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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. [3H]Benzylpenicillin was used to label the penicillin-binding proteins (PBPs) in the unicellular cyanobacterium Synechococcus 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 a absorption at 678 nm (A390/A678 > 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 [3H]benzylpenicillin followed by SDS-PAGE. The Mr values of the Synechococcus PBPs differed from those in Staphylococcus aureus or Escherichia coli 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 Mr 78000. This fraction may contain contact zones between the different membrane types.

Penicillin-binding Proteins in the Unicellular Cyanobacterium Synechococcus sp. Strain R2 (PCC 7942): Implications for Membrane Identification