CARTENOIDS OF PHOTOSYNTHETIC BACTERIA 31 



The structural features of the eight reaction types discussed are 

 summarized in Table 3. 



In Fig. 7 is presented the postulated pathway of carotenoid biosyn- 

 thesis in photosynthetic bacteria. The numbers at the arrows refer 

 to the reaction types. The structures of the carotenoids with Roman 

 numerals and those of 7- and /?-carotene are known. In the longest 

 vertical column the sequence from the colourless phytoene to spiril- 

 loxanthin is depicted. The right-hand branch shows the alternative 

 pathway to spirilloxanthin estahlishedinRhodopseudomonasgelatinosa, 

 and to the far right the sequence of the keto- carotenoids in aerated 

 cells of Rkodopseudomonas gelatinosa is presented. On the basis of 

 considerations of the distribution pattern in various mutants with 

 deranged carotenoid systems we (23) have earlier assumed that one 

 single enzyme is responsible for the three type 1 dehydrogenations 

 (la) from phytoene to neurosporene, and that a different enzyme 

 carries out the dehydrogenation lb. Examination of the quantitative 

 distribution pattern in Rhodopseudomonas gelatinosa and Rliodo- 

 pseudomonas spheroides suggests that the lb reaction is a limiting 

 factor in these organisms. 



The keto- carotenoids so far unique for the two Chromatium spp. 

 should probably be placed somewhere at the left of the scheme, and 

 the precursors of the cyclic carotenoids (indicated by an enclosure) 

 are not known. 



As the number and range of observations is extended, new facts 

 may come to light which will invalidate some of the suggestions made 

 here. I hope, however, that this short review has given the reader a 

 certain insight into the current interpretations of the data available, 

 the tentative speculations still lacking experimental verification, and 





8T 



CYCLIZATION STEP 



Fig. 6. Possible interpretations of the cyclizati on step 

 involved in carotenoid biosynthesis. 



