PROCEEDINGS

                          OF THE
NATIONALACADEMYOFSCIENCES

Volume 33           September 15. 1947

Number 9

REACTIVATION OF IRRADIATED BACTERIOPHAGE BY
   TRANSFER OF SELF-REPRODUCING UNITS

BY S. E. LURIA

DEPARTMENT OF BACTERIOLOGY, INDIANA UNIVERSITY, BLOOMINGTON, INDIANA

Communicated June 13, 1947

In investigating certain peculiarities in the titration of the activity of
bacteriophages after ultraviolet irradiation, a mechanism was discovered
that offers direct support for interpreting inactivation of these viruses by
radiation as due to lethal mutations.  Moreover, this mechanism reveals
unexpected features of the process of virus reproduction.  We summarize
here the results of this work and discuss some of their implications. They
will be published in detail elsewhere.

Our work employed the coli bacteriophages, Tl--T7, their "r" mutants
(i.e., with rapid lysis, and large plaque), the host strain, Escherichia coli B,
and a number of bacterial mutants resistant to one or another of the bac-
teriophages.`* 2

The rate of inactivation of these bacteriophages exposed to ultraviolet
light (wave-length 2537 A) in a non-absorbent medium is a simple logarith-
mic function of the dose of irradiation (Fig. 1).3*4 This indicates a "one
hit" mechanism of inactivation, one quantum being the effective
inactivating hit.  The phage survival is generally measured by plaque
count, by diluting the irradiated phage rather heavily in broth and plating
the diluted samples with sensitive bacteria.  Each phage particle that re-
mains active will give one plaque (phage colony).

It was, however, noticed by Delbrtick and Bailey (personal communica-
tion) that the active titre of an irradiated suspension, as determined by
plaque count, is dependent on the concentration of the irradiated sample
when first placed in contact with the bacterial cells.  For example, an ir-
radiated sample can be diluted 1: 10 into a heavy bacterial suspepsion,
and after a few minutes-during which no phage liberation occurs-diluted
again 1: lo4 in broth and plated. The count will be much higher than if the
same sample had first been diluted 1: 10' in broth, then 1: 10 in bacterial
suspension. This phenomenon seemed to indicate the presence in irradi-


254              BACTERIOLOGY: S. E. LURIA

PROC. N. A. S.

ated phage suspensions of some "factor," besides the phage itself, capable of
causing reactivation of inactivated phage particles if allowed to act on
the same bacterial cells. A study of this factor was the original purpose of
the work reported here.

It has been shown previously,6 and again in the experiments discussed in
this paper, that ultra-violet inactivated phage particles are still adsorbed
by sensitive bacteria at the same rate as active particles, even after having
received as many as 40-50 lethal hits.  This can be demonstrated because

~66 400  600  800   200  400  600 600  (COO  1200 1400   100  400  600 ERG?

                   FIGURE 1

w'=

Survival curves for phages T2, T6, T4, T5 in synthetic medium.  P/PO = proportion
of active phage particles after irradiation.  I = In PdP = average number of lethal hits
per particle.  The deviations for high doses arc due to the reactivation (described in this
paper) taking place on the assay plates.

each bacterial cell that adsorbs one phage particle fails to divide and to pro-
duce a colony after plating.  Incidentally, this fact makes it possible to
calculate the number and rate of adsorption of ultraviolet inactivated
phage particles. The unknown reactivating "factor" must be something
that, when acting on a bacterial cell that has adsorbed an inactive phage
particle, causes the production of active phage.

We first established that reactivation only occurs for the large-particle
phages T2, T4, T6, T5, and their mutants, not the small phages TI
and T7. It was found next that the "factor" is partially phage specific, in
the sense that even a concentrated suspension of one phage can generally
not reactivate the particles of another irradiated phage. This can be tested
by preparing mixtures of host cells with two phages in various proportions,
the irradiated phage whose reactivat.ion is to be tested being so diluted that


VOL. 33, 1947        BACTERIOLOGY: S.E.LlJRIA              255

little or no reactivation would take place in the absence of the other phage
suspension. After a few minutes of contact, the mixtures are diluted and
plated with bacteria sensitive only to the. phage whose reactivation is
being tested.  Since all phage suspensions are lysates of the same host cells,
the specificity of reactivation suggests that the "factor" is not of bacterial
origin.

