Abstract
The similarity of sex pili mediated by F factors and R(fi(+)) factors and the ability of R(fi(+)) factors to control by repression the functioning of pilus genes encoded by the F factor suggested that F factors and R(fi(+)) factors are closely related. Further comparisons of the episomal properties of F factors and R(fi(+)) factors, however, indicated many differences. F factors contain information for a restriction system for phages phiII and T7. Cells containing R factors are sensitive to these phages. Furthermore, R(fi(+)) factors do not repress the F factor phiII restriction system in cells containing both an R(fi(+)) factor and an F factor. R factors and F factors are heteroimmune episomes. In addition, an R(fi(+)) factor in cells containing both an R factor and an F factor does not fully repress the expression of F-factor immunity to an incoming second F factor. R-factor and F-factor replication systems are not identical. Wild-type F-factor replication genes will complement the mutant F(ts114)lac(+) replication genes in cells containing two F factors. The F(ts114)lac(+) episome is retained when these cells are grown at 42 C; however, cells containing an R(fi(+)) factor and F(ts114)lac(+) lose the F(ts114)lac(+) when grown at 42 C, at the same rate as cells containing only the F(ts114)lac(+). The replication system of the R(fi(+)) factor will not complement the mutant F(ts114)lac(+) replication system.