Copyright 0 1991 by the Genetic? Society of America

Perspectives

Anecdotal, Historical and Critical Commentaries on Genetics
      Edited by James F. Crow and William F. Dove

The Gene (H. J. MULLER 1947)

    Joshua Lederberg

Rockefeller University, New York, Nm York 10021

66 T HE Gene" was H. J. MULLER'S Pilgrim Trust
   lecture, delivered before the Royal Society of

London on November 1, 1945. World War II was
barely over, but sea travel was still hazardous. A storm
had dislodged a number of floating mines, and the
transit to port of SS Queen Mary was something of an
adventure (CARLSON 1981). Published in 1947, "The
Gene" is the finest exposition of the state of develop-
ment of genetics at the very dawn of its turning
molecular in the decade spanned by AVERY, MACLEOD
and MCCARTY (1944)and WATSON and CRICK(~~~~).
The explosive transition was of course closely linked
to contemporaneous history: the burgeoning commit-
ment to scientific research, now strongly supported
by government in a style inherited from wartime
experience. I will be reviewing this paper in a retro-
spective mood: "now" (as opposed to "today") will
mean 1945. The contrast of 1945 with 1991 gives us
a chance to reflect how much we have learned in 46
years, and how much was anticipated. It is especially
instructive to reread this work in company with DU-
BOS's The BacteriaE Cell (1945), a noted microbiolo-
gist's synthesis that almost converges with "The
Gene."

MULLER'S leading argument is whether there is
"even such a thing as genetic material at all, as distinct
from other constituents of living matter." He responds
that the simplest observation of the developmental life
cycle points to some conserved invariant that persists
from fertilization, through embryonic development
and the formation of gametes, returning to the fertil-
ized egg. This is then complicated by the requirement
for accurate duplication of that invariant, whatever it
may be, under its own influence. Discrete mutations
are then further evidence of correspondingly discrete
particles as the material basis of inheritance. However,
the knowledge of chromosomes now enables an appeal
to much more direct pragmatic evidence, if not yet of
the material composition of the gene, at least of its

Cknetm 129: 313-316 (October, 1991)

cytological location. Most of the genetic research dur-
ing 1900-I 945 was indeed devoted to chromosome
mechanics; today we view Mendelian ratios less as a
fundamental law of biology than of the idiosyncrasies
of chromosome partition in material carefully chosen
for the avoidance of particularities like meiotic drive,
nondisjunction, gene conversion or paternal imprint-
ing.

MULLER turns to the chemical composition of chro-
mosomes as predominantly "nucleoprotein, a com-
pound of protein with nucleic acid, [as] was shown in
analyses of sperm chromosomes by MIESCHER, 1897."
The reference is to a compilation of MIESCHER'S work
for the previous 30 years. MULLER remarks that "only
recently has it become reasonably certain-through
the analogous finding in viruses-that it is really this
major component rather than some elusive accompa-
niment of it which constitutes the genetic material
itself." Protein, rather than monotonous nucleic acid,
is presumably the information-bearer; however, "nu-
cleic acid also exists in highly polymerized form . . . as
may be very significant."

Much of MULLER'S own research had concerned
mutagenesis, including that induced by X-rays. Again,
the gene is a particle with highly circumscribed local-
ity. Mutation can alter one allele and leave its homol-
ogous partner "lying but a fraction of a micron
away. . . undisturbed. " "Blindness and molar indeter-
minacy" characterize mutation. How this can lead to
constructive evolution is usually through the action of
natural selection on ensembles of mutations each with
small effect, and therefore unlikely to be disastrous.
MULLER reaches hard to extract useful hints on the
chemistry of the gene from X-ray mutagenesis. At
least it is internally nonrepetitive or "aperiodic" (per
SCHR~DINGER; but most molecules are), on the feeble
argument that mutation is a discrete event, not a
protracted instability that might speak for continued
internal reshuffling.