An important exception to specificity is that cross-reactivation occurs
between suspensions of phages of the T-even group (T2, T4, T6).  These
phages, although representing different wild types and distinguishable by a
number of characteristics, are known to be serologically related and mor-
phologically similar. Moreover, particles of these phages are known to be
capable of mutual transfer of hereditary characteristics when adsorbed by
the same host cell." Reactivation of one of these T-even phages can be in-
duced both by ir;adiated and non-irradiated suspensions of any of the other'
T-even phages.

These results suggested that the reactivating "factor" might simply be
phage itself, in the sense that an inactive phage particle, if adsorbed
on the same cell with another particle of the same or of a related
strain, could be reactivated by transfer from the latter of the genetic locus
or loci at which lethal mutations had occurred.

This hypothesis Gas qualitatively supported by the finding that cross-
reactivation between two related phages can only occur in the presence of
bacteria capable of adsorbing both phages, not in the presence of bacteria
sensitive only ,to one of them. For example, T6 can reactivate T2 in pres-
ence of strain B, sensitive to both, but not in presence of bacteria B/6, sensi-
tive to T2 and resistant to T6.

That reactivation is not caused by some other "factor" in the lysates was
also supported by the finding that reactivation occurred with phage that
had been purified from extraneous material by differential centrifugation
(Strain T4r, kindly supplied by Dr. T. F. Anderson).      .
For quantitative testing, our hypothesis can be formulated more exactly

by `stating that reactivation should only occur inside bacterial cells that
adsorb two or more bacteriophage particles (multiple-infected bacteria).
This expectation can be tested for each phage by using several mixtures of
bacteria and irradiated phage in different proportions, and calculating for
each mixture the number of bacteria that adsorb more than one phage par-
ticle. If the average number of phage particles adsorbed per cell is X, the
proportion of cells with more than one phage particle is given by the ex-
pression : 1 - (x + 1)~~. For x small, thisexpression corresponds very
closely to the proportion of cells with two phage particles,  The values for
multiple-infected bacteria thus obtained can then be compared with the ac-
tual numbers of bacteria that liberate phage in each mixture. These num-
bers are measured by plating a sample for phaqe plaque count before lysis


TABLE 1

      RELATION OF THE N~BER OF BACTERIA YIELDING PKAGE TO THE NIJMBER OF MULTIPLE-INFECTED BACTERIA
Series of mixtures containing the same number of bacteria and different numbers of phage particles were kept 10 minutes at 37'c., then
plated for plaque count. The number of multiple-infected bacteria in each mixture was calculated from the formula
                      [B > lP] = [l - (zr + l)~?][Bl,

where [B] -=`total number of bacteria per ml.; [B > lP] = multiple-infected bacteria per ml.;  x = ratio "adsorbed phage/bactcria" in
each mixture.

Expt. Izo..   41

Phage..   T4
Dose, erg x mm. -2..  600

      LB > lP1
047  1.7 (Calc,) x 108
0.33    5.0 x 10'
0.135   1.1 x 10'
01067   2.6 X lo6
0.033   6.8 y 105
0.013   1.7 x loj

Count
(Exper.)
9 x 10'
3 x 10'
6 X 106
1.3 x 106
3 x 105
1 x 106

Av.
c

Ratio l/y
(C&J                      Ratio l/y
             [B > IPI     Count     (Calc.)
(Exper.)     x      (Calc.)    (Exper.)    (Exper.)

1.9
1.8
1.9
2.0
2.1
1.7

1.92
0.16

0.95   1.9 X 10s  1.7 X 107    11
0.48   6.5 X 10'  5.4 X lo6    12
0.24   1.9X10'  1.5x106   13
0.09   3.2 X 10b  2.2 X lo5    14
0.05   9.0 X 105  8.0 X 10'    11

Av.        12.2
0          1.2

              38

              T6
             500

                     Ratio l/y
      [B > PI     Coullt     (Calc.)
0.;2     (talc.)    (Exper.)  `(Exper.)
     1.6 X lo8  3.9 X lo7   4.1
0.31   3.6 X lo7  1.2 X 10'   3.0
0.155   1.3 X 10'  3.2 X lo6    4.1
0.062   2.4 X lo6  6.7 X.105   3.6
0.031   5.4 x 105  1.3 x 105   4.2
0.015   1.7 x 10'  4.1 x 10"   4.1

