METABOLIC ASPECTS 133 



little doubt that there must be numerous qualitative and quantitative 

 metabolic differences between cells (of the same strain) growing under 

 the two sets of conditions. First of all, we can expect that different 

 pathways of carbon metabolism will predominate. The types of changes 

 that might be anticipated are illustrated by the studies of H, L. Rom- 

 berg and his colleagues (21) with the facultative chemosynthetic auto- 

 troph Micrococcus deuitrificans . Their results indicate that when the 

 micrococcus grows autotrophically the reductive pentose cycle oper- 

 ates as a primary mechanism. On the other hand, cells cultivated as 

 heterotrophs on acetate do not contain significant quantities of ribulose 

 diphosphate carboxylase or other enzymes of the cycle for which tests 

 were made. In place of the pentose cycle, cells growing on acetate 

 apparently utilize the glyoxylate cycle as an important means of ob- 

 taining carbon skeletons for biosynthesis of cellular constituents. One 

 of the key enzymes of the glyoxylate cycle is isocitratase, and this 

 enzyme is found in high concentration in acetate-grown cells of M. 

 denitrificans and other organisms which use this particular cycle. 

 Formation of isocitratase in such bacteria is ordinarily greatly sup- 

 pressed when the carbon source is a C4 dicarboxylic acid, and this 

 effect is believed to involve repression or related mechanisms of 

 regulatory control. 



We can confidently predict that in the photosynthetic bacteria the 

 transition from autotrophic to autoheterotrophic growth, or y/ce versa, 

 will also evoke a number of enzymatic changes governed by repres- 

 sion, derepression, or induction mechanisms. Although the data 

 available are relatively limited, it appears that appreciably different 

 patterns of carbon metabolism are found in different photosynthetic 

 bacteria and, consequently, alteration of the carbon source used for 

 growth leads to varied responses. Autotrophically grown cells of 

 Chromatium contain a high level of ribulose diphosphate carboxylase, 

 but the isocitratase content is low (22), Growth on acetate is charac- 

 terized by a striking increase in isocitratase and a significant de- 

 crease in the carboxylase level. It seems that when acetate is the 

 carbon source for autoheterotrophic growth a modified type of glyoxy- 

 late cycle becomes a prominent pathway inChromatium (22). The non- 

 sulfur purple bacteria Rhodopseudo monas palustris and Rhodopseudo- 

 monas capsidatiis resemble Chromatium (and M. denitrificans) in that 

 they also contain isocitratase in large amount when grown on acetate 

 (23). On the other hand, only traces of this enzyme are found in acetate- 

 grown cells of Rhodopseudo monas sphcroides and /?. rubrum (23); in 

 these particular organisms, the glyoxylate cycle evidently does not 

 function to a quantitatively significant extent. 



The general picture emerging from studies on carbon pathways is 

 that the mechanisms used by photosynthetic bacteria may vary signifi- 

 cantly, depending on the nutritional conditions, and may range from 



