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J Bacteriol. 1970 July; 103(1): 81-88
Copyright © 1970 American Society for Microbiology. All Rights Reserved.

Comparisons of F Factors and R Factors: Existence of Independent Regulation Groups in F Factors

¹ Department of Molecular Biology and Microbiology, Tufts University School of Medicine, Boston, Massachusetts 02111

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 II and T7. Cells containing R factors are sensitive to these phages. Furthermore, R(fi⁺) factors do not repress the F factor II 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_ts114lac⁺ replication genes in cells containing two F factors. The F_ts114lac⁺ episome is retained when these cells are grown at 42 C; however, cells containing an R(fi⁺) factor and F_ts114lac⁺ lose the F_ts114lac⁺ when grown at 42 C, at the same rate as cells containing only the F_ts114lac⁺. The replication system of the R(fi⁺) factor will not complement the mutant F_ts114lac⁺ replication system.

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