From strain c, made F by infection with the sex factor F normally carried by strain K-12, several Hfr (‘high frequency of recombination’) strains were derived. Among these, four were found which exhibited a defective growth pattern on minimal media at 37°. Reversion to the F state was accompanied by re-establishment of normal growth habit. In the case best studied (strain c-132) the Hfr bacteria form colonies smaller than normal, acquire a rough surface upon prolonged incubation, and are unable to grow at 42°. Growth is normal at room temperature and on rich media; it can be improved by the addition of methionine to minimal media. The rate of reversion from the Hfr to the F state (i.e. from defective to normal growth) is of the order of 1/20,000 per generation. Defective growth is not due to a genetic peculiarity of the F factor, nor is it dependent on the map location of the ‘origin’ or leading end of the particular Hfr strain, or on its direction of chromosome transfer; possibly it results from the manner in which the F factor is integrated at any given chromosomal site.


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