136 METABOLISM AND PHYSIOLOGY 



Although a competition of this sort may still be entertained as a 

 first approximation, the effect is evidently more complex since the 

 amount of N2 actually utilized in short-term experiments is consider- 

 ably less than the predicted quantity (38), In other words, N2 appears 

 to inhibit by two mechanisms, one of which is essentially "catalytic," 

 It seems quite possible that the "catalytic" effect of N2 on H2 produc- 

 tion may be a regulatory cut-off device similar to the so-called 

 "allosteric inhibition" (39) observed in other enzyme systems. Further 

 investigations will be required to unravel this interesting puzzle. 



The overall effect of N2, or ammonia, on the photometabolism of 

 organic substrates by hydrogen-producing cells of R. nihrum is to 

 divert carbon from the dissimilatory anaerobic citric acid cycle, and 

 toward assimilatory pathways (5,24), This is hardly unexpected be- 

 havior for a cell with an abundant energy supply suddenly faced with a 

 utilizable nitrogen source. The absence of H2 production during active 

 growth in media containing excess NH4"'" is partly understandable 

 on the basis of the fact that reduction is required to convert this 

 inorganic nitrogen source to the level of the amino group. There is, 

 however, an additional mechanism which ensures the prevention of 

 hydrogen evolution during growth in ammonium salt media. Substantial 

 evidence has been obtained (33) indicating that NH4+, or a metabolic 

 derivative, effectively represses the formation of one or more com- 

 ponents of the hydrogen- evolving system. Accordingly, cells harvested 

 from a malate + NH4+ medium before the nitrogen source is exhausted 

 are initially incapable of producing hydrogen. After a period of con- 

 tinued illumination in the presence of suitable organic substrates, 

 however, H2 evolution begins and gradually increases in rate (see Fig. 



1). 



Experiments (33) with chloramphenicol and other inhibitors indicate 

 that the derepression of hydrogen production involves protein synthe- 

 sis, which presumably can occur at the expense of amino acids in the 

 endogenous pool. In agreement with expectations, cells harvested sev- 

 eral hours after exhaustion of the nitrogen source from cultures 

 grown with limiting amounts of ammonia are immediately capable of 

 photoproducing hydrogen. 



This brief review of the carbon and nitrogen metabolism of photo- 

 synthetic bacteria should serve, in part, to underline the dynamic and 

 variable aspects of their metabolic behavior. In this respect, the 

 photosynthetic bacteria obviously do not differ materially from other 

 microorganisms. Their metabolic variability is emphasized here be- 

 cause of a tendency to regard photosynthetic bacteria as "laboratory 

 reagents" to be used primarily as a convenient source of photoactive 

 subcellular particles. It is probable that the composition and proper- 

 ties of such particles will differ considerably depending on the organism 

 and its nutritional history, and it is prudent not to underestimate the 

 possibility (5,40,41) that violent disengagement of the photochemical 