             Av.        3.85
              rs         0.46


VOL. 33, 1947       BACTERIOLOGY: S. E. LURIA             257

of multiple-infected bacteria and of bacteria that yield active phage.  We
shall assume that a phage particle contains a certain number of different
self-reproducing "units" (loci), each capable of undergoing a lethal muta-
tion under the action of radiation. Reactivation depends on transfer of
these units, the requirement for reactivation being that a given locus does
nof carry a lethal mutation in all of the particles that infect the same cell.
The probability that this requirement is satisfied will depend on the dose
of radiation, since the probability that a lethal hit takes place at the same
locus in all inf &ting particles will increase with increasing doses of radiation.
This hypothesis is graphically described in figure 2.  It can be formulated
mathematically by assuming that there are in each particle of a given
phage n units each capable of giving a lethal mutation. We further as-
sume, in first approximation, that the sensitivity of all units to radiation is
the same..

I         II        III        lx

FIGURE 2

Schematic representation of phage particles, each carrying three lethal mutations,
distributed at random among ten loci.

h A bacterium infected with particles (I + II) will yield active phage.
A bacterium infected with particles (I + III) will not yield active phage.
A bacterium infected with particles, (I + IV) will not yield active phage.
A bacterium infected with particles (III + IV) will not yield active phage.
A bacterium infected tith particles (I + III + IV) will yield active phage.

The probability yk that none of the n units carries a lethal mutation in all
of k particles adsorbed by the same cell is then found7 to be

yk = [l - (1 - e-r'x)k]n

(1)

where r is the average number of lethal hits or mutations per particle.  The
value of r can be read directly from the regular survival curve, as given in
figure 1, Y being the natural logarithm of the ratio between the initial titre
and the survival : r = In PO/P.

For r/n < l-that is, for low doses of radiation-the formula (1) can
be simplified with good approximation to


258            BA CTERIOLOG Y: S. E. L UlilA

PR0C.N. A.S.

Yk s e -,k/,,k-'

In particular, for bacteria that adsorb only two phage particles (K = 2), the
probability becomes

yz = [l - (1 - e-r'n)21n

It must be noticed that for low doses, when the probability that a lethal
mutation at any given locus occurs in all particles is small, the values of y
should tend to one if the efficiency of the transfer mechanism is complete,
that is,, if there is full recombination of active units to form active phage
particles.

The formulas (1, 1') and (2, 2') can be tested experimentally by studying
the dependence of the probability of reactivation (see table 1, columns
l/y) on the dose of radiation. In particular, yr can be obtained from experi-
ments with bacteria in such excess that the probability of infection higher
than double can be neglected. From the values of Y and y, n can be deter-
mined. If our hypofhesis is correct, y should tend to one for low doses, and
the values of n obtained from all experiments should be constant for each
phage. A large number of experiments gave results agreeing with these

                           TABLE 2

CALCULATION OFTHENUMBEROF"UNITS" PER PHME PARTICLE (EXPERIMENTS WITH
                  Low DOSES)
I = lethal hits per particle; y = proportion of multiple-infected bacteria that yield
phase ; n = number of units per particle calculated from the formula
   y = [l - (1 - e+In)sln = e-"1" (for r/n < 1).  1z = ra/ln(l/y).

   PHIGB       I      l/Y      n       PH.4GIE           l/Y      ?I
TZ-TZr     2.6     1
         1.6 49'       T6  3*.6     1      . .
           4.3                        7.9     3.4 51
           7.7     3.5 47              9.4     5.7    50
           11.0     19      41               11.7     17      49
        14.3 93     45            12.6 40     43
                Av.     45.5          15.5 70
                                        Av.  z;


  T4-T4r     2.8     1       . .        T5     2.8     1.9     12
           5.1     2.8 26             6.1 16      13
           7.8     5.7 36             9.2 67      19
         10.0 21      33                    Av.     15;

                Av.     32
* Values uncertain, due to very low adsorption rate of this phage.

expectations. Some experiments are shown in table 2, where l/y is given
instead of y for convenience. We may then conclude that active phage is


VOL. 33, 1947        BACTERIOLOGY: S.E.LUR.IA               259

formed from inactive particles by a highly efficient mechanism of transfer of
any one of a relatively large number of independently transferable units:
45-50 for phages T2 and T6, 30-35 for phage, T4, possibly around 15 for
phage T5.

Formula (1) requires that the probability of reactivation increases with
increasing values of k, that is, of the number of phage particles adsorbed per
bacterium. This was verified by experiments in which bacteria were mixed
with larger amounts of irradiated,phage (x higher),,s so that the probability
of three or more phage particles being adsorbed by the same cell became
appreciable. Representative data are given in table 3, and are seen to agree
with the expectation that the values of y increase with increasing values of
x (and of k).                                     

