REFLECTIONS ON RECENT THEORIES CONCERNING 

 THE MECHANISM OF BACTERIAL PHOTOSYNTHESIS^ 



Albert W. Frenkel 



Department of Botany 



University of Minnesota, Minneapolis, Minnesota 



The basic features which distinguish bacterial photosynthesis from 

 that of oxygen-producing plants have been described in great detail 

 by van Niel (12, 18), and his unified theory of photosynthesis has 

 been an effective guidepost in attempts to correlate observations on 

 all types of photosyntheses carried out by chlorophyll-containing or- 

 ganisms and by particulate structures derived from them. Van Niel's 

 theory basically envisions that oxidizing and reducing equivalents 

 are produced as the result of the primary photochemical act; that the 

 photochemical reductant can bring about the reduction of COg 

 through suitable metabolic intermediates; and that the photochemical 

 oxidant either dismutates to produce molecular oxygen as in plants 

 containing chlorophyll a, or must react with a suitable electron donor, 

 as is the case in photosynthetic bacteria. Cell-free preparations of 

 photosynthetic bacteria carry out a number of light-induced reac- 

 tions which, like the light-induced reactions of isolated chloroplasts, 

 can be interpreted in terms of a modification of van Niel's theory of 

 photosynthesis (10) . 



Recently, the importance of the photochemically produced reductant 

 for bacterial photosynthesis, in bringing about the reduction of meta- 

 bolic intermediates, has been questioned by Stanier and coworkers 



(17) , and by Arnon (1) . Arnon argues that in bacteria we are ob- 

 serving a primitive photosynthetic system whose principal product is 

 high-energy phosphate, and that the electrons which are necessary 

 for CO2 reduction must be derived from the externally supplied re- 

 ductant. Arnon's argument is based on observations concerned with 

 light-induced phosphorylation reactions carried out by chloroplasts 

 and bacterial chromatophores. In a number of contributions Arnon 



(1-3) has described what he considers as two different types of phos- 

 phorylation processes carried out by illuminated chloroplasts. In 

 "cyclic photophosphorylation" by chloroplasts the production of 



^Work supported by a grant from the Graduate School of the University of 

 Nfinnesota, and l)y grants from the National Institute of Allergy and Infectious 

 Diseases (£-2218), and the National Science Foundation (G9888). 



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