froth I$'ATuI&, Vol. 158, pclge 558, October 19, 19&L],
       ;
       .

Gene Recombination in Escherichia coli

AKALYSIS of mixed cultures. sf nutritional mutants has revctlled
the presence of new types which strongiy suggest the occurrence of a
sexual process in the bacterium; EsciumcJ~ia coti.

The mutants consist of strains which differ from their parent wild
type, strain I.-l'& in lacking the ahilitv to synthesize growth-factors. .
As a result of these dejiciencies they <ill only grow in media supple-
mented with their specific nutritional requirements. In these mutants
single nutritional requirements are .established at single mutational
steps under the influence of X-ray or ultra-violet1n2. By successive
treatments, `strains with several requirements have been obtained.

In the recombination studies here reported, two triple mutants
have been used : Y-10, requiring threonine, leucine and thiamin, and
Y-24, requiring biotin, phenylalanine and cystine. These strains were
grown in mixed culture in `Bacto' yeast-beef broth.. When fully grown,,
the cells were washed with sterile water and inoculated heavily into
synthetic agarmedium, to which various supplements had been added
to allow the growth of colonies of, various nutritional types. - This
procedure. readily allows the detection of very small numbers of cell
types different from the parental forms.
The only new types found in `pure' cultures of the individual
mutants were occasional forms which had reverted for a single factor,
giving stmins which required only two of the original three substances.
In mixed cultures, however, a variety of types ]las been found. These
include wild-type strains with no growth-factor deficiencies and single
mutant types requiring only thiamin or phenylalasine. In addition,
double requirement types have been obtained; including strains de-
ficient in th.e syntheses of biotin and leucine, biotin and threonine,
and bibtin and thiamin respectively. The wild-type strains have been ;
studied most intensively, and several independent lines of evidence
.have indicated their stability and homogeneity.
In other espcriments, using the triple mutants mentioned, CXC$lJt
that one was resistant to the coli phage TZ (obtained by the proccdurc
of Luria and ,Delbriick3), nutritionally `wild-type strains were found
both in sensitive and in resistant categories. Similarly, .recombinat,ions
between biochemical requirements and phage resistance have frequently
been found.

These types can most reasonably be interpreted as instances of the
assortment of genes in new combinations.  In order that various
genes `may have the opportunit*y to recombine, a cell fusion would be
required. The only apparent alternative to this interpretation would
be the occurrence in the medium of transforming factors capable
of inducing the mutation of genes, bilaterally, both to and from
the wild condition. Attempts at the induction of transformations in
single cultures by the use of sterile filtrates have been unsuccessful.
The fusion presumably occurs only rarely, since in the cultures
investigated only one cell in a million can be classified as a recom->
bination type.
logically.   The hypothetical zygote has not b,een detected cyto;
Thcsc experiments imply thk occurrence of a sexual process in the
bacteriunj Escherichia colz ; they will be reported in more detail
elsewhere.

This work was supported .& part by .a grant from the Jane Coftin
Childs Memorial Fund for Medical Research.
            ~~~I~;~~~~~RIIBRO'
                                                 . .

* Fellow of the Jane Cof%in Childs Memorial Fund for Medical
Research.

l Tatum, E. Il., Proc. Nat. Acad. Sci., 3i, i15 (1945).
a Tatum E. L., Cold Spring Harbor, Symposia Qgant. Biol., vol. 11
(in )the press, 1946).

a Luria, S. E.? and Delbriick? M., CeFetics, 28, 491 (1943).