40 THE BACTERIAL PHOTOCHEMICAL APPARATUS 



Smaller amounts of magnesium protoporphyrin monomethyl ester 

 have been observed in Rps. capsulata and Rps. spheroides (18,17, and 

 Lascelles, unpublished observations). Excretion of chlorophyll deriva- 

 tives by mutant strains of Athiorhodaceae is discussed later. 



The accumulation of porphyrins and derivatives by the Athiorho- 

 daceae is connected with the formation of bacteriochlorophyll. It has 

 not been observed in cultures growing aerobically in the dark, when 

 little or no photosynthetic pigments are formed. Also the inverse re- 

 lation between porphyrin and bacteriochlorophyll, influenced by the 

 iron concentration of the medium, points to an association in the syn- 

 thesis of these pigments. 



BIOSYNTHESIS OF PORPHYRINS AND CHLOROPHYLLS 



Pathway to protoporphyrin. 



The discovery by Granick (19) that mutants of Chlorella blocked 

 in chlorophyll synthesis accumulated free porphyrins gave the first 

 indication that the formation of chlorophylls and hemes proceeded via 

 a common pathway up to the stage of protoporphyrin. The basic outline 

 of the pathway to protoporphyrin is now known (Scheme 1), This has 

 been achieved initially by studies with preparations from avian ery- 

 throcytes (reviewed by Granick & Mauzerall (20)) and it is only com- 

 paratively recently that bacteria, in particular Rps. spheroides , have 

 been exploited (reviewed by Lascelles (16,21)), All the evidence points 

 to a common pathway in animals and photosynthetic bacteria and the 

 exceptional ability of Rps. spheroides to form tetrapyrroles has made 

 it a fruitful source for enzymic studies. 



Glycine 



pyridoxal PO^ 



-^-aminolevulinic 



-8CO2 

 Succinyl-CoA 



-2CO2, -4H 



coproporphyrin in uroporphyrin HI uroporphyrinogen I 



-6H 



uroporphyrin I 



Hemes, chlorophylls 



Scheme 1. Path of tetrapyrrole synthesis. 