3 1 4

,J. Lederberg

News of chemical mutagenesis was just trickling in,
especially of the war gas, mustard (AUERBACH, ROB-
SON and CARR 1947). These studies were inspired by
similarities between mustard gas burns and radiation
damage. MULLER was aware of the mustard gas work
at the time of his lecture, but military restrictions
prevented his mentioning it (CROW 1990). MULLER
also recited more dubious claims from Caltech of
antibody-induced mutations in Neurospora (EMERSON
1944) which, together with PAULINC and CAMPBELL'S
(1942) claims for antibody synthesis by protein folding
in vitro, have been consigned to oblivion. Alkylation
mutagenesis remains a lively research topic today and,
indeed, "These . . . experiments constitute the first
decided break in the impasse that had developed in
studies directed toward the chemistry of the mutation
process." Nevertheless, genetic chemistry has contrib-
uted more to the rather intricate and still problemat-
ical mechanism of mutagenesis than the converse
(DRAKE 1989).

Historically, the validation of chemical mutagenesis
seems long overdue, considering that almost every
molecule is suspect today. This can be attributed in
part to the tediousness of methods, requiring elabo-
rate statistical validation, before bacterial systems
were developed. Furthermore, few of MCTLLER'S con-
temporaries were intellectually positioned to be able
to marry concepts from genetics and chemistry;
MULLER was by no means a sophisticated chemist, but
used an aggressive and insightful imagination in bor-
rowing from the insights of other disciplines.

The new horizon of chemical mutagenesis offered
no obstacle as yet to the concept of evolutionary
indeterminacy. Despite effects on "the frequency of
gene mutation in general, . . . each individual muta-
tion remains a chance and uncontrollable event, from
the macroscopic standpoint." This has remained ge-
netic orthodoxy to the present day, bolstered by re-
vulsion about the criminal excesses of the Lysenkoist
counter-doctrine, It deserves reexamination in the
light of the intricacies of DNA conformation and its
secondary structure, which are indisputably coupled
to regulated gene expression (DAVIS 1989; LEDER-
BERG 1989). MULLER'S consideration of heterochro-
matin position effect as a &-acting influence of chro-
matin coiling on gene expression is a harbinger of
today's second look.

Despite the molar indeterminacy of evolution, and
the disruptive "bad" consequence of most mutations,
"the Maxwell demon of natural selection. . brings
order out of mutation's chaos despite itself." MULLER
could not yet know of the plethora of phenotypically
silent mutations in DNA which today support a much
greater role of mutation pressure and genetic drift in
evolution (KIMURA 1991).

MULLER then turns to nonchromosomal genes.

Chloroplasts are the best worked out; but animal cells
can do without them. Uniparental inheritance con-
strains the diversification of chloroplast genomes, and
their limited content suggests they have a correspond-
ingly small role in evolution. The chloroplast probably
"had a common ancestry with the chromosomal genes,
dating back to the period before the latter had become
organized into typical nuclear chromosomes." This
conjecture was voiced on the brink of a new and
successful cycle of evolutionary attribution of chloro-
plasts to endosymbiotic cyaiobacteria (LEDERBERG
1952; MARGUIJS 1981). LINDEGREN and SPIEGEL-
MAN'S yeast "cytogenes" are also mentioned, but with
cautious reservations about the "links of the evidence"
for the regular production of self-reproducing replicas
from a chromosomal gene. This caution was amply
vindicated. However, proviruses, from lambda to HI\
today, securely occupy that niche. Some exceptional
instances of gene amplification may follow a similar
pattern (STARK et al. 1989).

For nonchromosomal genes in animal cells, MULLER
attends to DARLINGTON and ALTENBURG'S specula-
tions about plasmagenes and viroids. These had some
support from SONNEBORN'S work 011 kappa in Para-
mecium, and this is extensively discussed. But while
"The Gene" was in press, SONNEBORN revised his prior
formulation of a chromosomal origin of kappa, and
MULLER footnotes this. The polemics about such par-
ticles being viruses, symbionts or genes were the im-
mediate stimulant for the overarching concept of plas-
mids (LEDERBERG 1952), which has largely dissolved
the controversy.

Gene duplication within the chromosome is uncon-
troversial. After subsequent divergent mutation, "the
germ plasm becomes not merely more compound but
more complex and . . . the possibilities of organiza-
tional complexity for the body in general should rise
also."

