SUMMARY: Ultraviolet irradiation of k 12 bacteria, carrying an autonomous F factor or an F-prime factor, enhances the fertility of the population by increasing the number of cells which can transfer the bacterial chromosome. In contrast, under similar conditions the fertility of irradiated Hfr populations falls in proportion to the survivors. Following irradiation, the effect begins to develop after about 30 min. incubation in broth at 37°, reaches a peak at about 90 min., and thereafter slowly declines. The effect develops with similar kinetics during post-irradiation incubation in minimal medium as in broth, provided the bacteria have been minimal-grown; in the case of broth-grown cells, appearance of the effect in minimal medium is greatly delayed. A comparison of the kinetics of the effect with the growth of the population as a whole shows that the u.v.-induced donor state is not inheritable. Mitomycin C, which resembles u.v. radiation in producing DNA damage repairable by a mechanism involving excision of single-stranded fragments, also induces new donor bacteria. Other agents such as X-rays and methyl methansulphonate (MMS) do not stimulate the production of new donors but may enhance the recombination frequency since cells killed by them may continue to act as chromosome donors. The effect is not shown either by mutants (unable to excise thymine dimers) or by mutants (unable to mediate recombination) which carry an F-prime factor. A possible mechanism is suggested whereby the excision of single-stranded fragments of the bacterial chromosome, during the repair of u.v. damage, facilitates pairing with homologous regions of the complementary sex factor strand. A recombination event, mediated by breakage and covalent bonding, then joins a free end of the excised DNA strand to the paired sex factor strand. In this way, recombination connects sex factor and chromosome by only a single strand instead of by the two strands which normally leads to insertion and the formation of an Hfr chromosome. It is postulated that such a structure can be transferred at conjugation, but is incapable of more than one cycle of replication.


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