48 THE BACTERIAL PHOTOCHEMICAL APPARATUS 



phyll content in response respectively to decreased or increased light 

 intensity. This is analogous to the response of animals which form 

 more hemoglobin under diminished oxygen tensions. 



Such adjustments in chlorophyll content occur in the plant kingdom 

 as shown by the higher levels in shade leaves compared with sun 

 leaves, while unicellular algae groAvn in dim light are richer in chloro- 

 phyll than those grown in bright light (10). 



The elegant experiments of Cohen- Bazire et al. (42) have shown 

 that synthesis of photosynthetic pigments by cultures of Athiorhodaceae 

 is regulated by light intensity. In Rps . spheroides the rate of synthesis 

 of bacteriochlorophyll is inversely proportional to the light intensity 

 and the pigment content of cells grown in dim light (50 ft-c) is about 

 eight times higher than in those grown in bright light (5000 ft-c). On 

 transfer from dim to bright light or vice versa cultures rapidly adjust 

 their pigment level by preferential synthesis or by transient repres- 

 sion of pigment formation. 



In the Athiorhodaceae oxygen exerts a spectacular control over 

 pigment synthesis as shown by the almost complete absence of bac- 

 teriochlorophyll and carotenoids in organisms grown aerobically in the 

 dark. Introduction of oxygen into cultures growing in the light results 

 in an immediate arrest of pigment synthesis and this is reversed by 

 restoration of anaerobic conditions (42). These experiments suggest 

 that absence of pigment in dark- aerobic cultures might be due to re- 

 pression of their synthesis by oxygen rather than to an obligatory re- 

 quirement for light. This was confirmed by showing that Athiorhodaceae 

 can indeed form bacteriochlorophyll and carotenoids in the dark pro- 

 vided that the oxygen tension is reduced (43). With suspensions of Rps. 

 spheroides forming bacteriochlorophyll in the dark the oxygen tension 

 which permits synthesis is critical and must presumably be sufficient 

 for general metabolism (e.g. to supply ATP by oxidative phosphoryla- 

 tion) yet insufficient to cause repression of pigment formation. 



In an attempt to understand the mechanism by which oxygen re- 

 presses bacteriochlorophyll synthesis, attention has been given to the 

 key intermediate, succinyl CoA. In organisms such 3.S Rps. spheroides 

 with a tricarboxylic acid cycle, ALA synthetase has to compete for 

 succinyl CoA with enzymes which would pull it through the cycle; utili- 

 sation via the cycle might be favored by high oxygen tensions since 

 there is evidence that oxidation of succinate becomes rate- limiting 

 under anaerobic conditions (51). Increasing the level of the synthetase 

 could favor diversion of the succinyl CoA towards tetrapyrrole syn- 

 thesis. There is in fact a strong correlation between level of the sjm- 

 thetase and ability to form bacteriochlorophyll; the enzyme is five to 

 ten times higher in Rps. spheroides grown anaerobically in the light 

 than when grown aerobically (43). In addition, synthesis of the 

 enzyme is repressed by high oxygen tensions, though, like bac- 

 teriochlorophyll, it is formed at a maximum rate under low oxygen 



