304 DOUGLAS C. PRATT, ALBERT W. FRENKEL, AND DONALD D. HICKMAN 



13. Vernon, L. P., and Ash, O. K.,y. biol. Chem. 235, 2721 (i960). 



14. Frenkel, A. W., J. biol. Chem. 222, 823 (1956). 



15. Frenkel, A. W., Brookhaven Symp. Biol. 11, 276 (i959)- 



16. Frenkel, A. W., Plant Physiol. 33, Suppl. xvii (1958). 



17. San Pietro, A., and Lang, H. M., J. biol. Chem. 231, 211 (1958)- 



18. Frenkel, A. W.,7. Amer. chem. Soc. 80, 3479 (1958). 



19. Klingenberg, M., and Biicher, T., Anmi. Rev. Biochem. 29, 669 (i960). 



20. Gibbs, M., Nature, Loud. 170, 164 (1952). 



21. Arnon, D. I., Science 116, 635 (1952). 



22. Arnon, D. I., Rosenberg, L. L., and Whatley, F. R., Nature, Lond. 173, 1132 



(1954)- 



23. Rosenberg, L. L., and Arnon, D. \.,y. biol. Chem. 217, 361 (i955)- 



24. Arnon, D. I., Brookhaven Symp. Biol. ll, 181 (1959). 



25. Oh-hama, T., and Miyachi, S., Plant Cell. Physiol. I, 155 (i960). 



26. Moses, v., and Calvin, M., Biochim. biophys. Acta 33, 297 (i959)- 



27. van Niel, C. B., Advanc. Enzymol. I, 263 (1941). 



28. Johnston, J. A., and Brown, A. H., Plant Physiol. 29, 177 (i954)- 



29. Brown, A. H., Amer. J. Bot. 40, 719 (i953)- 



30. Brown, A. H., and Weis, D., Plant Physiol. 34, 224 (i959)- 



31. Frenkel, A. W., and Hickman, D. D.,jf. biophys. bioclmn. Cytol. 6, 285 (i959)- 



Discussion 



Bergeron : In the small particle preparations from the dark-grown cells where 

 you get pyridine nucleotide reduction if you add magnesium, is this reduction 

 inhibited if substrates for phosphorylation are present as it is with the regular 

 Rhodospirillum rubrum chromatophores ? 



Frenkel : We have not worked with the small particle preparations. 



Arnon : Dr. Frenkel said that Rhodospirillumrubrumwould not grow on acetate in 

 the dark but we have grownC/;raw<7^m?«, aphotosynthetic sulphur bacterium, on ace- 

 tate without added CO^ or under conditions when COo, which might be formed from 

 acetate, would be swept out by continuously bubbled gas. Under these conditions 

 there would be enough COo for it to act as a catalyst but not as a substrate. 

 Apparently, there are differences between these two organisms. My second point 

 concerns the reduction of pyridine nucleotides in the dark, i.e. a case when the 

 photosynthetic process becomes limited to ATP formation or to what we call 

 cyclic photophosphorylation. As we shall discuss this afternoon, Chromatium can 

 use hydrogen gas to reduce pyridine nucleotides in the dark. In Chromatium 

 supplied with hydrogen gas light is required only for ATP formation. If exogenous 

 ATP is substituted for light, then Chromatium, which unlike R. rubrum is normally 

 a strict phototroph, now becomes able to assimilate carbon dioxide in the dark. 



Frenkel : I was very surprised to hear that Chrcjmatium can get along without 

 CO.j. We have found that Rhodospirillum, when grown on standard media in the 

 dark or light, has a definite CO2 requirement which saturates at about i per cent 

 COo. Thus far we have not been able to replace this COo requirement by inter- 

 mediates of the Krebs tricarboxylic acid cycle, or by a number of other well- 

 defined chemicals. Only high concentrations of yeast extract or casein hydrolysate 

 were effective in relieving this CO., requirement. With regard to Dr. Arnon's 



