VI. RNA AND CODING PROBLEMS 277 



like the homologous DNA in iiianiinalian systems (Boulanger and 

 Montreuil, 1952; Hokin and Hokin, 1954; Logan and Davidson, 1957). 

 Here again, such observations were attainable only where relatively 

 short periods of isotope feeding w'cre employed and the composition of 

 newly formed RNA was an apparent one, evaluated from the specific 

 radioactivity of its mononucleotides. With the advent of autoradiogra- 

 phy, particularly where tritium-labeled precursors are involved, experi- 

 mentation with a large variety of organisms makes it increasingly clear 

 that most, if not all, RNA synthesis of the cell starts within the nucleus 

 — the initial product may be exclusively a DNA-like RNA (see Chapter 

 II). 



III. Mechanism of Messenger RNA Function 

 Based on in Vivo Experiments 



A. ENZYME INDUCTION 



Perhaps the most striking characteristic of messenger RNA function 

 in microbial systems is its short lifetime with respect to activity. That 

 is, if it behaves either as a template or a catalyst, it must be an 

 extremely unstable one. Evidence by way of independent techniques 

 suggests that inactivation of messenger RNA is a direct result of its 

 action, and, as a matter of fact, a reasonable degree of stoichiometry 

 between messenger RNA and biologic product has been demonstrated. 



The synthesis of the enzyme ^S-galactosidase is initiated almost im- 

 mediately at a maximum rate by addition of the inducer, while synthesis 

 is stopped in an equally short time interval by removal of the inducer 

 (Monod, 1958; Boezi and Cowie, 1961; Roberts et al, 1961). Essentially 

 similar kinetics of ;8-galactosidase synthesis are obtained by adding 

 and removing the responsible gene (Pardee ef al., 1959; Riley et al., 

 1960). Insertion of the ^g-galactosidase gene by bacterial conjugation 

 results in enzyme synthesis within 2 minutes; moreover, the subsequent 

 rate of enzyme synthesis per zygote is constant. Experiments with 

 heavily labeled (P^-) male parental bacteria, involving ample time for 

 gene expression after insertion, followed by various periods of P''- decay, 

 reveal that the rate of enzyme synthesis decreases as a function of P-'- 

 atoms decayed. Thus the integrity of the z'^ gene is constantly required 

 for continued enzyme synthesis. These data on /?-galactosidase induction 

 are interpreted by Jacob and Monod (1961) to mean that the structural 

 messenger of the gene is rapidly formed and exists as a short-lived 

 intermediate. The group at the Carnegie Institution of Washington 

 (Roberts et al., 1961; Boezi et al.. 1961) has described similar kinetics 

 for the induction of /3-galactosidase and has suggested a model for the 



