Volume 3

Gene expression / Benjamin Lewin.

  • Lewin, Benjamin.
Date:
1974-1977
Catalogue details

Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: Gene expression / Benjamin Lewin. Source: Wellcome Collection.

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    material. An F+ strain is therefore able only to donate genetic material; an F strain is able only to receive it. When the same cross is performed between a male donor and either a male or female recipient, the cross F + x F + gives some twentyfold fewer prototrophs than the cross PxF. This is consistent with the idea that crosses take place only between an F+ donor and F~ recipient; but although F^ cells cannot act as recipients, the presence in the F+ population of some cells that are F~ allows conjugation to proceed, although at a rate that is reduced in accordance with the low concentration of true recipients. Two consequences ensue when an culture is mixed with F~ cells. One is the transfer of genetic information from donor to recipient with the subsequent formation of recombinants. A second is the transfer of the F^ genotype. This was first noted when Lederberg et al. (1952) tested the compatibilities of the progeny of F+ x F~ crosses and found that all were compatible, that is F+. That F 4 is transmitted infectiously, independently of any genetic transfer, was revealed by an experiment in which three strains were mixed: an F + met~thr~leu~, an F~met + , and an F~thr + leu + . Although the two F~ strains cannot yield prototrophs when mixed to gether, they are able to do so when mixed in the presence of the F+ strain. Since the F 4 strain does not contribute positive markers to help form prototrophs, its action must be to convert cells of one or both F strains to the F^ mating type, so that they in turn can conjugate with the auxotroph. Subsequent experiments showed that most or all of the cells of an F culture may be converted to the F + state upon inoculation with some F 4 cells; the efficiency with which mating type is transferred is thus very much greater (about lO^fold) than the frequency of genetic recombi nation. The sexual characteristic of E.coli is thus infectious. Another feature of F+ cells is that when cultures are grown to high concentrations, the cells may behave as though F~ although their genetic constitution remains unaltered— phenocopies have been produced. Linkage between Unselected and Selected F + Markers Following transfer of donor genetic material, the recipient cell passes through a transient diploid stage, during which genetic recombination may take place between donor and recipient DNA. Although usually it is not possible to isolate cells in the diploid condition, Lederberg (1949) found that after crosses in which one or both parents possess a mutant allele termed het, the recombinant prototrophs behaved as heterozygous diploids; after a cross of bio~met~ lac + x thr~ leu~ lac~, for example, the bio + met + thr + leu + prototrophs plated on the indicator medium EMB- lactose give variegated colonies instead of dark lac + or light lac~. Unlike