I08 NITROGEN METABOLISM 



presence of the specific substrate [44], and that the action 

 spectrum for light of different wavelengths resembles the 

 absorption spectra of the nucleic acids [43]. 



Such experiments have naturally focused attention on the 

 possible role of the nucleic acids in protein synthesis and so 

 far two theories have been proposed: one suggests that the 

 bond energy of the nucleic acid phosphate groups is used 

 for the synthesis of peptide bonds, whilst the other regards 

 the nucleic acids as being the fundamental components of 

 the organized systems controlling the sequence in which 

 amino-acids are joined together. Proteases appear to possess 

 well-defined specificity with regard to the peptide bonds 

 they attack (p. 113), and if they can in fact function syntheti- 

 cally the same specificity is to be expected in the reverse 

 reactions. If such enzymes can be organized so that they act 

 in a predetermined sequence, a mechanism can be envisaged 

 which possesses the ability to synthesize a peptide chain with 

 the required serial arrangement of amino-acid residues. 

 Since nucleic acids differ in their composition and structure 

 and readily form complexes with proteins, it is possible that 

 different nucleic acids combine specifically with different 

 proteins. Hence an organized system of nucleic acids may 

 provide a framework on to which enzymes are adsorbed in 

 a particular order, with the result that they then direct the 

 synthesis of a specific substance [17]. It is also feasible that 

 the nucleic acids are structures on to which amino-acids 

 rather than enzymes are adsorbed, and that differences in 

 nucleic acid structure give rise to different sequences of 

 amino-acids [3]. If there is any truth in such speculations, 

 it is to be expected that disruption of nucleic acid meta- 

 bolism will immediately result in the cessation of protein 

 synthesis. It could therefore be argued that the effects of 

 ultraviolet light described above are due to disruption or 

 degradation of the organized nucleic acid systems directing 

 adaptive enzyme synthesis. Bacteriophage are composed of 

 nucleoprotein and they can infect and reproduce in irradi- 

 ated cells of Esch. coli, although such cells are unable to 

 form the adaptive enzyme ^^-galatosidase [19], and this may 

 be construed to mean that irradiated cells can still synthesize 



