BIOSYNTHESIS OF MACROMOLECULES 619 
RNA molecule has specific segments or areas, that will attract and hold specific 
activated amino acids in appropriate spatial relations to form a chain with a given 
sequential order. The events that precede completion of a polypeptide chain might 
be of various kinds. For example, it is possible that activated substrate molecules 
find their places in random temporal order; when the template is full, the formation 
of all peptide bonds is somehow completed. The most plausible alternative is the 
deposition of the amino acids in sequential order, peptide bonds being formed one 
at a time. 
We have shown’ that turnover rate studies alone are unlikely to discriminate 
between these two major alternatives. However, there is some experimental evidence 
favoring the second choice’.*. We shall therefore adopt it as a working hypothesis. 
To this we shall add the assumption that a single enzyme, specific merely for the 
formation of the peptide bond, is involved at every step. This is modelled on the 
findings of KORNBERG et al.!° on DNA synthesis, where a single polymerase forms all 
the phosphodiester linkages, the sequential specificity being determined by a DNA 
template, usually termed primer. The final step of protein synthesis is conceived 
as the release of the completed polypeptide chain from the template; this step 
might very well be non-enzymatic. 
THE BIOSYNTHETIC EQUATIONS 
We now propose to write down the reaction equations that represent our model, 
and the differential equations that are their consequences. But first a little about 
notation. We shall adorn the various symbols with subscripts, primes, or other 
affixes as may be suitable to make distinctions and denote relationships. We shall 
denote the amino acid precursors by x with suitable subscripts; thus x, shall stand 
for the precursor of the first amino acid of some polypeptide chain, x, for the pre- 
cursor of the second, and so forth. The corresponding amino acids themselves will 
be y,, V2, and so on, while the activated forms will be distinguished by primes (’), 
yy’, Vy’, etc. Concentrations outside the cell will be marked by the added subscript o: 
Xo, for the external concentration of x,, and so on. The S-RNA classes will have 
the subscripts of their amino acids, so that 7, will stand for the S-RNA that couples 
to amino acid #1. All enzymes will be denoted by the letter E with suitable affixes; 
in particular the amino acid-activating enzymes will be £,, Ey, etc., numbered 
according to their specific substrates, while the polymerase that forms peptide bonds 
will be E’. Permeases will be marked by the letter P instead of E. As for the ribosomal 
RNA template, the empty template will be 7), the template with the first amino 
acid added 7,, and so forth to the template with completed chain, 7, if there are 
mn amino acids in the chain. 
The same symbol will be used to refer to the substances in the stoichiometric 
reaction equations, and to their molar concentrations in the kinetic differential 
equations, a usage which has been adopted™ to save superfluous brackets and 
parentheses, inasmuch as occasions for confusing the senses are extremely rare. 
Classes of substances distinguished by subscripts may be referred to in general by 
literal subscripts: e.g., 7; is any of the S-RNA series, E; might be any of the activating 
enzymes, and the like. The usual conventions will apply to the use of the summa- 
tion sign X’, as applied to expressions with such variable subscripts. 
References p. 632 
