On the Evolution of Biochemical Syntheses

Abstract

Although it has been recognized for a long time that the biochemistry of the organism is conditioned by its genetic constitution, a more precise definition of this dependence has not been possible until recently.A con- siderable amount of evidence now exists for the view that there is a one-to- one correspondence between genes and biochemical reactions.This con- cept, foreshadowed in the work of Garrod' on human alcaptonuria, ac- counts in a satisfactory way for the inheritance of pigment formation in guinea pigs,2 insects3 and flowers,4 and the synthesis of essential growth factors in Neurospora.5It appears from these studies that each synthesis is controlled by' a set of non-allelic genes, each gene governing a different step in the synthesis.As to the nature of this control, it is probable that the primary action of the gene is concerned with enzyme production.That genes can direct the specificities of proteins has been shown in the case of many antigens,6 while several mutations demonstrably affecting the production of enzymes have been reported.6Evidence on the postulated gene-enzyme relationship is in most cases, however, still circumstantial; this is partly because of technical difficulties involved in the study of synthetic, or free-energy consuming reactions in vitro, and partly because of the insufficiency of biochemical information on those reactions which happen to be susceptible of genetic analysis.As a corollary of the above hypothesis, each.biosynthesis depends on the direct participation of a number of genes equal to the number of different, enzymatically catalyzed steps in the reaction chain.In at- tempting to account for the evolutionary development of such a reaction chain one meets in a clear form the problem of explaining macroevolution- ary changes in terms of microevolutionary steps.The individual reactions making up the chain are of value to the organism only when considered collectively and in view of the ultimate product.Regarded individually, intermediate substances cannot, in general, be assumed to have physio-