STRUCTURAL AND CHEMICAL ARCHITECTURE OF HOST CELLS 117 



other cell constituents. Lethal consequences of unbalanced growth as 

 a result of the inability to produce cytoplasmic constituents have not 

 yet been clearJy documented, with the exception of the case described 

 by Griffiths et at. (1955) and Cohen-Bazire et at. (1957), in which the 

 synthesis of a cytoplasmic particulate, a chromatophore, is distorted 

 in carotenoidless mutants of Rhodopseudomonas spheroides. This organism 

 is a photosynthetic nonsulfur purple bacterium (family Athiorhodaceae), 

 which normally may grow aerobically in the dark on a suitable 

 substrate or anaerobicaUy in the light. In the light, the normal pigment 

 system of carotenoids and bacteriochlorophyU, centered in particulate 

 chromophores (50 m^u, in diameter), facihtates photosynthetic fixation 

 of COa. 



Mutants were isolated in which, instead of the two normal red and yellow 

 carotenoids, the chromatophores contained the colorless C^q polyene, 

 phytoene, which is considered to be a precursor of the more unsaturated 

 carotenoids. The spectra of the mutant chromatophores are altered in the 

 position of the chlorophyll band and it is beheved that this change arose from 

 structural inadequacies, even holes, in the particles. The mutants synthesize 

 porphyrin at the normal rate, but unlike wild type organisms, excrete 

 significant amounts of these substances, indicating an inabihty to pack them 

 efficiently into the chromatophores. The mutant grows normally in the 

 dark, and anaerobicaUy in the hght, although less rapidly than the wild 



type. 



Air only inhibits pigment synthesis in the wild type in the light, respiration 

 replacing photosynthesis. In the mutant, however, air kills the cells in the 

 light. This result has been interpreted as a photodynamic action due to the 

 presence of porphyrins, in which the oxidizing fragment (OH), generated by 

 the photolytic decomposition of water, is not removed as a result of the 

 absence of carotenoids. It is proposed that the protection of cells from 

 photodynamic destruction by chlorophyll is a major function of carotenoids, 

 accounting for its ubiquitous association with the photosynthetic 

 apparatus. 



In still another instance, then, the apparent integration of polymer 

 synthesis in the production of cellular components does not reflect a com- 

 pulsory interlacing of these activities at the enzymatic level, but reflects the 

 fact that these independent syntheses must occur concomitantly or have 

 pathological consequences detrimental to cell survival. It is of interest that 

 the cytopathology observed in liver, in response to infection by ectromelia 

 virus, has been ascribed to an initial inhibition of mitosis, followed by a 

 continuing production of cytoplasm (DeBurgh and Miller, 1955). Subsequent 

 cell damage is attributed by the authors to this abnormal form of 

 growth. 



