126 CONTROL MECHANISMS IN CELLULAR PROCESSES 



that high protein production can be achieved in insect cells only by 

 increasing the complement of genes, either by polyploidy or by 

 polyteny, as in the salivary glands of Diptera. It is, therefore, even 

 more imperative that all of the genes thus multiplied do not remain 

 active. As was discussed previously, the gene activity can be seen 

 directly, in salivary gland chromosomes, by the appearance of "puffs" 

 (Beerman, 1952; Breuer and Pavan, 1955), indicating that only few 

 loci actively produce RNA. 



Control of RNA Activity 



In the preceding chapter, the amount of RNA and protein syn- 

 thesis in the cell was shown to depend on the rate of RNA produc- 

 tion by the genes. It was assumed, to simplify the argument, that 

 RNA, once formed, produces proteins without further impairment. 

 In reality, many factors inside and outside the cell can also con- 

 tribute to the control of protein production and influence the func- 

 tioning of RNA. 



The level of RNA present in the cytoplasm will depend on an 

 equilibrium between the formation rate and destruction rate. If 

 RNA once formed remained active in the cell for the rest of cell life, 

 it may continue to produce proteins which in later cell life are un- 

 necessary or undesirable. This would interfere with the effective 

 control of cell activity and, therefore, RNA should have a limited 

 life. Different rates of destruction could effectivelv control the 

 amounts of various types of RNA present in the cytoplasm, inde- 

 pendent of direct gene control. To postulate such differential rates 

 would mean another set of regulators for RNA activity, specific for 

 each type of RNA molecule. Such control would require a redupli- 

 cation of genie controls in cytoplasm, whicli seems improbable. It 

 may be, however, that the RNA particle has its own life span deter- 

 mined by its structure and activity. High protein production rate 

 may accelerate the inactivation of the RNA particle, or the life of 

 the particle may be proportional to the number of protein molecules 

 made. It is interestins; to note that ribosomes have their own ribo- 

 nuclease built in the particle (Elson, 1959). This enzyme is inactive 

 as long as the particle is intact, but becomes active after certain 

 chemical treatments. Apart from such a possibility of differential 

 destruction rates for different types of RNA, it may be also that all 

 the RNA in the cytoplasm has a certain half-life, independent of its 



