TETRAPYRROLES 49 



tension in the dark (43,50), ALA dehydratase is also repressed by 

 oxygen, suggesting that co-ordinate repression by oxygen occurs in 

 the tetrapyrrole pathway. 



Regulation of bacteriochlorophyll synthesis by oxygen and by light 

 intensity may operate by a similar mechanism (42), In support of this 

 the levels of ALA synthetase and dehydratase in Rps , spheroides are 

 affected by the light intensity just as they are influenced by the oxygen 

 pressure; their rates of formation in growing cultures are inversely 

 proportional to the light intensity (50), 



These observations suggest that one of the ways in which oxygen 

 and light may influence the formation of bacteriochlorophyll is by 

 repressing synthesis of enzymes concerned in early stages of the 

 biosynthetic pathway. They tell us nothing of the mechanism by which 

 the repressing effect is exerted. Nor do they fully account for all the 

 observed effects of oxygen. Control by enzyme repression only would 

 result, upon the introduction of oxygen, in a gradual fall in the dif- 

 ferential rate of bacteriochlorophyll synthesis by cultures growing in 

 the light; this would occur as the enzymes already present became 

 diluted out. In fact, oxygen produces an immediate and complete stop- 

 page of pigment synthesis (42,50), This suggests that oxygen is inhibit- 

 ing the action of one or more enzymes on the biosynthetic path. Since 

 porphyrins do not accumulate in oxygen- repressed cultures it seems 

 that an early stage is either directly or indirectly inhibited by oxygen. 



In addition to the effects of oxygen and light on tetrapyrrole forma- 

 tion, consideration must be given to their action on the carotenoids. 

 These respond to the environment in the same way as bacteriochlor- 

 ophyll, yet the biosynthetic pathways have nothing in common, except 

 perhaps for the phytol residual of the bacteriochlorophyll. 



A full understanding of the response of the pigment system to the 

 environment can only come when we know more about the steps in the 

 formation of the chlorophylls and carotenoids and about the stage where 

 their synthesisbecomesinterwoven with the lipoproteins of the chroma- 

 tophore and chloroplast structures. 



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Fischer, H., and Stern, A., Die Chemie des Pyrrols, Vol. II, part 2. Akad- 



emische Verlagsgesellschaft M. B. H., Leipzig, 1940. 



Golden, J. H., Linstead, R. R.,and Whitham, G. M., Chlorophyll and related 



compounds. VII. The structure of bacteriochlorophyll. J. Chem. Soc, 195S, 



1725. 



Stanier, R., and Smith, J. H. C, The chlorophylls of green bacteria, Bio- 



chiw. Biophys. Acta, 41, 478 (1960). 



Holt, A, S,, and Hughes, D. W., Studies of Chlorobium chlorophylls. III. 



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