MULLER was the first high peer in genetics to enun-
ciate that "virus particles . . . which fulfil the deflni-
tion of genes in being self-determining in their repro-
duction and capable of transmitting their mutations,
are composed of. . . nothing but nucleoprotein." He
finds appealing DELBRCCK'S early ideas (1941) about
polypeptide template-directed assembly "by means of
a resonance . . . at peptide links followed up by a
finishing up of the peptide connections, and associated
undoing of the" bonding of the old and new chains.
To be sure, there is no evidence that gene synthesis
involves peptide links: and protamines are too simple.
However, some kind of steric complementarity might
pertain between basic proteins and acid DNA. By
1953, WATSON and CRICK would model DNA-DNA
complementarity as the core of modern molecular
genetics. In 1947, chromosome synapsis was a possible
clue to recognitional mechanisms in gene duplication,


Perspectives

though MULLER is quick to emphasize that karyologi-
cal synapsis involves forces over microscopically visible
distances. Especially perplexing (then and today) is
meiosis in Neurospora, where "the chromosomes
come into contact while still in a condensed, closely
coiled condition . . ." MULLER'S response here is an
invocation of temporal vibrations, a premonition of
holography, better left without further comment,
though similar ideas still recur in neurobiological spec-
ulation. Many aspects of synapsis, and the example of
Neurospora's ability to scan for duplicated segments
(SELKER 1990), remain a challenging mystery today.

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Akin to virus replication is pneumococcal transfor-
mation. MULLER'S endorsement was an important tes-
timonial for geneticists of that decade:

In my opinion, the most probable interpretation of these
virus and Pneumococcus results then becomes that of actual
entrance of the foreign genetic material already there, by a
process essentially of the type of crossing over, though on a
more minute scale . that is, there were, in effect, still
viable bacterial "chromosomes" or parts of chromosomes
floating free in the medium used. These might, in my
opinion, have penetrated the capsuleless bacteria and in part
at least taken root there, perhaps after having undergone a
kind of crossing over with the chromosomes of the host. In
view of the transfer of only a part of the genetic material at
a time, at least in the viruses, a method appears to be
provided whereby the gene constitution of these forms can
be analyzed, much as in the cross-breeding test on higher
organisms. However, unlike what has so far been possible
in higher organisms, viable chromosome threads could also
be obtained from these lower forms for in vitro observation,
chenlical analysis, and determination of the genetic effects
of treatment.

This emboldened me to posit a close analogy to the
newly discovered phenomenon of genetic exchange
in bacteria (LEDERBERG 1947). But he could not yet
accept that the transforming activity had been proven
to be pure DNA (contra "nucleoprotein"`). PHOEBES
LEVENE had laid the groundwork of DNA chemical
structure with the elucidation of the constituent de-
oxyribonucleotides and their linkage through phos-
photriester bonds. But the model closest to hand was
that of a monotonous tetranucleotide, which left little
room for genetic informational variety. MULLER left
several hints that larger polymers might alter our
perceptions, but he had no platform for more detailed
chemical modeling or experiment.

How do genes work? MULLER cautions against too
facile a depiction of the gene or its primary product
as an enzyme. I translate his two arguments: that the
known primary gene product is another gene, and
this has properties not shared by known enzymes; and
that developmental pathways will almost always show
pleiotropic complications, viz. several genes affecting
one enzyme even if this is not seen in initial surveys
("new methods will be needed before the primary
gene products can be identified"). I shared this skep-
ticism about the ultimate rigor of the "one-gene:one-

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enzyme" theory,  to the irritation of BEADLE and
HOROWITZ (LEDERBERG 1956). In retrospect, it ws
an indispensable heuristic, and complications like the
intervention of mRNA, RN.4 splicing and editing,
and post-translational modifications could be left for
later historical superimposition on the initial skeleton
of colinearity of DNA with protein.

MULLER was among the first to extrapolate from
basic scientific knowledge of genetic mutation and
evolution to their human implications. Mutational
disorder will eventually afflict the human genome as
a result of the blunting of natural selection by culture;
but this process will take centuries and \ve have time
to educate ourselves in countermeasures of genetic
hygiene. His estimate of one lethal equivalent as the


3 1 6

J. Lederberg

genetic load of recessive mutation in the contempo-
rary human still stands.

