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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    Figure 2.17, the bacterial chromosome should have lost the ade, tsx, and lac loci during excision, and with them the ability to integrate an F factor at this site. Scaife and Pekhov (1964) demonstrated that this F' factor does not seem to be able to integrate readily at its ancestral site, although when transferred into other F~ recipients (which possess this chromo some region), it sponsors chromosome transfer with its ancestral orienta tion. Its failure to do so in the parental strain must thus be due to deletion of the sequences usually recognized at this site by the sex factor. Cuzin and Jacob (1964) also isolated a strain in which deletion prevented inser tion of F' at its ancestral site. In both type I and type II excisions material is lost from the bacterial chromosome. Scaife and Pekhov (1964) confirmed that this makes it impossible to derive ¥~ cells under appropriate selective conditions, a conclusion reached also by Pittard and Ramakrishnan (1964). In further studies, Scaife (1966) demonstrated that when cells are grown on medium in which a lac + genetic function is required, attempts at curing F13 with acridine orange do not produce any F~ cells; the only lac + survivors represent Hfr cells in which the F' factor has been integrated into the bacterial chromosome. Since this F' factor was generated by a type II excision, although the sites of integration varied they never corre sponded to the original site. In these Hfr strains, the integration of F13 at these various sites always resulted in transfer of ade + as an early marker; so transfer following integration starts with the bacterial material carried by the original F' factor. Because a type II F' factor possesses both proximal and distal markers, on entering a new recipient possessing both these regions it should be able to undergo a double recombination. This explains why cells carrying type II F factors often revert to an Hfr state identical to that of the ancestral strain; two recombination events, one in the proximal and one in the distal marker region, can insert the F factor at its original site, freeing fragments that in toto correspond to the duplicate set of markers, and which pre sumably are lost at cell division. This double recombination thus gener ates a haploid Hfr strain (see Hayes, 1968). When a reciprocal exchange occurs within a circular structure, two consequences are possible, depending on the type of reunion that follows the breakage. As Figure 2.18 demonstrates, a homodirectional reunion causes the formation of two circular structures; this corresponds to the type II excision shown in Figure 2.17. A heterodirectional reunion, how ever, causes an inversion, in which the sequence containing the sex factor is replaced in the bacterial chromosome in the opposite orientation to its previous one. Both have the result that a donor is produced in which previously late markers are now transferred early, in the first case as part