686 F. C. STEWARD AND R. G. S. BIDWELL 
also marked by irreversible changes which eventually lead to death (for references see 
STEWARD AND THOMPSON’, p. 519). Here again there is a critical event which dis- 
rupts what would otherwise be the normal and stable level of turn-over. 
At this point it is profitable to return to the work of WRIGHT AND ANDERSON’? on 
slime molds. In addition to their finding that there are different protein moieties in 
the slime molds which have different rates of turnover, these authors also reach the 
interesting conclusion that the pace of that turnover is itself a function of develop- 
ment. In response to starvation, the slime mold enters upon a reproductive phase 
and its total protein content declines, but the rate of turnover of that protein which 
remains is actually much increased. This maximum rate of turnover is accompanied 
by a corresponding increase in the activity of enzymes in respiration, as measured 
by oxygen consumption, in carbohydrate synthesis and in the exchange of amino 
acids in the soluble pool with labeled amino acids that are supplied exogenously 
(Fig. 2). The interesting and applicable idea is that the rate of turnover in slime 
molds is associated with their morphogenetic development. In the carrot tissue 
cultures also the rate of protein turnover is associated with a degree of growth 
induction which, in free cells, leads on to such morphogenetic developments that 
whole plants may finally emerge*®. 
The work on growth induction in plant cells has, however, provided even more 
direct evidence. As already stated, the cells which are treated with coconut milk 
synthesize protein in bulk, and they display accentuated turnover of their meta- 
bolically active protein. In addition, however, they synthesize a metabolically in- 
active moiety which has the distinctive property that it acquires an unusual hydroxy- 
proline content by oxidation of proline after the latter has been built into the protein 
molecule®!. This result is of special significance in two ways. First, there is no direct 
incorporation into this protein of any free hydroxyproline which they may absorb 
from the ambient medium?*; second, this hydroxyproline-containing protein is quite 
essential for the growth induction and the morphogenesis which follows. By the 
use of either L-hydroxyproline or some of its derivatives, the incorporation of L- 
proline into the protein of the growing cells may be reversibly inhibited and, when 
this occurs, growth also stops*?. 
SECTION VI. SOME IMPLICATIONS OF THE MECHANISM OF PROTEIN SYNTHESIS 
Any ideas upon protein synthesis now tend to be dominated by the general working 
hypothesis that the DNA of the nucleus controls the synthesis of the RNA of the 
cytoplasm, and this in turn acts as the template which governs the sequence of 
amino acids in the protein that is synthesized. However, not all the protein synthesis 
may proceed over the same pathways. It would indeed be surprising if, in plants, 
the resting proteins of seeds which are elaborated from preformed nitrogenous com- 
pounds; the large amount of chloroplast protein in a leaf; the storage protein in the 
vacuoles of fleshy storage organs, and the metabolically active proteins formed in 
growing, dividing cells, as in a growing point, were all made by an identical type of 
chemical machinery. 
Indeed, even in the animal kingdom there is some evidence to show that more than 
one method exists for the incorporation of amino acids into protein. Here one has to 
References p. 692/693 
