PHOTOSYNTHETIC MACROMOLECULES OF CHLOROBIUM THIOSULFATOPHILUM 317 



three such slower components. Thus, the resemblance of Chlorobium 

 thiosulfatophiliim to E. coli which was observed in the electron micro- 

 graphs is carried over into the components present in extracts. The 

 obvious distinction is in composition. In these extracts the 50 S component 

 bears photosynthetic pigments and appears to be free of nucleic acids. 



THE PURIFIED PIGMENTED COMPONENT 



Characterization of the physiochemical properties, composition and 

 photochemical activity of the pigmented component is incomplete but 

 sufficies to place limits on a number of pertinent variables. We have two 

 indices of the homogeneity of the fraction ; sedimentation behaviour and 

 electron microscopical observation. Although refined analysis may reveal 

 complexities, it is clear that the photosynthetic pigments do sediment with 

 one component which exhibits a well-defined spike. To this extent, the 

 preparation appears to be monodisperse with regard to the photosynthetic 

 pigment. When the pellet of a pigmented fraction that sedimented essen- 

 tially as a single component is removed from the analytical ultracentrifuge 

 and processed for electron microscopy, the thin sections (Fig. 8) reveal a 

 rather uniform population of particles. These particles resemble the 

 particles seen in the cytoplasm of the cell ; that is, the maximum extension 

 of the image in any direction is about 150 A. If these particles are slightly 

 elongated the range of deviation is probably between 100 and 150 A. The 

 sedimentation of the pigmented fraction depends to a considerable degree 

 upon concentration and the data are still inadequate for an accurate cal- 

 culation of the sedimentation constant; however, the maximum value 

 obtained from the purest preparations at high dilution is 50 Svedberg 

 units when converted to 20' in water. It is of interest to compare the 

 direct and indirect data on particle size and obtain some idea of the agree- 

 ment. If we use a sedimentation constant of 50 S and the conventional 

 assumption of a density of about 1-2 g./ml., then the particle diameter 

 calculated from Stokes relation is 173 A. This degree of agreement with 

 the electron microscopical observation is reassuring. A spherical particle 

 ot this size and density would have a molecular weight between i -3 and 

 I -6 million. This value can be used as a first estimate of the molecular 

 weight. 



An interesting but troublesome property of the 50 S particle is the 

 tendency to aggregate into a series of more rapidly sedimenting com- 

 ponents as the degree of purification becomes relatively high. Electron 

 microscopic examination of thin sections of pellets of these aggregates 

 (Fig. 9) reveal elongated profiles with diameters up to 400 A and lengths 

 of thousands of A. When such preparations are sprayed upon on specimen 

 supports and shadowed with metal the electron micrographs reveal rigid 



