Characteristics of E. coli K12 strains carrying both an F prime and an R factor.

General rights Copyright for the publications made accessible via the Edinburgh Research Explorer is retained by the author(s) and / or other copyright owners and it is a condition of accessing these publications that users recognise and abide by the legal requirements associated with these rights. Take down policy The University of Edinburgh has made every reasonable effort to ensure that Edinburgh Research Explorer content complies with UK legislation. If you believe that the public display of this file breaches copyright please contact [email protected] providing details, and we will remove access to the work immediately and investigate your claim. SUMMARY The interrelationship between an F prime and an F-like or I-like R factor was studied in Escherichia coli K 12 strains carrying both. The donorability of such strains suggested that these plasmids were transferred independently by their own transfer systems. Plaque formation by several male-and female-specific phage was tested; the inhibition by Flac of several female-specific phage was not affected by an F-like R factor. Neither did an R factor affect the incompatibility between Flac and F7ii,s. However, the surface exclusion characteristic of IFlac was lost in the presence of an F-like R factor. This is probably closely related to the simultaneous loss of donorability, the F-pilus, and the f+ antigen: all may be the result of the inhibition by an R product of the formation or activity of a single F product. 1. INTRODUCTION It has been known for some time that cells can carry both an F-prime factor and a transferable drug resistance factor (R), showing that there is no incompatibility between these two types of plasmid (Watanabe & Fukasawa, 1962). In such strains fi + , or F-like, R factors seem to prevent the formation of both R-pili and F-pili, leading to poor donorability and resistance to male-specific phage (Wata-nabe & Fukasawa, 1962; Hirota et al. 1964; Nishimura et al. 1967). On the other hand, fir, or I-like R factors prevent the formation of their own pili, but not of F-pili. Mutants of both types of R factor are known, the 'depressed' mutants, which do not prevent pilus formation. We have examined in more detail the relationship between an F-prime factor and R factors of the various types mentioned above, in strains carrying both, with particular reference to properties of F which might be affected by the presence of an R factor.