STRUCTURE OF PHOTOSYNTHETIC BACTERIA 107 



TABLE 2. 



Chlorophyll content of green bacteria grown at low light intensity 

 (<40 foot-candles) 



. 2 Chlorophyll^ content of cells , 



^^^'^^ l^g/mg cellular protein 



Chlorobium limicola R (660) 142 



Chlorobium limicola ML (650) 120 



Chlorobium limiCola 17 CR (650) 118 



Chlorobium thiosulfatophilum , B (660) 115 



Chlorobium thiosulfatophilum Tassajara (660) 190 



Chlorobium thiosulfatophilum 6 CR (660) 100 



1 The absorption coefficients for chlorobium chlorophylls 650 and 660 in meth- 

 anol were taken from the publication of R. Y. Stanier and J. H. C. Smith, The 

 chlorophylls of green bacteria, Biochim. Biophys. Acta, 41, 478 (1960). 



2 The values in parentheses represent the type of chlorobium chlorophyll pres- 

 ent in each particular strain. 



broken in the French pressure cell, and after a low-speed centrifuga- 

 tion to remove residual intact cells and large fragments, the extract 

 was centrifuged at 100,000 x g for 2 hours. The pigment system was 

 completely sedimented. A very small fraction of the total pigment was 

 contained in a loose layer overlying the pellet. After removal of this 

 layer, the pellet was resuspended and subjected to centrifugation 

 through a linear sucrose gradient (0,5-2,0 M) for 2 hours at 25,000 

 rpm. At the end of this period, most of the pigment was contained in a 

 broad, deep-green band. This material had a considerably higher spe- 

 cific chlorophyll content (325 /Ug/mg protein) than the original cells 

 (190 /ig/mg protein). It proved to be structurally heterogeneous upon 

 examination in the electron microscope after negative staining with 

 phosphotungstate (Fig, 16), However, the bulk of the material consisted 

 of vesicles 1000-1800 A long and 500-750 A wide. They are accordingly 

 similar in form and dimensions to the peripheral chlorobium vesicles 

 which are such a constant feature of thin sections of intact cells. Al- 

 though this evidence certainly cannot be considered conclusive, it 

 suggests that the chlorobium vesicles could be the major site in the 

 cell of the photosynthetic pigment system. If this supposition is con- 

 firmed by subsequent work, the 150 A holochrome particles isolated 

 by Bergeron and Fuller must either be contained within the vesicles or 

 else derived from them by comminution. 



