Abstract
Putative membrane invagination sites at which intracytoplasmic photosynthetic membrane growth is initiated in Rhodopseudomonas sphaeroides can be isolated in an upper pigmented fraction by rate-zone sedimentation. The intracellular localization of membranes present in the isolated fraction was investigated with the impermeant surface-labeling reagent pyridoxal 5'-phosphate, which has been shown to diffuse into the periplasmic space and to label proteins of both the peripheral cytoplasmic membrane and the mature intracytoplasmic membrane. A comparison of the extent of labeling at 25 and 0 degrees C was consistent with the possibility that membranes present in the upper pigmented fraction arise from sites near the cell periphery. Pronase digestion of the surface-labeled membranes suggested further that the purified upper fraction consisted largely of open membrane fragments and that the majority of the intracytoplasmic membrane is labeled by this procedure. The pigmented membrane growth initiation sites were separated partially from undifferentiated respiratory cytoplasmic membrane also present in the upper fraction.