Photosynthetic membrane development in Rhodopseudomonas spheroides. II. Correlation of pigment incorporation with morphological aspects of thylakoid formation.

Abstract

Cells ofRhodopseudomonas spheroides were depigmented by aerobic growth in the light and then transferred to 4% oxygen in the dark to induce pigment synthesis. Pigment synthesis and photochemical activity were measured fluorometrically. In conjunction with the fluorescence studies, thylakoid morphogenesis was followed by electron microscopy of thin sections of cells fixed during the repigmentation process.Both bacteriochlorophyli and the onset of photochemical activity were detected before distinct thylakoids were observed. Subsequent bacteriochlorophyll synthesis was associated with a gradual increase in the thylakoid content throughout the developmental process.The results obtained strongly indicate that initially the cytoplasmic membrane is modified by pigment incorporation, possibly at specific sites, and that the bacteriochlorophyll is photochemically active in the pigmented cytoplasmic membrane or in the early stages of invagination.Finally, in a confirmation of previous hypotheses, these studies provide evidence for the origin of the thylakoids as a protrusion and invagination of the cytoplasmic membrane. This is followed by constriction and subsequent proliferation and branching to form a continuous membrane system which gives rise to chromatophores upon cellular disruption.