STRUCTURE AND FUNCTION OF CHLOROPLASTS AND CHROMATOPHORES 275 



ing how hydroxylated xanthophylls are formed in higher plants and algae, 

 but it is clear that /S-carotene epoxides are formed in excised leaves by 

 epoxidation of the parent hydrocarbon. 



Having given a very brief summary of what we know about the 

 mechanism of carotenoid formation, I shall conclude by mentioning two 

 specific problems which should be of direct interest to this symposium. 

 Both are concerned with the action of light on carotenoid formation in 

 photosynthetic organisms and both are only in the embryonic state of 

 development. The first problem is the synthesis of carotenoids in illu- 

 minated etiolated maize seedlings. Etiolated seedlings produce only 

 small amounts of xanthophylls ; on illumination they immediately begin to 

 synthesize the typical plastid carotenoids, mainly /3-carotene, lutein, and 

 neoxanthin, along with the chlorophylls as the functional chloroplasts 

 develop. Isotope experiments show that mevalonate and acetate are 

 ineffectively incorporated into /3-carotene during this period, but that CO^ 

 is specificallv incorporated. Etiolated seedlings synthesize considerable 

 amounts of sterols and mevalonate and acetate are incorporated into these 

 compounds both in the dark and on illumination of the seedlings ; CO2 on 

 the other hand is less effectively incorporated into the sterols than into the 

 carotenoids [5]. We are now trying to find out the biochemical reason for 

 the sudden switch of terpenoid precursors from steroid synthesis to 

 carotenoid synthesis and for the effectiveness of CO2 as a carotenoid 

 precursor. A possible explanation is that TPNHo is required for the later 

 stages of carotenoid synthesis ( } dehydrogenation) and this would, of 

 course, become available in increasing amounts in the developing chloro- 

 plasts. Furthermore, it has recently been observed (H. Yokoyama, personal 

 communication) that in an enzyme preparation from PJiycomyces blakeslee- 

 anus which incorporates labelled mevalonate into /3-carotene and ergosterol, 

 the addition of TPNH., to the suspending medium results in relatively 

 more label appearing in the /3-carotene fraction. 



The second problem deals with the purple photosynthetic bacterium 

 Rlwdospirillum riihrum. When grown photosynthetically this normally 

 produces in its unsaponifiable fraction a carotenoid spirilloxanthin and a 

 terpenoid recognized at the moment only by its Rf value [6]. When R. 

 riibnim is grown heterotrophically in the dark it is colourless and syn- 

 thesizes only the terpenoid compound; on illumination spirilloxanthin and 

 bacteriochlorophyll are synthesized together as functional chromatophores 

 develop. This situation is obviously very similar to that encountered in 

 etiolated seedlings. However, a somewhat different situation can be 

 demonstrated under suitable conditions. Dr. June Lascelles m Oxford [7] 

 showed that washed colourless Rhodopseudomonas spheroides resuspended 

 in a medium containing small amounts of glycine, a-ketoglutarate, 

 fumarate and salts, including Fe^ +, and with a gas phase containing b^/^y 



