SKX IN BACTFRIA— GF.NETIC STUDIES 23 



that E. coJi K-12 was homothallic. The crossahlc strains arc all de- 

 rived from a single pure culture. Several workers had suggested that 

 a niating-tvpe system might be obscured by mutations from one mat- 

 ing type to the other, as occurs in certain yeasts. However, this 

 hxpothesis \\as rejected because no segregation of mating preferences 

 was observed from heterozygous diploids, as would have been ex- 

 pected from a heterothallic mating. It has since been discovered that 

 a unique compatibility mechanism does operate in Escherichia coli 

 (Lederberg, Cavalli, and Lederberg, 1952). The involved history of 

 this discovery must be detailed elsewhere, and only the general 

 conclusions can be given here. 



Wild type K-12 carries a hereditary factor, F+, which is re- 

 quired for mating. Similarly, most of the auxotrophic mutants of K-12 

 are f +, and therefore mutually compatible, but the much used line 

 descended from the threonine-leucine mutant, 679-680 (Tatum, 1945), 

 is F—. Because most of the other tester cultures are F+, however, the 

 F— "mutation" was not detected in earlier experiments. The empiri- 

 cal definition of F— is that crosses of two F— parents are completely 

 sterile, although comparable crosses in which one or both parents is 

 F-{- are productive. The self-incompatibility of F— has been detected 

 in two \vays: sublines of 679-680 are mutually incompatible, and new 

 occurrences of the F— "mutation" have been discovered which are 

 incompatible with 679-680. The F— "mutation" is given in quotation 

 marks because its inheritance and transmission set it apart from all 

 of the other markers so far studied. 



All the progeny of crosses within strain K-12 are F4-, whether the 

 parents were F— x F+ or F+ x F+ [F— xF— cannot, of course, be 

 tested]. This was explained by the finding that F+ was contagious, 

 that is that growing F— cells in mixture with F-f- resulted in many 

 of the former (identified by other genetic markers) becoming per- 

 manently F+. As many as 50 per cent of the originally F— cells may 

 become F-f by this conditioning process within a few hours. "F+" 

 is therefore tentatively regarded as an infective, virus-like agent, but 

 this has not yet been confirmed by the isolation of "F-f" in a cell-free 

 preparation. The transmission of F+ is not accompanied by the trans- 

 fer of any other marker, so far as is known. 



The virus-like properties of the compatibility factor have led to 

 some speculation on its relationship to a virus known to be present in 

 E. coli K-12, the latent bacteriophage X. This question has been 



