Transposition derivatives of an Hfr strain of Escherichia coli K-12.

HE genome in Escherichia coli K-12 exists normally as one ring of doublestranded DNA (CAIRINS 1963). The fertility factor, F, is responsible for the ability of certain strains to act as genetic donors. F may exist as an autonomously replicating element (in Ff strains) or be integrated into and replicate with bacterial genome (in Hfr strains) (WOLLMAN, JACOB, and HAYES 1956). Multiplication of the autonomous F factor is inhibited by growth in a medium containing acridine orange: this converts an F+ culture to an Fculture. Acridine orange, however, does not affect the duplication of the integrated F factor (HIROTA 1960). Hfr cells transfer their chromosomes to suitable recipients in an oriented manner, beginning at the site of F factor integration (the origin). The F factor itself is the last marker transferred. Since there is spontaneous breakage of the donor chromosome during transfer, the probability of a marker being introduced into a recipient decreases with its distance from the origin of chromosome transfer (JACOB and WOLLMAN 1958). JACOB and ADELBERG (1 959) found that matings interrupted before the entire Hfr chromosome could be transferred still yielded a few recombinants for a distal Hfr marker. Many of these recombinants were able to donate this marker with high frequency to Frecipients, which became, in turn, high frequency donors. These were found to possess the selected distal marker associated with the F factor in an autonomous genetic element. Such elements are termed F-prime (F’) factors. I? factors, like F factors, may be removed from strains such as those described by JACOB and ADELBERG by growth in medium containing acridine orange (HIROTA and SNEATH 1961). CAMPBELL (1962) provided a framework for understanding the properties of I?’ factors when he proposed that autonomous elements (episomes) were circular structures. He suggested that episomes become associated with the bacterial chromosome by a single reciprocal crossover which integrates the episome into the chromosome. Thus, a reversal of the integrating exchange would release the