STRUCTURAL AND CHEMICAL ARCHITECTURE OP HOST CELLS 139 



the cell regardless of the presence of an exogenous mducer. The validity of 

 this qualitative distinction between classes of enzymes is stiU obscure for the 

 following reasons: 



a. It is possible that the inducers for "constitutive" systems are generated 

 endogenously. 



b. No inducible enzyme systems are known wliicli are entirely missing 

 when cells are grown in the absence of an exogenous uiducer. 



c. By a process of selection, organisms may be obtained in which relatively 

 high levels of inducible enzymes are formed in the absence of exogenous 

 inducer, i.e., inducible enzymes may become constitutive. 



d. Many constitutive enzymes may be synthesized in greater amounts if 

 an inducer is present under appropriate conditions. 



Nevertheless, the distinction is useful in describing quantitative relations 

 among enzymes in the presence or absence of specific inducers. Thus, E. coli 

 will be able to metabohze glucose without delay and contams constitutive 

 hexokmase whether grown on glucose or D-xylose. On the other hand, if 

 grown in glucose, E. coli will contain the merest trace of xylose isomerase and 

 subsequent systems for metabolizing xylulose, the first reaction product 

 of xylose utilization. Such cells vvdU not be able to handle xylose at 

 a significant rate unless a far greater biosynthesis of these enzymes is 

 induced. It may be suggested as one hypothesis that organisms do indeed 

 have a more or less fixed battery of metabolic systems ("constitutive" 

 enzymes) and that the mducible systems represent more or less disposable 

 baggage whose main function is to insert substrates into constitutive systems 

 when such substrates become available. It would be inefiicient to have to 

 make such enzymes even when these substrates are not present. On the other 

 hand, it will be seen below that enzyme levels for the synthesis of even 

 essential metabohtes may vary considerably as a result of the action of 

 various controllmg mechanisms. 



In one instance different constitutive and inducible enzymes are known for 

 the cleavage of the same substrate, maltose, in the same bacterium, E. coli 

 stram K12 (Pontieri, 1955). Thus, the constitutive maltase is an a-gluco- 

 sidase, converting maltose to 2 moles of glucose. The adaptive maltase is an 



under suitable conditions of gratuitj^ it has been observed that the synthesis of the new 

 enzyme, j8-galactosidase, does indeed occur at similar rate in all the cells (Benzer, 1953). 

 This study was done by takmg advantage of the fact that bacteria infected by certam 

 phages are no longer inducible (Monod and Wollman, 1947). Infection at different times 

 after induction thus fixes the enzyme content of the cell and this in turn governs the 

 rate of utilization of lactose, the rate of production of virus, and the develoj^ment of 

 cell lysis. The extent of lysis may be quantitated by estimating the amount of enzyme 

 liberated into the medium. 



A different result has been obtained at low inducer concentrations in which only some 

 cells are induced (Novick and \^'iener, 1957). 



