Genetic variation in the sex factor of Escherichia coli.

The mating type of Escherichia coli is determined by the presence or absence of the sex factor (F). Cells which lack F are genetic recipients or females, and are designated F-; cells which possess F may be genetic donors, or males (Lederberg et al., 1952). Two types of males have been described: F+, in which the sex factor behaves as though it were a population of autonomous, extrachromosomal particles, transmissible by cell-to-cell contact; and Hfr, in which the sex factor is firmly bound to the chromosome and is not transmissible except as a chromosomal locus (Jacob and Wollman, 1957). The name "episome" has been proposed for genetic determinants of this type, which have alternative states of existence as cytoplasmic particles or as attached chromosomal elements (Jacob and WVollman, 1958). The genetic material of a temperate phage is another example of an episome. Male cells of E. coli appear to transfer their chromosome to females only when F is in the attached state. Thus, all Hfr cells transmit chromosome, but in a population of F+ cells, only those few in which F is attached act as donors. Such donor cells probably include both stable Hfr mutants as well as cells in which the attachment is unstable and transitory. The chromosome of the Ft cell forms a closed circle of genetic markers (figure 1); the attachment of F causes a break in the chromosome such that F remains attached to one broken end, whereas the other end becomes the leading point or "origin" in chromosome transfer. Thus, in chromosome transfer by an Hfr, attached sex factor is always the last marker to be transferred (Jacob and Wollman, 1957). Since conjugation may be interrupted at any time by spontaneous separation of the mating couple, partial transfer of the chromosome is the general rule. As a consequence, the farther away a marker is from