Gene Action — Growth, Differentiation, and Development 



505 



rolase and glucose-6-phosphatase." ; The 

 type of protein synthesized by the recipient 

 cell seems to have some if not all of the 

 specificity produced by the RNA-donor cell. 

 Some of the introduced RNA seems to func- 

 tion as messenger RNA for at least an hour. 

 When the responses to stimuli do not in- 

 volve learning, the neurons of rats show an 

 increase in nuclear RNA but no shift in base 

 ratios. However, when rats are placed in a 

 learning situation (involving balance), not 

 only does nuclear RNA increase but also 

 the A/U ratio increases and C decreases. 

 The RNA content and base ratios can be 

 studied in single cortical neurons of right- 

 handed rats forced to use the left hand to 

 obtain food. Neurons serving the learning 

 side not only show an increased RNA con- 

 tent but an increased — — — ratio and a 

 C + U 

 G + C 



decreased 



ratio as compared with 



A + U 



that of control neurons in the contralateral 

 part of the same cortex. In Parkinson's 

 disease profound changes in RNA base ra- 

 tios arise in the nervous tissue; interference 

 with RNA synthesis in the brain sometimes 

 impairs learning in rats. These results 

 strongly suggest that the learning process 

 is associated with production of messenger 

 RNA. 17 



Both protein and RNA synthesis can be 

 stimulated by estrogens (in uterine tissue); 

 testosterone (in the prostate gland); and by 

 a flowering hormone, presumably a sterol 

 (in a plant bud). The flowering hormone 

 also reduces the histone to DNA ratio, 

 which suggests that steroids can bring about 

 the removal of histone from chromatin. 18 

 Low concentrations of thyroxine in a cell- 

 free system prepared from rat liver will 

 stimulate the incorporation of amino acids 



16 See M. C. Niu, C. C. Cordova, L. C. Niu, and 

 C. L. Radbill (1962), and A. H. Evans (1964). 



17 See H. Hyden and E. Egyhazi (1963, 1964). 



18 See reference to J. Bonner and P. O. P. Ts'o 

 on p. 499. See also T. H. Hamilton (1964). 



into protein. This effect is dependent upon 

 the presence of mitochondria and an oxidiz- 

 able substrate, is independent of DNA-de- 

 pendent RNA polymerase activity or mRNA 

 synthesis, and seems to involve the transfer 

 of sRNA-bound amino acid to protein syn- 

 thesizing ribosomes. 1 '' 



Differentiation in Paramecium -° 



Although Paramecium is normally a single 

 animal, or singlet, double animals, or dou- 

 blets, occur. Singlets and doublets repro- 

 duce true to type through numerous fissions. 

 A doublet can also conjugate with two sin- 

 glets and each singlet exconjugant regularly 

 produces singlet clones and the doublet 

 exconjugant, a doublet clone. The singlet- 

 doublet difference cannot be due to micro- 

 nuclear genes since exconjugants are iden- 

 tical in this respect. This same phenotypic 

 result is obtained even when a cytoplasmic 

 bridge lasts long enough to permit an ex- 

 tensive exchange of cytoplasm. Conse- 

 quently, the difference between doublet and 

 singlet does not have a basis in any cyto- 

 plasmic component free to migrate. Other 

 evidence seems to exclude the macronucleus 

 from being involved. The only portion of 

 the cell unaccounted for then is the immo- 

 bile 0.5 micron-thick outer layer of ecto- 

 plasm, the cortex. 



In one experiment, after cytoplasmic 

 bridge formation between a singlet and 

 doublet, a rare free singlet exconjugant was 

 found bearing a conspicuous extra piece of 

 cortex. The doublet exconjugant, on the 

 other hand, showed a corresponding nick 

 in its cortex. The extra piece in the sin- 

 glet later flattened out and, after fission, one 

 of the two daughter cells gave rise to a 

 clone phenotypically intermediate between 

 singlets and doublets. This natural grafting 

 of only a small piece of a Paramecium's 



19 See L. Sokoloff, C. M. Francis, and P. L. Camp- 

 bell (1964). 

 -°See T. M. Sonneborn (1963, 1964). 



