Q 38

Reprinted—from Abstramm, 7th iIntexnaetene! Congress
of Microbiolegy, Stockholm, 1958, pp. 58-60.
Extranuclear transmission of the 7 compatibility fac-
to in E. coli. Joshua Lederberg,”Departments of
Genetics and Medical Genetics, University of Wiscon-
sin, Madison 6, Wisconsin, U.S.A.

The mating types of E. coli include F females
and F males, the latter being determined by the
presence of a readily contagious factor "F". A
number of lines of evidence converge to suggest that
this is inherited outside the chromosomal system of
linked markers which encompasses most of the known
heredity of E. coli: 1) in crosses of FY x F’, virtue
ally all of the progeny arc F° while other markers
segregate in characteristic ratios. 2) When an F
culture is seeded with a few F’ cells, the F° charac-
ter spreads through the entire population, indicating
that it multiplies more rapidly than the other genes.
3) After pairing with an F' cell, an F cell gives rise
to a pura F clone; in comparable matings of F x Hfr
(high frequency of recombination with respect to other
markers) the exconjugant F clone invariably segregates
a mixture of unaltered parental and recombinant types.
k) FY is transferred more rapidly and efficiently than
any other known markers, and in F’ x F crosses, shows
no linkage to any other marker. 5) As described by
Y. Hirota, the F" factor can be efficiently removed
from a population of F° cells by treating them with.
acridine dyes, especially acridine orange. These
findings suagest that the Ft cell carries an oxtra-
nuclear factor, i.e., a plasmid, which is rapidly and
efficiently transferred to F cells during bricf con-
tact. Ia Tany respects, the inheritance of F is
analogous to that of the factor which governs the
trait colicinogeny as analysed by Fredericq. The
mating behavior of Hfr mutants, the instability of
some of these to give infective Ft reversions auld
the segregation of the_Hfr in various linkage re

lationships in Hfr x F crosses suggest that the.
Hfr mating types represent the fixation of the F
factor at specific chromosomal loci.

The deerth of recombinants given by FY com-
pared to Hfr has provoked the hypothesis that F
is instrinsically sterile, and that its recombin-
ational fertility depends on spontaneous mutation
to Hfr. Such mutants undoubtedly occur, but it
is difficult to assess their role in F fertility
without an accurate measure of the mutation rate.
The regularity with which F x F crosses give
F' progeny argues against the necessary role of
stable, non-infective Hfr mutants similar to the
type. Hfro, Control] experiments have shown
that the Cavallarity of F’ progeny is not duc to
secondary reinfection. It is still possible that
in addition to occasional Hfr mutations, the more
fertile cells in an F° culture have experienced
more transient changes in the quality or position
of the F agent they carry.