STRUCTURE OF PHOTOSYNTHETIC BACTERIA 101 



fur bacteria; the sections were prepared from cells grown in very dim 

 light (<40 foot- candles) and depleted of internal sulfur deposits prior 

 to fixation. The cytoplasmic region is in each case densely filled with 

 vesicles bounded by unit membranes. The individual vesicles are 

 approximately 500A in diameter and are thus structurally indistinguish- 

 able from the vesicles of R.rubrum andRhodopseudomonas spheroides. 

 A markedly different internal membrane structure occurs in the 

 small, spherical cells of Thiocapsa (Fig, 10). These cells were also 

 taken from a culture grown in very dim light. Just as in the case of the 

 larger purple sulfur bacteria, much of the cytoplasm is filled with 

 typical 500A vesicles surroundedbyunit membranes; however, in many 

 sections, a large area is occupied by an extensive system of parallel 

 paired lamellae. 



THE STRUCTURE OF ISOLATED MEMBRANE FRACTIONS 



The interpretation which has been offered for the structure of the 

 photosynthetic apparatus in purple bacteria implies that the chromato- 

 phores which can be isolated from broken cells constitute fragments 

 derived from an initially continuous membrane system, no doubt fre- 

 quently contaminated with associated wall material. In the case of R. 

 ruhrum, the presence of numerous wall fragments in crude chromato- 

 phore preparations can be readily established by the examination of 

 material negatively stained with phosphotungstic acid, since the wall 

 of this species has the distinctive fine structure characteristic of the 

 wall of spirilla. This probably explains the observation of Newton (23) 

 that antisera prepared against chromatophore material are capable of 

 agglutinating intact cells, 



K one avoids methods of cell breakage that cause considerable com- 

 minution, and fractionates the extract by sucrose gradient centrifuga- 

 tion, it is possible to obtain from R. rubrum, though in small yields, 

 membrane fractions that are essentially devoid of wall material. Sec- 

 tions show that the unit membrane structure is well preserved in such 

 material (Fig. 12), The geometric form of these membrane fragments 

 is revealed by negative staining with phosphotungstate. Fig. 11 shows 

 our best preparation of this kind to date, made with membrane material 

 prepared from a large Chromatium species provided by Dr. Pfennig. 

 Similar preparations from R. ruhrum give a virtually indistinguishable 

 picture. To judge from such preparations, most of the membrane frag- 

 ments have the form of cups or hemispheres, flattened to a greater or 

 lesser extent by drying. We have thus far been unable to detect any 

 fine structure in them. 



