriae.