Classic Papers in Genetics

CtASSIC PAPERS IN

GENETieS '/' ,

^""s

Edited by

JAMES

-^-^

A.

^^^^

PETERS

\ (2.H

c LASSIC

TAPERS IN

G ENETICS J3

eO Ol

;

I

E

James A. Peters

C3

Associate Professor

(0-

2?

Edited by

=

the In

•

is

IT

DR — DD r—

Fi,—r-,

respectively, heterozygous

zygous dominants

in Fo.

,

are,

and homo-

instance,

R

means that the bird which can be

origin,

may mean R from

DR X

appearing in Fi (see Report

for

R, 1,

must be indicated, but they do not seem to p.

116), etc.

These

distinctions

p.p., r.r., mean that the bird was proved to be pure pea or pure rose; i.r., i.p. and p.s., that it was proved to

contain single.

Doub. means the longitudinally

split

described later. r.p. is the rose-pea, or "walnut" comb, whether natural (Malay) or

sijigle,

artificial.

The same bird is given as DD in its matings with a single, and as r.r. or p.p., in its matings with one of the dominants.

BATESON AND PUNNETT

45

C bo >. c c

.2^

*c

o bo

Volkin, E. K., personal communication.

«

Watson,

J.

D.,

and Crick,

F.

H. C. Cold

Spring Harbor Symposia Quant. Biol. 123-131, 1953.

18:

BENZER cleotide

273 pairs.

We

wish to translate

linkage distances, as derived from gen-

recombination experiments, into molecular units. This cannot be done very precisely at present. It is not etic

known whether phage particle material.

Nor

all

tified bv"

in

a

indispensable genetic

is

it

known whether

phage "chromosome" counterpart of

DNA

the

is

a

(i.e.,

a

the physical

linkage group iden-

genetic means)

composed

is

DNA

fiber or (duplex) whether genetic recombination is equally probable in all chromosomal regions. For the purpose of a rough calculation, however, these notions

of

a

single

assumed to be

true.

Thus we

place the total linkage

map

correspondence with

X

of T4 in nucleo-

will be

2

DNA. The

tide pairs of

10''

known

total

signed to extend genetic studies to the molecular (nucleotide) level. r Aiutcmts.—The wild-type phages

T2, T4, and with rough strain

B

T6 produce

small plaques edges when plated on of Escherichia coli. From sec-

tors of clearing in these plaques,

mu-

can be readily isolated which produce large, sharp-edged plaques (Hershey "). These mutants have been designated "r" for rapid lysis; they differ from the wild type by a failure to tants

cause

"lysis

(Doermann

inhibition"

on

strain

B

The

wild type has a selective advantage over r mutants when the two types grow together on B. The genetics of r mutants was studied by Hershey and Rotman,^^ ^^).

who found three regions in the linkage map of T2 in which various mutaphenotype were

length of the three linkage groups in phage T4 amounts to some 100 units

tions

(one unit = 1 per cent recombination standard cross). In addition, there is evidence ^ for roughly another 100 units of length connecting two of the groups. Therefore, if we assume 200 recombination units to correspond to

of mutants which were shown to be genetically distinct from one another. The genetic study of T4 by Doermann showed r regions correand Hill

'^

in a

2

X

recom-

10^ nucleotide pairs, the

10"^ per phage two given That to say, cent. is mutants whose mutations are localized in their chromosomes at sites only one bination per nucleotide pair

is

nucleotide pair apart, a cross between these mutants should give rise to a progeny population in which one par-

causing the

r

located, including one large "cluster"

''

sponding to two of those in T2. T6 also has at least two such r regions. The rll Group.— For all three phases, T2, T4, and T6, the r mutants can be separated into groups on the basis of their behavior on strains other than B. This paper will be concerned only with one group, which will be called the "rll group." A^Iutants of the

between the mutations (provided, of

group are distinguished from those of other groups, and from wild type, by a failure to produce plaques on cer-

course, that recombination

tain

ticle in 10^ results

from recombination is

possible

between adjacent nucleotide pairs). This computation is an exceedingly rough one and is only intended to indicate the order of magnitude of the scale factor.

Some preliminary

are here presented of a

results

program de-

rll

lysogenic

strains ^-

of

E.

coli

As shown in which carry phage Table 1, a mutant of the rll group >-.

9

Hershey,

A.

D.,

Genetics

31:620-640,

1946. 10

Doermann, A. H.,

/.

Bacterial. 55:257-

276, 1948. 1

Doermann, A. H., and HUl, M.

Jietics

B.,

Ge-

38:79-90, 1953.

s Streisinger,

communication.

G., and Bruce, V., personal

11

Hershey, A. D., and Rotman,

R.,

Ge-

netics 34:44^71, 1949. 12 Lederberg, E. M., and Lederberg, Genetics 38:51-64, 1953.

J.,

BENZER

274 Table

plating are approximately equal

three strains, except, of course, for rll on K12S (^). The three bacterial

Phenotype Plaque Morphology of T4 Wild and rll Mutant Plated on Various Hosts

Host Strain

T4 wild type T4 rll mutant

E. colt

E. coli

B

K12S Wild Wild

KUSiX)

WUd Type

strains will

Wild

produces r-type plaques on strain B, wild-type plaques on strain K12S (nonlysogenic strain sensitive to >^), and no plaques on K12S (^) (derived from K12S by Ivsogenization with 'k). The wild-type phage produces similar plaques on all three strains. In the case of T4, with which we shall be concerned in this paper, the efficiencies of

m

r

rl

=10%

as

"B,"

Approximately two-thirds of the independently arising r mutants isolated on B are of the rll type. This group includes the "cluster" of r mutants of T2 described by Hershey and Rotman and the r47 and r51 mutants described by Doermann and Hill in the corresponding map region of T4 but does not include r mutants located outside that region.

Similarly,

all

newly

isolated

mutants showing the rll character have turned out to fall within the same region, as indicated in Figure 1. The properties of the rll group are

tu

r

42 47 51 /o

be here designated

"S," and "K." n

E. colt

r

on the

1

41

region

map of T4 (Doermann), indicating the location of the rll tu designate "minute plaque" and "turbid plaque" mutations. The circular inset shows, diagrammatically, the corresponding dimensions of the chain magnified 1,000 diameters.

Fig.

1.

region,

Partial linkage

m and

DNA

especially favorable for detailed gene-

An

mutant has three different phenotypes on the three host tic

study.

strains

rll

(Table

1):

(1) altered plaque indistinguish-

morphology on B, (2) able from wild type on

and (3) unable to produce plaques on K. These properties are

all

S,

useful.

their altered plaque type tants are readily isolated,

By

virtue of

on B, r muand those of

the rll group are identified by testing on K. Where it is desired to avoid a

compared with wild type, e.g., in measuring mutation rates, S can be used as a nondiscriminating host. The failure of rll mutants to plate on K enables one to detect

selective disadvantage

very small proportions of wild-type particles due to reversion or due to recombination between different rll mutants.

Mutants in K.— Wildmutants adsorb equally well to strains S and K. Whereas the wild type provokes lysis and liberation of a burst of progeny on both strains, the rll mutant grows normally only on S. Infection of K with an rll mutant provokes very little (and/or very late) lysis, although all infected cells are killed. The block in grow th of rll mutant is associated with the presence of the carried phage ^. The reason for this association is unknown.

Fate of type and

rll

rll

BENZER Quantitative Differences in Phefio-

type.— While all rll mutants show the same phenotvpic effect of poor multiplication on K, thev' differ in the degree of this effect. A certain proportion of K infected with rll actually liberates some progeny, which can be detected

on

by

The

R.

\'ieldincT

plating the infected cells

fraction of infected cells

progeny defines

a "transmis-

sion coefficient" characteristic of the

mutant.

The

transmission coefficient

is

the multiplicity of infection but depends strongly upon the physiological state of the bacteria (K) insensitive

to

and upon temperature. Under given conditions, however, the coefficient can be used as a comparative index of degree of phenotypic effect, a "leaky" mutant having a high coefficient. As can be seen in Table 2, a wide range of values

is

found.

Table Properties of rll

2

T4 Mutants Group

of the

*

Reversion Index

Mutant

Map

Transmission

(units

Number

Position

Coefficient

of IQ-**)

0.03