472 APPENDIX 



standing the light- activated system, which could be a variant form of 

 the respiratory enzyme complex (10). 



On the other hand, studies by Sistrom (11,12) show clearly that 

 pigment formation cannot take place in the absence of protein synthesis 

 in Rps. spheroides , even in cells which were forming pigments before 

 protein synthesis was inhibited and, hence, contain the requisite 

 enzymes for bacteriochlorophyll formation. To reconcile this obser- 

 vation with the other data indicating that chromatophore formation does 

 not take place de novo, Sistrom suggests that chromatophore material 

 once formed cannot be modified and that subsequent material formed 

 under different growth conditions has a different composition. Con- 

 sequently the average composition will vary, but this average repre- 

 sents a variety of chromatophore material of greatly different com- 

 position. If his suggestion is correct, the different species of particles 

 might be physically separable because they do vary greatly in compo- 

 sition. 



None of the present models of chromatophore formation satisfactor- 

 ily account for the observed fact that chromatophore material con- 

 tains unique antigens not present in the cell grown in the dark (9), If 

 the chromatophore is not formed de novo, then some unique structural 

 feature must be added to it during chlorophyll insertion. Similarly, any 

 hypothesis concerning chromatophore compositional variability must 

 take into account the fact that the basic structure does contain a color- 

 less component not present in dark- grown cells. Conceivably a chloro- 

 phyll bearer moiety is in the chromatophore; it is the unique chroma- 

 tophore antigen and its formation is required coincident with chloro- 

 phyll deposition in the photochemical apparatus; hence the requirement 

 for protein synthesis during chlorophyll formation. If this antigen 

 represents a small fraction of the total protein of chromatophore ma- 

 terial, then its addition to the structure would not be readily detectable 

 by gross compositional analysis. 



Electron Transport Components . 



Table 4 lists the relative concentrations of chlorophyll and various 

 electron transport constituents frequently associated with chromato- 

 phore fractions. Except for quinone content, all the data were obtained 

 on the same preparation and should not be subject to errors of extra- 

 polation from one set of growth conditions to another. Apparently the 

 preparation contains the minimum requirements of a "conventional" 

 electron transport system. In early experience with such analyses, the 

 cytochrome c component proved the most tenaciously bound constituent 

 of the particles and was the only one consistently enriched by in- 

 creasing chlorophyll content of the fraction. 



These early data indicated the presence of large amounts of non- 

 heme iron in the chromatophore preparations. This simple approach 

 had forecast the possible involvement of nonheme iron components 