                           TABLE 3

DEPENDENCE OF THE PROBABILITY OF REACTIVATION ON THE MULTIPLICITY OF
                        INPECTI~N

DOSI, ERG X MM.-=. . . ,  400        000       800        1000

l/Y

l/Y

71       T6      0.11       .       8.1      4i     ~   . . .
              0.55       .       7.8      25       130
              1.1               4.1      18        44
              2.7         :       5.1      15        44

77       T2      0.39     2.6      10.6      60        .
              0.78      2.4       9.3      35        
              3.9      2.0       3.4      10
              7.8       1.65      2.15      5.3.   :::

These data permitted us to obtain a rough independent estimate of n
from the probability of reactivation in bacteria with more than two phages.
The values thus obtained for n are in sufficient agreement with those given
in table 2, although generally a little lower.. This systematic deviation
seems to indicate that the efficiency of transfer in the case of infection with
more than two particles is layer than 100%.

Our next question is the following: Does all inactivation of phage par-
ticles by ultraviolet light occur through production of lethal mutations in
transferable units, or will some other mechanism of inactivation appear for
very high doses? This point was investigated by using heavy doses (up to
40-50 lethal hits per particle) and comparing the results with the expecta-
tions from formula (2) for double infection. Any additional mechanism of
inactivation appearing at high doses should result in a systematic devia-
.tion in the sense of less reactivation than predicted by the formula.  No
such deviation was found; this indicated that, even for large doses of radia-
tion, all effect can be accounted for by lethal mutations in the transferable
units.


260             BACTERIOLOGY: S. E. LURIA

PROC. N. A. S.

This conclusion also has another interesting feature: it indicates that
there is no appreciable amount of "linkage" in the transfer process.  Any
tendency of units to be transferred in groups rather than individually
should give a deviation in the direction of too low reactivation at high
doses, when the probability of producing two or more lethals within a link-
age group becomes appreciable.

The next step was an attempt to elucidate the mechanism of transfer.
The obvious analogy with crossing-over might suggest that the two entering
particles exchange units directly, before any multiplication takes place,
and that what then multiplies is a fully active particle resulting by ex-
change. This possibility is difficult to reconcile with the very high efficiency
of reactivation:. in case of crossing-over we would hardly expect that all
active units be collected into one active particle.  Another test can be made
by the cross described in figure 3. Remembering that an active particle of
one phage may reactivate an inactive particle of another phage,
we may expect that transfer will also occur between active and inactive
particles of the same phage. If there is direct crossing-over, an active par-
ticle, `if adsorbed with inactive ones onto the same cell, should often be in-
activated by transfer of some "lethal" unit.

INACTIVE

ACTIVE          INACTIVE

       FIGURE 3

+

INACTIVE

Schematic representation of the expectation in case of "cross" between a heavily ir-
radiated particle and an active particle, assuming transfer of units by direct crossing-
over between the two infecting particles.

In the actual experiments, phage suspensions were given large doses of
radiation (20-50 hits per particle). Mixtures were set up containing bac-
teria, inactive phage, and active phage in proportion such that a large
number of cells received one active and one or more inactive particles.
If there were direct exchanges before reproduction, the result should be
the one described in figure 3. A high proportion of the infected cells should
end up with two inactive particles and, therefore, should yield no active


VOL. 33, 1947        BACTERIOLOGY: S. E. LURIA               261

phage. This was not found to occur in any case : an active phage particle
was never inactivated by *`crossing" with an inactive one.g

A possible alternative interpretation of the transfer is that each of the ac-
tive units can reproduce copies of itself in excess of the number needed for
multiplication of the phage particle as a whole.  The copies of each unit
might then either become incorporated into other phage particles that
missed them, or come together to reconstitute active phage particles, inde-
pendently of the origin of the individual units from one or another of the
infecting particles. We have no evidence as yet available whether the
"lethal" units do not multiply, or multiply in a way that makes them unfit
for their niche in the phage particle. In view of the high efficiency of the
reactivation process and of the approximately normal yield of phage par-
ticles, it seems certain that, if the inactive units reproduce at all, they do not
have any appreciable chance of becoming incorporated into the newly
formed particles.

