Summary: The isolation and identification of cytoplasmic membranes from cyanobacteria is complicated by the presence of multiple photosynthetic membranes (thylakoids), and by a lack of biochemical markers characteristic of the cytoplasmic membranes of heterotrophic pro-karyotes. [H]Benzylpenicillin was used to label the penicillin-binding proteins (PBPs) in the unicellular cyanobacterium sp. strain R2 (PCC 7942), in spheroplasts and in fractionated cell membranes. Spheroplasts were made with lysozyme/EDTA, and were broken by shaking with glass beads in hypo-osmotic buffer at 4 °C. Sequential glycerol density gradients were then used to purify the membrane fractions. A low-density membrane (LDM) fraction was isolated which, when compared to the thylakoid fraction, was enriched in a unique carotenoid with an absorption maximum at 390 nm and low in chlorophyll absorption at 678 nm ( / > 10). This fraction had a characteristic polypeptide profile on denaturing polyacrylamide gels (SDS-PAGE), and specific enrichment of PBPs, as detected by fluorography of membrane fractions incubated with [H]benzylpenicillin followed by SDS-PAGE. The values of the PBPs differed from those in or membranes. The high content of specific PBPs in LDM suggested that this fraction was highly enriched in cytoplasmic membrane. A membrane fraction with density between that of the thylakoid and LDM fractions, previously considered to be a mixture of these membranes, showed specific enrichment of a PBP of 78000. This fraction may contain contact zones between the different membrane types.


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