BIOSYNTHESIS OF CHLOROPHYLL; THE PROTOCHLOROPHYLL 1767 



the seeds had been germinated in the presence of streptomycin. This 

 antibiotic seems to prevent the formation of the enzyme that converts 

 protochlorophyll into chlorophyll. 



It must be kept in mind, however, that this interpretation (whether 

 applied to x-ray mutants, or to the action of streptomycin) takes it for 

 granted that all chlorophyll formation occurs via protochlorophyll — a 

 question which we have left open. 



The relation between the appearance of chlorophyll and the beginning of 

 photosynthesis was mentioned in Chapter 32 (section 3). Additional ob- 

 servations have been since reported by Blaauw-Jansen, Komen and 

 Thomas (1950), and Smith (1954). Blaauw Jansen et al. found that 

 etiolated oat leaves had only a slight capacity for photosynthesis when first 

 illuminated. This capacity increased, upon exposure to light, faster than 

 the chlorophyll content; and it was noted that it grew parallel with the 

 increase in the [h]:[a] ratio, until the latter attained its normal value. 

 Smith (1954) found, by the phosphorescence-quenching method, that even 

 after 85% of the protochlorophyll present in etiolated barley leaves had 

 been converted into chlorophyll a by illumination in pure hydrogen, the 

 leaves were still incapable of photosynthetic oxygen production. If such 

 leaves (containing only chlorophyll derived from the original reservoir of 

 protochlorophyll) were exposed to air in the dark, they acquired a shght 

 capacity for photosynthesis. This capacity was greatly increased by brief 

 illumination — the increase being out of all proportion with the accumula- 

 tion of new chlorophyll. It thus seems that the chlorophyll formed from 

 protochlorophyll in the first minute or two of exposure of etiolated leaves 

 to light is photosynthetically inactive. This inactivity should be due to 

 difference, either in chemical structure, or in the composition of the pigment 

 complex, or in the arrangement of the pigment molecules. (For example, 

 activity may require the formation of monomolecular layers of the pigment 

 on protein discs, as suggested in Chapter 37A.) 



Alternatively, the delay in the acquisition of photosynthetic capacity could be 

 caused by deficiency of a catalytic component other than chlorophyll. (This component 

 must be rapidly formed in the cycle: brief anaerobic irradiation — exposure to air in the 

 dark — brief aerobic irradiation; and only more slowly in continuous light.) 



As mentioned above, Krasnovsky and Kosobutskaya (1953) found 

 that the first chlorophyll formed has an absorption peak at 670 m/i 

 while the main mass of the pigment, formed later, has a peak at 678 m^. 

 They suggested that the (more easily photoxidized) first batch ("Chi 670") 

 is the only "photoactive" part of the pigment; while the subsequently 

 formed bulk ("Chi 678"), is in an inactive, polymeric form, and contributes 

 to photosynthesis only by energy transfer to the "active" form. The 



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