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CHAPTER 36 



REGULATOR 

 GENE 



REPRESSOR 

 SUBSTANCE 



T_n_r 



OPERON 



_V^ 



Operator Structural 



gene genes 



. O . A B 



WSAA/S/N Messenger 

 RNA 



Proteins 



f \J\j 



Metabolite 

 removes 

 repressor 



figure 36-2. Relationships 

 between a regulator gene and 

 the operator and structural 

 genes of an o per on. 



to specify a chemical product (such as an 

 amino acid sequence in a polypeptide ) but 

 to control the function of other genes. 

 Consequently, operator genes can be called 

 genes for function in contrast to those which 

 specify chemical structures and are accord- 

 ingly called genes for structure (Figure 36- 

 2). 



An operator gene coordinates the expres- 

 sion of linear gene neighbors. In our model 

 the genes controlled are related in that both 

 structural genes affect the biochemical path- 

 way involving lactose utilization. This sit- 

 uation suggests that there is, at least in some 

 cases, a unit of gene function, intermediate 

 in size between the gene and the chromo- 

 some, which can be called an operon. An 

 operon is a linear group of genes whose 

 structural activity is coordinated by a func- 

 tional gene, or operator, located at one end? 

 An operon probably represents the length 

 of genetic material whose complementary 

 RNA comprises one strip of messenger 

 RNA; the operon, therefore, may well be 

 a unit of transcription. When the operon 



- The discussion of operons and operator genes is 

 based mainly upon work of F. Jacob, D. Perrin, 

 C. Sanchez, and J. Monod (1960), and F. Jacob 

 and J. Monod ( 1961 ). 



is functioning, a strip of messenger RNA 

 is produced containing information for all 

 the structural genes in the operon. The 

 messenger RNA is then translated starting 

 from one end. 



A leucine auxotroph, leu 500, is caused 

 by an operator mutant, o' , in the leucine 

 operon of Salmonella typhimurium. 3 Using 

 transduction to replace this mutant with the 

 wild-type operator o + restores leucine pro- 

 totrophy. When leu 500 (o' ) individuals 

 are plated on complete medium without leu- 

 cine, some large colonies are formed as a 

 result of reverse mutation to o + . Some 

 small colonies, however, are also produced. 

 When tested by transduction, the small col- 

 onies all prove to have a common property 

 — all are mutant at the same locus, sup- 

 pressor of leu 500 (su leu 500), located 

 outside the leucine operon between the tryp- 

 tophan and cysteine operons. These mu- 

 tants of su leu 500 are of point mutation 

 or deletion type; they no longer completely 

 suppress o' , and, consequently, partial leu- 

 cine prototrophy is restored. Su leu 500 

 is not the normal regulator gene for the leu- 

 cine operon; when su leu 500 is deleted, 



:i See F. H. Mukai and P. Margolin (1963). 



