SUMMARY: Analysis of a pair of K-12 mutants which had ceased to show genetic recombination after storage, implicated mutant 58-161, which had previously behaved as a gene donor strain, as the infertile parent. Infertile 58-161 failed to display recombination when crossed with a gene acceptor strain (W-677) but was able to mate successfully with wild-type K-12 and prototrophic recombinant donor strains, i.e. it had become a gene acceptor. The terms ‘F+’ and ‘F -’ have been adopted (after Lederberg, Cavalli & Lederberg, 1952) to denote donor and acceptor strains respectively. Growth of either 58-161/F - or W-677/F - in mixed broth culture with 58-161/F + resulted in the conversion to F + of up to 75% of re-isolated colonies of the initially F - strain. F - strains converted to F + by strains of dissimilar genotype showed no phenotypic alteration and, therefore, were not recombinants. Washed, mixed cultures on minimal agar yielded an F + conversion rate of only 3.6%, while 100% recombinants were F + under the same conditions. The F + agent could be transmitted serially through F - strains and was not filterable. While F - x F - crosses were sterile, F + x F - crosses showed maximum fertility. F + x F + crosses were . 10–20 times less productive than F + x F -. The F + agent had a determining effect on the phenotype of recombinants. Thus, when the F + F - relationship was reversed in F + x F - crosses between the same pair of mutants, almost all recombinants which did not show new patterns of unselected marker characters had the phenotype of the F - parent. Among recombinants from F + x F + crosses, the phenotypes of both parents were represented though not always equally. This effect of F + on the phenotype of segregants invalidates much of the evidence for genetic linkage in K-12. Reversal of F + potential in otherwise similar crosses also had a marked effect on the efficiency of prototroph formation on minimal agar supplemented with various growth factors required by one of the parent auxotrophs. A tentative theory of the mechanism of recombination is presented on the basis of this and previous work. This supposes that F + is a non-lytic infectious agent, harboured by F + cells and absent from F - cells, which becomes effectively associated with a part (or parts) of the chromosomes of a small proportion of the cells it inhabits. The F + agent thus acts as a gene carrier in the transfer of genetic material from F + to F - cells.


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