Differentiation and Morphogenesis in Insects 245 



Control of gvnetie information 

 in microbial systems 



The central dogma has, more recently, heen extrapolated to account 

 for the moment-to-moment control of enzyme synthesis in microbial 

 systems. This extrapolation takes the following form (Jacoh and 

 Monod, 1961; Jacoh and Wollman, 1961) : 



There are certain enzymes which are necessary for the survival of 

 the cell at all times, for example, all the enzymes which lie on the 

 main pathways of intermediate metabolism and energy transforma- 

 tions. These so-called "constitutive enzymes" are continuously syn- 

 thesized — a state of affairs which signals the activity of the corre- 

 sponding genes. By contrast, there are a large number of other enzymes 

 which are needed only under certain conditions and circumstances. 

 These "inductive enzymes'* are not synthesized until they are needed; 

 therefore, the genes for their synthesis are ordinarily maintained in 

 a state of chronic repression. 



The theory tries to account for the ability of specific metabolites 

 to induce the synthesis of specific enzyme systems. It is postulated 

 that the synthetic reactions for groups of enzymes serving a given bio- 

 chemical pathway are programmed by a collection of genes which 

 are under the control of a single "operator gene," the whole complex 

 being known as an "operon."' But under most conditions the operator 

 gene is in combination with a specific "repressor substance." and the 

 corresponding operon is thereby repressed. 



Each repressor substance is the synthetic product of a particular 

 "regulator gene." The repressor substance is freely diffusible. If all 

 goes well, it combines with and represses its specific target. But on 

 its way to the operator gene, the repressor substance is subject to 

 inactivation if it encounters and combines with a substrate of the 

 particular enzyme system that is repressed. In this case the operator 

 gene escapes from repression and can go ahead and activate the col- 

 lection of genes in its operon. So, in this indirect manner, a given 

 substrate induces the synthesis of an appropriate enzyme system. 



It is also proposed that certain regulator genes produce inactive re- 

 pressor substances which can be activated only by combination with 

 specific small molecules. This accounts for the ability of metabolic 

 end products to suppress the on-going synthesis of specific enzyme 

 systems. 



In this bird's-eye view of the theory, one cannot fail to be impressed 

 by the increasing attention that centers on mechanisms of control. 



