136 THE BIOSYNTHESIS OF PROTEINS 



nucleotide sequence. In view of the present knowledge of the interactions 

 between polynucleotides a sensible hypothesis is that the active repressing 

 agent contains a nucleotide sequence complementary to that of the operator 

 or to its cytoplasmic replica. The polynucleotidic repressing agent could be 

 made by the controlling gene (i+ for galactosidase) which indeed does not 

 seem to control the structure of any protein (Pardee and Prestidge, 1959). 

 A more difficult matter is to fit the exogeneous, or endogeneous, inducers 

 like lactose into the picture ; do they cause or prevent the production of the 

 repressor at the level of the gene, do they become associated with a specific 

 nucleotide sequence (Szilard, 1960), butthenby what enzyme action? Where 

 does the competition take place? Thus we are confronted with the same kind 

 of questions as before. But because they are asked in new terms, these 

 questions will certainly suggest new experim.ents which will throw new 

 light upon the mechanism of induction and repression of enzyme synthesis. 



Torriani (1956) and Halvorson and Jackson (1956) observed that the 

 early phase of induction is exceptionally sensitive to ultraviolet irradiation. 

 During the lag period of maltase induction by maltose in resting yeast, a 

 fraction of RNA becomes acid soluble without being degraded to free 

 nucleotides, and this fraction is reincorporated into acid insoluble RNA 

 when the synthesis of the enzyme begins (Gobert — see Chantrenne, 1958). 

 This small RNA fraction is released from the ribosomes (Gobert, unpubl.). 

 One may wonder whether these observations have some bearing on the 

 transitory participation of fragile nucleotidic compounds in the process 

 which triggers enzyme synthesis. 



Finally, it may be remarked that if induction and repression of the 

 synthesis of enzymes is a process in which the specific templates are put to 

 operation or switched off^, the rate of production of an enzyme must be 

 determined by the number of templates which are in operation. In a con- 

 stitutive strain, the rate of production of an enzyme must depend on the 

 total number of templates corresponding to the enzyme. Almost nothing 

 is known about the number of identical templates in the cell, neither is 

 it known when the templates are produced: are they being continuously 

 produced or only at one stage of cell division (Mitchinson, 1958; Plaut, 

 personal communication)? Do all the structural genes produce the same 

 number of cytoplasmic replica, or is another regulation mechanism operat- 

 ing on the production of templates? In haemoglobin synthesis in man, the 

 templates depending on two homologous genes present in a heterozygote 

 produce their specific protein at about the same rate, since the amounts of 

 normal and sickle cell haemoglobins produced are comparable. 



It will be noticed that a large part of the present picture of induction and 

 repression of enzyme synthesis is derived from studies on a few micro- 

 organisms and a few enzymes. By far the most thoroughly analysed case is 

 ^-galactosidase synthesis in E. coli; it has been the object of exceptionally 



