determination of enzyme structure. The "central dogma," in fact,
states that the gene (DNA) carries the code of the amino acid
sequence of its product enzyme and that a specific messenger RNA
transfers this code from DNA to the site of enzyme formation. Thus,
both DNA and RNA might be considered as regulatory elements of
the cell. Discussions of regulation by the nucleic acids are presented
in two chapters. It is now clear, however, that the action of all
genetic material cannot be accounted for simply in these terms. Genes
have been found which control quantitative aspects of enzyme
formation. Enzyme repression has been known for a number of
years, but it has recently been shown that repression is genetically
determined by a locus distinct from the locus controlling the struc-
tural characteristics of the formed enzyme. Distinct genetic ele-
ments therefore appear to control the qualitative and quantitative
characteristics of enzyme formation. Enzyme repression together
with its genetic control thus represents a system of major interest in
terms of regulatory mechanisms; this is treated in the second chapter.

All regulation is not directly gene-controlled. Control of bio-
chemical reactions and biochemical sequences by feedback provides
an elegant mechanism for the maintenance of metabolic balance.
This topic is discussed in a separate chapter. The biochemical basis
of regulation by hormones is receiving intensive attention at present,
as are regulatory mechanisms in glycolysis. Two chapters deal with
hormonal regulation in animals and in plants; another examines
regulation in energy metabolism. Light and time as regulatory
factors are additional fields of active inquiry; discussions of each are
included.

As convener of this symposium, I should like to express my ap-
preciation to the contributing authors for their cooperation in the
preparation of this volume. On behalf of the Society of General
Physiologists, I wish to express appreciation to the National Insti-
tutes of Health, United States Public Health Service, whose financial
support made possible the symposium and this volume.

DAVID M. BONNER

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