Modification of conjugation in Escherichia coli K-12 by ultraviolet irradiation.

ONJUGATION in Escherichia coli K-12 results from a sequence of consecutive steps: (1) an effective union between two cells of opposite mating types; (2) a polarized transfer of at least a portion of a male chromosome into the recipient cell, the precise order of transfer depending only on the donor strain utilized; (3) an integration of the genetic information from both parental strains into a recombinant chromosome; and (4) the segregation of the recombinant chromosome from the residual genetic information within the zygote with the subsequent production of a recombinant clone (see JACOB and WOLLMAN 1961; CLARK and ADELBERG 1962, for reviews). In previous studies the effects of ionizing radiations delivered before mating to either donor or recipient cells have been described (MARCOVICH 1961; WOOD and MARCOVICH 1964). Briefly, irradiation of the male produces lesions which prevent the transfer of the male chromosome to the female; no radiation lesions are produced which hinder the ability of the cells to form an effective union and no detectable effects on the processes of integration and segregation have been observed. The appearance of a specific male marker decreases exponentially with the dose to which the donor cells are exposed, and the rate of loss of appearance increases with the distance of a particular male marker from the Origin (the portion of the male chromosome which first enters the female). I t is possible to construct chromosome maps using radiosensitivity as an index of marker position that are identical to those generated by the interrupted mating technique (JACOB and WOLLMAN 1961). Irradiation of the recipient cells, on the other hand, does not affect detectably the transfer step but affects primarily the steps of union and integration. In the production of recombinants, there is a selective utilization of genetic information from the nonirradiated male parent as if there were an advantage conferred on those zygotes that receive larger portions of the male chromosome during mating. This results in an apparent increase in linkage between the male markers. JACOB and WOLLMAN (1961) have studied the effects of ultraviolet (UV) irradiation on conjugation in the K-12 mating system. They have reported that: