Chapter 36 



REGULATION OF GENE 

 ACTION-OPERONS 



E 



xtensive study of any organ- 

 ism reveals a large number 

 of alternative traits with a 

 genetic basis. Some of these alternatives 

 result from the presence or absence of ge- 

 netic material (for example, in Paramecium 

 "cytoplasmic DNA" can depend upon the 

 presence of kappa); other alternative traits 

 involve the relocation of genetic material 

 (for example, changes in episomal state or 

 the inversion of a chromosomal segment). 

 But the presence, absence, and location of 

 genetic material do not describe the mecha- 

 nism operating on the affected cell or organ- 

 ism, or the ways genetic material performs 

 a function. 



We are, therefore, especially interested in 

 studying those alternative traits resulting 

 from some action by or involving genetic 

 material. Self-replication, one action typi- 

 cal of what has been defined as genetic 

 material, must have some phenotypic conse- 

 quences due to the removal of gene pre- 

 cursor material from the pool of metabolic 

 substances and to the presence of new ge- 

 netic material. (Gene control mechanisms 

 which act via gene replication were consid- 

 ered in the previous chapter.) 



Evidence for the occurrence of gene ac- 

 tion without gene replication is provided in 

 numerous cases, including abortive trans- 

 duction and highly functional cells which 

 never divide again (neurons, for example). 

 We already know that whenever phenotypes 

 are dependent upon protein synthesis, gene 

 457 



action requires the formation of messenger 

 RNA. Using DNA to make complemen- 

 tary DNA — that is, for gene replication 

 — may inhibit its use in making comple- 

 mentary messenger RNA — that is, for gene 

 functioning via polypeptide synthesis. This 

 may be true even if DNA polymerase uses 

 the major groove and RNA polymerase the 

 minor groove of double-helix DNA. That 

 gene action is sometimes controlled by ge- 

 netic means was demonstrated by finding 

 regulator genes (such as Activator in maize, 

 p. 385). We should not exclude the possi- 

 bility that DNA genes can produce pheno- 

 typic effects using mechanisms other than 

 DNA replication and messenger RNA for- 

 mation. 



Gene action can be controlled nongenet- 

 ically. A series of enzymatic reactions is 

 usually required to produce a particular 

 metabolic end product. In many cases the 

 end product inhibits the functioning of one 

 of the first enzymes in the pathway. Such 

 end product inhibition of an enzyme, very 

 widespread in bacteria, provides immediate 

 and sensitive control of the rate of synthesis 

 of many metabolites. Enzyme inhibition by 

 end product is one example of controlling 

 gene action by a feedback mechanism. The 

 possibility also exists that gene action can be 

 regulated more directly — at a stage prior to 

 protein synthesis. 



Recall (p. 357) that the Lac segment of 

 the E. coli genetic map contains three re- 

 combinationally separate genes. The y + 

 gene specifies the structure of the enzyme 

 (3-galactoside permease; z + specifies the 

 structure of the enzyme (H-galactosidase 

 (certain alleles of z cause the synthesis of 

 a modified, enzymatically inactive protein, 

 called Cz. identified by its specific antigenic 

 characteristics); the third gene, /' + , specifies 

 the synthesis of a repressor substance which 

 prevents y + and z + from producing per- 

 mease and galactosidase. In the presence 

 of lactose (which supplies the substrate 



