120 CYTOCHEMISTRY OF PROTEINS 



Thus from the combination of biochemical and radiation stud- 

 ies it appears that new protein formation markedly precedes 

 new nucleic acid formation. But the formation of new genetically 

 active particles is not complete until new nucleic acid is formed. 



Further information may be derived from studies of bacteria 

 which have multiple infection, i.e., more than one phage particle 

 per cell. Lea and Salaman (1946) had concluded that a large 

 phage particle has a zone sensitive to X-rays which is composed 

 of at least 14 distinct units. Luria and Latarget (1947) and 

 Luria (1947), using ultraviolet irradiation, obtained results com- 

 patible with the conclusions drawn from the use of X-rays, and 

 showed that there are 30-50 dissociable loci in a phage particle, 

 all of which are necessary for successful reproduction of phage. 

 In cases of multiple infection, reproduction was still possible so 

 long as at least one of each type of locus remained in the cell, 

 i.e., new phage could be produced as a result of some form of col- 

 laboration between different loci of separate phage particles in 

 the same cell, although none of the phage particles taken singly 

 in a cell could reproduce. 



If it were permissible to generalise from the results on phage to 

 protein production in general, one would be tempted to ask 

 whether protein production is in fact determined by nucleic acid. 

 In the case of phage the evidence, though not definitive, provides 

 a hint that protein production may be independent of nucleic 

 acids, or alternatively that the nucleic acids may be responsible 

 merely for the organisation of synthesized protein, or for some 

 final stage in protein formation other than synthesis of peptide 

 bonds. It also seems certain that nucleic acid phosphorus is not 

 necessarily labile during protein synthesis. But to compare the 

 evidence obtained with phage with cytochemical studies is in any 

 event premature, because of a number of technical weaknesses in 

 the evidence available from cytochemical methods. These weak- 

 nesses include: 



(a) The fact that little work has been done on the cytochemi- 

 cal changes of individual cells through a cycle of growth and divi- 

 sion. Most of the results available have been obtained on fixed 

 material, the cells of which are in different stages of growth. The 

 exact placing of individual cells of a fixed preparation in a se- 

 quence corresponding to their growth stages is necessarily some- 

 what a matter of opinion, and to that extent unsatisfactory. 



