CHLOROPHYLL FORMATION 431 



These observations suggest that the formations of chlorophylls a and b 

 are independent processes. Schwartz (1949) has found that corn mutants 

 vary with respect to the rate at which chlorophyll b appears subsequent 

 to the appearance of chlorophyll a. In one luteus mutant "chlorophyll a 

 preceded the synthesis of chlorophyll b by 4 days when grown at 85°F. 

 .... At 62°F the retardation extended over a period of 11-14 days." 



Granick (1951) has summarized his effective use of biochemical 

 mutants to signalize the path of biogenesis of chlorophyll. By X irradi- 

 ation he has obtained several Chlorella mutants, each of which, in the 

 dark, accumulates a characteristic pigment closely related chemically to 

 protochlorophyll. In the order of their closeness to protochlorophyll, 

 these pigments are magnesium vinylpheoporphyrin-as (protochlorophyll 

 minus phytol) (Granick, 1950), magnesium protoporphyrin (Granick, 

 1948b), and protoporphyrin-9 (Granick, 1948a). 



These mutants vary in their response to irradiation. The mutant 

 colonies containing the magnesium protoporphyrin "developed on the 

 inorganic salts-glucose agar medium had a dull yellow color which turned 

 orange-brown in 4 to 7 days when grown either in the light or dark at 

 room temperature" (ibid.). As for the mutant cells that contain mag- 

 nesium vinylpheoporphyrin, "When grown in the dark on a solid medium 

 of agar, glucose, and inorganic salts, . . . they form deep yellow colo- 

 nies tinged faintly greenish When grown in the light, the cells 



become deep green In the light these mutants behave like wild 



type Chlorella, growing in the absence of glucose, i.e., photosynthesizing. 

 .... In contrast to the wild type, this yellow Chlorella mutant resem- 

 bles the higher plants in requiring light for the production of chlorophyll " 

 (Granick, 1950). 



9. MECHANISM OF CHLOROPHYLL BIOSYNTHESIS 



The various schemes proposed for the biosynthesis of chlorophyll have 

 been summarized by Rothemund (1935) and by Aronoff (1950). Many 

 of them are of only historical interest, and no attempt will be made here 

 to discuss them further. Rather, we will try to draw from the facts just 

 presented as full and as up-to-date a picture of this process as is possible. 



It is clear that protochlorophyll is the immediate precursor of chloro- 

 phyll a: its transformation to chlorophyll a is very rapid and almost 

 complete; its conversion is a molecule-for-molecule reaction; and the 

 action spectrum for its transmutation to chlorophyll a closely resembles 

 its absorption spectrum in organic solvents, provided allowance is made 

 for its absorption in the holochromatic state. 



The chemistry of protochlorophyll and chlorophyll, as far as is known, 

 indicates that the reaction is a hydrogenation of protochlorophyll— 

 effected by light in the higher plants. From what source the hydrogen 



