THE SUMMING UP 391 



somal multiplication would be controlled by the part which comes from 

 the chromosome. Although a dual information system is postulated, the 

 interaction of the two components within the nucleus greatly minimizes 

 the problem of "searching out and finding. " If the RNA component 

 is present in many replicates, the chance of random mutation of many 

 of them at once is vanishingly small. The chromosomal system would 

 provide the flexibility for evolution by mutation. 



Now let us see how these components might interact. Following the 

 discussion of Chapter 11, we consider a system in which the ability of 

 S, the class of functional genes, to act is controlled by O, the class 

 of linked controlling elements. The activation of O in turn is deter- 

 mined by a gene product from R, the class of unlinked controlling ele- 

 ments. Since we expect that normally these systems are regulatory in 

 nature, a feedback signal from the cytoplasm is required to complete the 

 circuit. 



How does the feedback operate? This question is equivalent to the 

 more conventionally phrased query: How does the cytoplasm influence 

 gene expression? Studies of enzyme formation have provided evidence 

 that the repressor-inducer ratio may represent the governing cytoplasmic 

 state which is ultimately responsible for regulating the rate of enzyme 

 formation in wild-type cells. So far as is known, repressors and inducers 

 are chemically related small molecules, metabolites, which can compete 

 for particular sites. 



Another class of compounds which have been identified by their action 

 are those which carry a signal from controlling elements R to O. On 

 the simplest scheme, the activation of R would be determined by the 

 cytoplasm (i.e., by the repressor-inducer ratio), and R would activate 

 O with no further regulation from the cytoplasm. Since O and R may 

 be far apart on the genome, however, it seems likely that the signal 

 from jR to O goes through the cytoplasm, and this may provide a second 

 point of cytoplasmic influence upon the course of gene action. Finally, 

 enzyme formation itself occurs in the cytoplasm, and it is likely that 

 some cytoplasmic control may be exerted at that point. 



It has been demonstrated in a striking way, both in bacteria and in 

 maize, that controlling elements act by repression of wild type gene 

 expression. Thus activation of R-O represses S, while inactivity of R-O 

 allows wild-type expression of S to occur. This is a new, unexpected, 

 and extremely important finding in the analysis of gene action. It makes 

 possible the formulation of much more precise questions for experimental 

 attack than have been posed before. For example, one may begin to pin- 

 point the site of repressor action, and thereby of cytoplasmic control of 

 gene expression. 



