iv PREFACE 



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 energv 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 



