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.

    66/952 (page 46)
    integration some tenfold less. High efficiency (HE) mutations thus inte grate with relative frequencies of 0.7-1.06; and low efficiency (LE) muta tions integrate only with frequencies of 0.07-0.24. In these one point crosses (so called because the two parents differ only at one amiA mutant site), all mutant sites except one showed the same efficiency of integration in whichever direction the cross was performed. This suggests that the parameter controlling the efficiency of integration is the difference between the sequences of the donor and recipient DNA molecules. Similar results were obtained by Lacks (1966) in mea surements of the efficiencies of integration of a series of mal mutants of pneumococcus that lack the enzyme amylomaltase, with the difference that four efficiency groups were found; taking into account the difference in integration efficiency of the reference markers used in these two studies, two of these groups correspond to those found at the amiA locus and the two additional groups possess intermediate efficiencies and very high efficiencies of integration. In these one point crosses, again only the relationship between the donor and recipient DNAs and not the orienta tion of the markers appeared to control the efficiency of integration (for review see Hotchkiss and Gabor, 1970). An orientation effect is observed, however, when two point crosses are made, that is, the donor and recipient parents differ at two sites. By crossing two mutants and scoring the wild type recombinants, Lacks and Hotchkiss (1960a) found that the presence of a low efficiency site in the recipient much reduces the frequency of integration compared with the cross in which the low efficiency site is present only in the donor DNA. In a detailed study of transformation at the mal locus, Lacks (1966) found that this effect depends upon the distance between the two mutant sites; the ability of a low efficiency site in the recipient genome to prevent integration at a second site declines with the distance between them, an effect reported also in the amiA system by Gray and Ephrussi-Taylor (1967). The result of this effect is to make it impossible to construct a genetic map by conventional determination of recombination frequencies (for review see Ephrussi-Taylor and Gray, 1966); for genetic mapping relies upon the concept that only the distance between the two sites, and not their molecular nature, determines the probability that a recombina tion event will occur between them. Lacks therefore proposed the use for mapping by transformation of a recombination index, in which the recombination frequency between two sites is divided by the efficiency of integration of the less efficient recipient marker. Although this was criticized by Ephrussi-Taylor et al. (1965), who found that it did not eliminate differences in reverse polarity in all their crosses with the amiA