44 THE BACTERIAL PHOTOCHEMICAL APPARATUS 



Four to five intermediate stages between magnesium protoporphyrin 

 monomethyl ester and magnesium vinyl pheoporphyrin a.^ (MgVP; 

 protochlorophyllide a; see Fig, 3) have been postulated by Granick but 

 these are completely unknown. MgVP is accumulated by a Chlorella 

 mutant, which, unlike the wild type, does not form chlorophyll vinless 

 illuminated. It also accumulates in etiolated leaves treated with ALA 

 in the dark (34), This pigment can exist in three forms (40), Type 1 

 has an absorption maximum at 631 m/i and is bleached by light; this is 

 the free pigment. Type 2 has an absorption maximum at 650 m/i and is 

 converted in the light to chlorophyllide a; this is attached to the holo- 

 chrome complex studied by Smith (41), Type 3 also has an absorption 

 maximum at 650 m/j. but is not transformed by light; this may be 

 attached to a holochrome which lacks a reducing component. 



Free MgVP when accumulated by etiolated leaves in the dark from 

 ALA is not utilised for chlorophyll synthesis upon subsequent illumina- 

 tion. For such a transformation to occur the pigment must presumably 

 be attached to the holochrome complex, at least in the higher plants. 

 It appears that free MgVP formed from added ALA cannot enter this 

 complex; it is possible that attachment of the tetrapyrrole component 

 to the holochrome may occur at a stage before MgVP, 



The protochlorophyll-like pigment isolated from a strain of i?/)s . 

 spheroides , unable to form bacteriochlorophyll, resembles type 1 

 MgVP; it has an absorption maximum at 623 m/i and it probably lacks 

 the phytol group (36), A similar, possibly identical pigment has been 

 identified spectroscopically in mutants of the same organism (37), 



Perhaps the greatest mystery in chlorophyll synthesis is the nature 

 of the light reaction which results in reduction of ring D of MgVP, The 

 fully functional holochrome complex presumably contains a light- 

 activated reducing system. Most of the simple algae form chlorophyll 

 in the dark and presumably contain an additional enzyme system which 

 catalyses the reduction without the intervention of light. Bacteria of 

 the Athiorhodaceae family must have a similar type of system since 

 they can form bacteriochlorophyll in the dark provided the oxygen 

 pressure is low (42,43). Protein-bound intermediates may participate 

 in the final stages of bacteriochlorophyll synthesis. The obligatory 

 association of pigment formation and protein synthesis or turnover 

 supports this (43,44,45), It seems likely that the enzymes for these 

 later stages are in the chromatophores; this is suggested by the obser- 

 vation of Tait & Gibson (39) that the magnesium protoporphyrin 

 methylating system is confined to the chromatophore fraction. 



The accumulation of pheophorbide a (Fig, 3) by the blue-green 

 carotenoidless mutant of Rps. spheroides provides additional evidence 

 that bacteriochlorophyll and chlorophyll synthesis proceeds by a com- 

 mon path, but the significance of the appearance of this magnesium- 

 free pigment is not clear. A compound similar if not identical with it 



