Chapter 35 



REGULATION 



OF GENE SYNTHESIS 



A' 



s revealed by in vitro studies, 

 DNA synthesis in biological 

 systems requires the follow- 

 ing: primer-template DNA; the nucleoside 

 5'-triphosphates of A, T, C, and G; Mg+ + 

 ions; and DNA polymerase in an aqueous 

 solution of proper pH and temperature. In 

 an extended in vitro synthesis of DNA using 

 E. coli DNA polymerase, we permit the re- 

 action to proceed in a largely uncontrolled, 

 unregulated manner until the supply of one 

 of the raw materials is exhausted or until 

 some other factor becomes limiting. The 

 synthesis of DNA in vitro can be controlled, 

 however, by changing one or a combination 

 of the required factors. For example, we 

 may choose to omit one of the triphosphates 

 containing a base present in the primer- 

 template. Reducing the amount of such a 

 triphosphate or utilizing one which contains 

 a base analog can control the rate and/or 

 amount of DNA synthesized. The synthetic 

 reaction can be slowed down or even partly 

 reversed by excessive additions of pyrophos- 

 phate. In other words, there are a large 

 number of ways by which the biochemist 

 can regulate the synthesis of DNA in vitro. 

 Such knowledge should be quite valuable 

 in helping us answer the question: In pres- 

 ent-day organisms how is gene synthesis 

 regulated in vivo? 



This question presupposes that DNA syn- 

 thesis in vivo is regulated, and ample evi- 

 dence, some of it already presented, sup- 

 ports such a view. The regulation of DNA 

 449 



synthesis at the cellular level is revealed by 

 observations that DNA synthesis occurs dur- 

 ing interphase and ceases during nuclear 

 division. Evidence of regulation at the 

 genomic level is provided by the fact that 

 when DNA synthesis stops, the nucleus is 

 euploid for DNA (± about 10%) — even 

 if the nucleus fails to divide and comes to 

 contain a multiple (polyploid or polynemic) 

 genome content. DNA synthesis is also 

 regulated chromosomally since largely het- 

 erochromatic chromosomes replicate at a 

 different time than largely euchromatic ones, 

 and intrachromosomally since the hetero- 

 chromatic and euchromatic regions within a 

 chromosome are synthesized at different 

 times during interphase. It should be noted 

 that the occurrence and amount of "natural 

 dAT" in different crabs is probably gene 

 controlled. 1 



DNA Synthesis in Uninfected and 

 Phage-lnfected Bacteria 



Let us discuss further the regulation of gene 

 synthesis by exploring the biochemical path- 

 ways (summarized in Figure 35-1 ) so impor- 

 tant in the synthesis of the four usual types 

 of deoxyriboside triphosphates in uninfected 

 E. coli. Since the hypothesis that genes 

 specify all protein synthesis in an organism 

 is now generally accepted, whenever these 

 reactions involve enzymes, the control of 

 gene synthesis by gene action is also in- 

 volved. 



In the presence of deoxyribosidase or re- 

 ductase, the riboside diphosphates of A, G, 

 C, and U are converted to the corresponding 

 deoxyriboside diphosphates by removal of 

 the O at the 2' position. 2 The energy source 

 for this reaction is ATP. The dTP is syn- 

 thesized from d\JP by adding a methyl group 

 at the 5 position in the presence of thymidy- 

 late synthetase. (A thymine-requiring strain 

 of E. coli lacking this enzyme is known. 



!See M. Smith (1963). 

 -' See A. Larsson (1963). 



