General Method for Isolating De-repressed Bacterial Sex Factors

Abstract

BACTEBIAL conjugation is now known to be regulated by repression in the same way as many other bacterial functions. Every cell in a culture may carry a sex factor but in only a minority will the factor chance to be de-repressed and the cell be capable of conjugation^1,2. The genetic and physiological analysis of conjugation requires mutant sex factors that are no longer subject to repression and which therefore enable all of their host cells to conjugate. Such de-repressed mutants can be selected by replica plating when the sex factor is linked to a selective marker like antibiotic-resistance, because those recipient cells acquiring the factor can be selected by their antibiotic resistance³. Many sex factors, however, are not associated with selective markers and replica plating cannot then be used. Mating in broth provides an alternative method that is applicable whether or not a selective marker is available and which, moreover, is more powerful than replica plating. The same argument underlies the qualitative method devised independently by Ohki and Ozeki⁴. The conditions described here give even greater selection than those of our original note⁵ and have been applied to the isolation of de-repressed mutants of colicin factor Ela (Col Ela: ref. 6) and of Col Ib-P9, well known as a sex factor in Salmonella typhimurium⁶.