On the multiple forms of cyanobacterial PS 1 complex

Five pigment—protein complexes (PPCs) were identified by electrophoretic separation of the extract obtained by solubilizing thylakoid membranes of the cyanobacterium Synechococcus elongatus using Triton X-100. Three of the complexes, CP1a, CP1c and CP1d, can be ascribed to photosystem 1 (PS 1). Treatment of the Triton extract with sodium dodecyl sulphate (SDS) causes the original PPCs of PS 1 to decompose into two new PPCs, CP1b and CP1e. This process becomes more distinct with an increasing SDS:chlorophyll ratio; the final decomposition product is CP1e. All PPCs were isolated by preparative electrophoresis and were characterized by absorption spectroscopy, circular dichroism (CD) spectra and electrophoresis. Re-electrophoresis of the PPCs shows that any CP1 complex of lower mobility can provide any complex of greater mobility. The dominant components, common to all complexes, are polypeptides with relative molecular weights M r of 71 and 67 kDa. CP1a and CP1b have identical polypeptide composition, containing in addition to the above-mentioned polypeptides, another five smaller polypeptides with M r = 10 – 20 kDa. The complexes CP1c, CP1d and CP1e differ in the content of low-molecular-weight polypeptides. Incubation of CP1a and CP1d with trypsin produces no structural changes in the complex; incubation with pronase converts the two complexes to CP1e. From the experimental data it follows that all CP1 complexes are various forms of the PPC of PS 1, derived from the high-molecular-weight complex CP1a. A model is proposed for the molecular organization of the PPC of PS 1 of the cyanobacterium Synechococcus elongatus.