We also have no evidence as yet as to whether the active units in an in-
active particle will proceed to multiply in single-infected cells.  The same
may be said for the question whether the active units remain in spatial rela-
tion to each other, as it were, in a "frame phage particle," while reproducing
excess copies that are incorporated into the new particles that are formed.
Work to attack some of these problems is now in progress.

A point of considerable interest is that in numerous experiments in which
phage particles were inactivated by x-rays instead of ultraviolet light, no
reactivation occurred, although the x-ray inactivated particles were still ad-
sorbed by the bacterial cells.

Discussion: These experiments have suggested a possible mechanism
of reproduction of phage particles inside the host cell, according to which
reproduction would take place, as it were, in an "atomistic" way, by
independent reproduction of a number of units and incorporation of
these into the final phage particles. A number of problems may be raised,
as to the nature of the self-reproducing units, the structure of the phage par-
ticle, and the relation of its mechanism of reproduction to that of other self-
reproducing entities (genes, plasmagenes).

In the first place, each unit seems to react as a distinct photochemical en-
tity in respect to ultraviolet quanta. The assumption of equal sensitivity
is very likely unjustified; a variation in sensitivity from locus to LOCUS
would tend to make the calculated number of loci per particle too small,
since the most sensitive units have a greater chance of being lethally hit
earlier in all infecting particles. This would result in less reactivation and
apparently lower number of loci.  Our estimates of the number of loci are
therefore minima. It is interesting to notice that phage T4, which is about
twice, as resistant to ultraviolet than the related phages T2 and T6, also
appears to have fewer loci. Its higher resistance seems, therefore, to be due


262              BACTERIOLOGY: S. E. LURIA

PROC. N. A. S.

to actual absence of a number of loci rather than to absence of some par-
ticularly sensitive locus.

Our results with the small phages Tl and T7, where no reactivation was
detected, do not prove that these phages do not possess a number of inde-
pendently transferable loci.  They only indicate that, if such loci occur,
they are too few to be detected in our experiments, that is, fewer than 8 or
10.  These phages are actually about 2.5 times more resistant to ultra-
violet than phage T4.

As for the failure of particles inactivated by ionizing radiation (x-rays)
to undergo reactivation, this may be interpreted as indicating, either
spread of the lethal effect of each ionization (or group of ionizations) to. a
large number of loci, or, less likely, an ability of the x-rays to cause other
changes than those produced by ultraviolet.  A projected study with
various monochromatic radiations may throw some light on this question.
It is interesting to notice that Lea and Salaman, lo extending earlier tiork on
the rate of inactivation of phage by ionizing radiation,li and analyzing their
results in terms of the density of ionization, recently concluded that the
"sensitive zone" of a large phage particle may be resolved into about fourteen
units (genes?). Because of the assumptions involved in their-calculations,
their results by themselves can hardly be considered as more than qualita-
tive indication of a geometrically complicated organization of the radiation
sensitive material of the phage particle.  In the light of our experiments,
however, the interpretation offered by Lea and Salaman appears quite
plausible, at least in assuming a multiplicity of sensitive units,

The incorporation of discrete units into organized phage particles is not
easy to visualize, in, view of the complex structure and morphology of the
latter.12 We can imagine that each active unit impresses its specificity on a
number of elements produced in excess inside the host cell, elements which
represent the raw material then utilized to build more phage particles.  The
units carrying lethal mutations may be unable to mold the substrate in
their own image and likeness.

It is plausible, although not yet experimentally demonstrated, that some
of the transferrableunits that can undergo lethal mutations are the same
gene-like entities responsible for determination of the transferable charac-
ters studied by Delbriick and Bailey6 and by Hershey'*.

The mechanism of transfer suggested for bacteriophages appears to differ
from that of crossing-over by being a transfer of units by "infection" rather
than an "exchange" of portions of gene strings. One might, however, con-
ceive that excess gene copies similar to the copies of our units are also pro-
duced in the cell nucleus, but have no opportunity of being incorporated by
"infection",into the chromosomal continuum because of a more rigid in-
tegration of the genes in the chromosome (needed to meet the requirements
of the sexual process) than of the units in the virus particle.  Excess pro-


VOL.38,1947       &iCTERIOidGY: i5'.$.LURIA            263

duction of full or partial gene copies has been suggested both on the basis of
cytological studies13 and in connection with theories of gene action.14

The transfer mechanism appears to represent a novel, and, in its way,
rather efficient means of effecting genetic recombina.tion between virus par-
ticles.