Meanwhile, we should be cautious about exposure
to X-rays and to "special chemicals." It is curious that
he makes no reference here to nuclear explosions.
Fallout was yet to enter the lexicon (after the H-bomb
tests), and even then MULLER was concerned that its
effects might be exaggerated in contrast to other
radiation hazards, and in ways that might erode nu-
clear deterrence of Soviet aggression (CARLSON 198 1).
Horrified by the "terrible Nazi perversion of ge-
netics," he believes that "any conscious guidance over
our own genetic processes" be deferred for voluntary
concern, understanding, and better developed social
consciousness. Many psychological traits, in particular,
are attributed to "training or by largely unwitting
conditioning." But eventually social wisdom should
allow "the self-reproduction of the gene and the self-
reproduction of intelligence [to] reinforce one an-
other in an ascending curve."

It will illustrate the impact of this article to list the
citations that appeared in 1947-l 954 (Figure 1).

LITERATURE CITED

AUERRACH, C., J. M. RORSON and J. G. CARR, 1947  The chemical
  production of mutations. Science 105: 243-247.
AVERY, 0. T., C. M. MACLEOD and M. MCCARTY, 1944 Studies
  on the chemical nature of the substance inducing transforma-
tion of pneumococcal types. J. Exp. Med. 79: 137-158.
CARLSON, E. A., 1981  Genes, Radiation, and Society. The Lzj2 and
  Work of H. J. Muller. Cornell University Press, Ithaca, N.Y.
CROW, J. F., 1990  H. J, Muller, scientist and humanist. Wis. Acad.
  Rev. 36(3): 19-22.
DAVIS, B. D., 1989  Transcriptional bias: a non-Lamarckian mech-

  anism for substrate-induced mutations. Proc. Natl. Acad. Sci.
  USA 86: 5005-5009.
DELBRUCK, M., 1941  A theory of autocatalytic synthesis of poly-
peptides and its application to the problem of chromosome
reproduction. Cold Spring Harbor Symp. Quant. Biol. 9: 122-
  126.

DRAKE, J. W., 1989 Mechanisms of mutagenesis. Environ. Mol.
  Mutagen. 14(16): 11-15.
DUBOS, R. J., 1945  The Bacterial Cell. Harvard University Press,
  Cambridge, Mass.
EMERSON, S., 1944  The induction of mutation by antibodies. Proc.
  Natl. Acad. Sci. USA 30: 179-183.
HAYNES, R. H., 1989  Genetics and the unity of biology. Genome
  31: l-7.

KIMURA, M., 1991 Recent development of the neutral theory
viewed from the Wrightian tradition of theoretical population
genetics. Proc. Nat]. Acad. Sci. USA 88: 5969-5973.
LEDERBERC, J., 1947  Gene recombination and linked segregations
  in Escherichia coli. Genetics 32: 505-525.
LEDERBERG, J., 1952  Cell genetics and hereditary symbiosis. Phys-
  iol. Rev. 32: 403-430.
LEDERBERG, J., 1956  Comments on gene-enzyme relationship, in
Enzymes: Units of Biological Structure and Function (Ford Hos-
pital Interational Symposia), edited by 0. H. GAEBLER. Aca-
  demic Press, New York.

LEDERBERG, J., 1989 Replica plating and indirect selection of
  bacterial mutants: isolation of preadaptive mutants in in bac-
teria by sib selection. Genetics 121: 395-399.
MARGULIS, L., 1981 Symbiosis in Cell Evolution. Freeman Press,
  San Francisco.
MULLER, H. J., 1947  The gene. Proc. R. Sot. Land. B 134: l-37.
PAULING, L., and D. H. CAMPBELL,, 1942 The manufacture of
antibodies in vitro. J. Exp. Med. 76: 2 1 l-220.
SELKER, E. U., 1990  Premeiotic instability of repeated sequences
  in Neurospora crassa. Annu. Rev. Genet. 24: 579-6 13.
STARK, G. R., M. DEBATISSE, E. GIULOTTO and G. M. WAHL,
  1989  Recent progress in understanding mechanisms of mam-
malian DNA amplification. Cell 57: 901-908.
WATSON, J. D., and F. H. C. CRICK, 1953  Molecular structure of
nucleic acids. A structure for deoxyribose nucleic acid. Nature
  171: 737-738.