Chapter *46 



GENE ACTION AND OPERONS 



T! 



|he extensive study of any or- 

 ganism reveals a large number 

 of alternative traits which have 

 a genetic basis. Some of these traits de- 

 scribe the presence or absence of genetic 

 material (for example, the trait "cytoplasmic 

 DNA" in Paramecium may be due to the 

 presence of kappa). Other traits involve the 

 relocation of genetic material (for example, 

 changes in episomal state, or the inversion 

 of a chromosomal segment). But such al- 

 ternatives as the presence, absence, and 

 movement of genetic material do not describe 

 how the cell or organism is affected, or in 

 what ways genetic material performs a func- 

 tion. 



We are especially interested in studying 

 those alternative traits which result from 

 some action by, or involving, genetic material. 

 One action, typical of what we have defined 

 as genetic material, is self-replication. You 

 will admit that self-replication must have 

 some phenotypic consequences due to the 

 removal of gene precursor material from the 

 pool of metabolic substances and to the 

 presence of new genetic material. Can all 

 genie action be ascribed to the metabolic 

 changes which take place because of genie 

 self-replication? We know of several kinds 

 of situations in which there is no evidence 

 of gene replication, yet there is evidence of 

 genie action. One example is found in the 

 case of abortive transduction; another is 

 provided by highly functional cells which 

 never divide again, for example, neurons. 

 We can conclude, therefore, that conserved 

 421 



genetic material also functions by some 

 mechanism other than self-replication. A 

 study of inborn errors of metabolism, and 

 of the pedigree of causes for pleiotropic ef- 

 fects of mutants, led us to hypothesize 

 (Chapter 32) that a gene has a single primary 

 function. You realize now that this hypoth- 

 esis refers to some action by genetic material 

 other than self-replication. Depending upon 

 the particular trait we consider to be primary, 

 the scope of genetic material essential for this 

 function, that is, the length of a cistron, will 

 vary (Chapter 42). The general hypothesis 

 of one cistron-one function (besides self- 

 replication) was tested in the specific form 

 of one cistron-one polypeptide chain (Chap- 

 ter 32). The results demonstrated that the 

 specific form of the general hypothesis is 

 acceptable. 



It was mentioned, on p. 376, that Salmo- 

 nella has at least eight closely hnked loci 

 (Figure 46-1), all having an effect upon the 

 sequence of chemical reactions leading to the 

 biosynthesis of histidine. Already four of 

 the eight loci have been correlated with 

 specific enzymes,^ thereby providing addi- 

 tional evidence for the hypothesis one poly- 

 peptide-one cistron. Although it was also 

 pointed out at that time that close linkage of 

 genes controlling different parts of a biosyn- 

 thetic sequence is not a universal phenom- 

 enon, let us consider some finer genetic de- 

 tails of the lactose, Lac, locus in E. coli which 

 is, in this respect, similar to the histidine 

 locus in Salmonella. 



You may remember, from p. 369, that the 

 Lac segment contains three cistrons. The 7+ 

 cistron specifies the structure of the enzyme 

 galactoside permease, while z+ is the gene 

 that specifies the structure of the enzyme 

 /3-galactosidase. (Certain alleles of z result 

 in the synthesis of a modified, enzymatically 

 inactive, protein, called Cz, which can be 

 identified by its specific antigenic charac- 



1 See P. E. Hartman, J. C. Loper, and D. Serman 

 (1960). 