Our results thus offer direct evidence for interpreting inactivation of
viruses by radiation as due to lethal mutations, and strengthen the concept
of a fundamental similarity between viruses and the genetic material of
other organisms. They indicate that a virus particle may rather be com-
parable to a gene complex than to an individual gene.. A virus particle can
undergo a number of independent mutations,2* l5 a property that may be
shared by certain "gene complexes," defined as cross-over units resolvable
into two independently mutable units. i6 The independent mutability of
' these subgenic units is, however, somewhat doubtful.i6 The virus par-
ticle might better be compared to a chromosome, although it behaves more
as a unit toward x-rays than the latter. The self -reproducing components
seem to possess in the virus particle a degree of solidarity intermediate be-
tween that of the components in the supposed gene complex and that of the
genes in the chromosome.

There are obvious similarities between the transfer phenomenon here
described and other phenomena in which a determinant of heredity is trans-
ferred into a new genetic complex, as in the case of type transformation in
pneumococci. lT Phage reproduction may be a particularly favorable ma-
terial on which to analyze the mechanisms involved.

One interesting application is the possibility of analyzing differences be-
tween related but independent wild-type phage strains which can be
"crossed" (like the T-even group) in terms of differences in the number of
units that can be exchanged (shared loci).  Preliminary work in this direc-
tion appears promising.

Summary.-Inactivation of large bacteriophages by ultraviolet light is
due to lethal mutations at a number of different loci.  Each of these loci
appears to be independently transferable from one phage particle to others
inside the same bacterium.  The transfer of loci between irradiated inactive
particles results in formation of active phage if the infecting particles, taken
as a group, possess at least one copy of each locus in active form.  The num-
ber of loci can be calculated for each phage, and is at least 30 to 50 for some
of them. Phage growth appears to take place by independent reproduction
of each of these "unit loci" inside the host cell.

This work was done under an American Cancer Society grant recom-
mended by the Committee on Growth of the National Research Council.
The author wishes to acknowledge the able collaboration of Miss Elizabeth
M. Miller in the course of these experiments, and to thank Drs. M. Del-
briick and H. J. Mu&r for fruitful discussions of the problems.


264         CHEMISTRY: SHERMAN AND PEARSON   PROC. N. A.$.

r Delbriick, M., Biol. Rev. (Cambridge), 21, 30 (1946).
* Hershey, A. D., Genetics, 31, G20 (1946).
r Latarjet, R., and Wahl, R., Ann. Inst. Pmt., `71, 336 (1945).

4 Luria, S. E., and Latarjct, R., J. Bncl., 53, 149 (1947).
6 Luria, S. E., and Delbriick, M., Arch. Biochem., 1,207 (1942).
6 Delbrtick, M., and Bailey, W. T., Jr., Cold .@ring Harbor Symfi., 11, 33 (1946).
r Formula (1) was derived as follows:
  Probability that a given locus receives no lethal mutation = e-"".
Probability that a given locus receives a lethal mutation = 1 - e-`I".
  Probability that a given locus receives a lethal mutation in all of k particles =
   (1 - e-`in)`.

Probability that a givenlocus does not receive a lethal mutation in all of K particles
=l - (1 - e-+y~,
Probability that none on n loci receives a lethal mutation in all of k particles =
yk = [l - (1 - e-r/n)k]"

* It should be noticed that x (average number of phage particles adsorbed per bac-
terium) is different from k (actual number of phage particles adsorbed by a given bac-
terium). The proportion of cells with k particles is (xk/k!) e-`.
0 These results differ from earlier observations6 of interference between inactive and
active particles of the same phage.  The older experiments, however, were done under
conditions not comparable with those discussed here.
10 Lea, D. E., and Salaman. M. H., PYOC. Royal Sot., B, 133, 434 (1946).
r* Wollman, E., Holweck, F., and Luria, S. E., Nature, 145, 935 (1940).  Luria, S. E.,
and Ewer, F. M., these PROCEEDINGS, 27,370 (1941).
*r Luria, S. E., and Anderson, T. F., Ibid., 28, 127 (1942).
la See Schultz, J., Co&id Chemistry, 5, 819 (1944).
14 See Emerson, S., Ann. Missouri Bot. Gauden, 32,243 (1945).
r6 Luria, S. E., Genetics, 30, 84 (1945).
16 Stadler, L. J., Ibid., 31,377 (1946).
17 See McCarty, M.. Bad. Rq., 10, 63 (1946).
18 Hershey, A. D., Cold Spring Harbor Symp.. 11, 67 (1946).