The Sll0606 Protein Is Required for Photosystem II Assembly/Stability in the Cyanobacterium Synechocystis sp. PCC 6803

Abstract

An insertional transposon mutation in the sll0606 gene was found to lead to a loss of photoautotrophy but not photoheterotrophy in the cyanobacterium Synechocystis sp. PCC 6803. Complementation analysis of this mutant (Tsll0606) indicated that an intact sll0606 gene could fully restore photoautotrophic growth. Gene organization in the vicinity of sll0606 indicates that it is not contained in an operon. No electron transport activity was detected in Tsll0606 using water as an electron donor and 2,6-dichlorobenzoquinone as an electron acceptor, indicating that Photosystem II (PS II) was defective. Electron transport activity using dichlorophenol indolephenol plus ascorbate as an electron donor to methyl viologen, however, was the same as observed in the control strain. This indicated that electron flow through Photosystem I was normal. Fluorescence induction and decay parameters verified that Photosystem II was highly compromised. The quantum yield for energy trapping by Photosystem II (F V /F M ) in the mutant was less than 10% of that observed in the control strain. The small variable fluorescence yield observed after a single saturating flash exhibited aberrant Q A ؊ reoxidation kinetics that were insensitive to dichloromethylurea. Immunological analysis indicated that whereas the D2 and CP47 proteins were modestly affected, the D1 and CP43 components were dramatically reduced. Analysis of twodimensional blue native/lithium dodecyl sulfate-polyacrylamide gels indicated that no intact PS II monomer or dimers were observed in the mutant. The CP43-less PS II monomer did accumulate to detectable levels. Our results indicate that the Sll0606 protein is required for the assembly/stability of a functionally competent Photosystem II.In higher plants, algae, and cyanobacteria, at least six intrinsic proteins appear to be required for oxygen evolution by PS II 2 (1-3). These are CP47, CP43, the D1 and D2 proteins, and the ␣ and ␤ subunits of cytochrome b 559 . Insertional inactivation or deletion of the genes for these components results in the absence of PS II complex assembly and the complete loss of oxygen evolution activity (for a review, see Ref. 4). For maximal rates of oxygen evolution in cyanobacteria, the extrinsic proteins PsbO, PsbU, PsbV, and PsbQ must also be present (5). Additionally, a large number of other intrinsic membrane components are present in PS II complexes (6 -8), although the functions of many of these proteins remain obscure. The most recent crystal structure of the thermophilic cyanobacterium Thermosynechococcus elongatus (9) indicates that PS II contains 20 protein components (it should be noted that PsbQ, which is essential for maximum PS II activity in cyanobacteria (10), is missing from the current crystal structure).PS II assembly and turnover requires a variety of other protein components (for a comprehensive review, see Ref. 11). Although many of these proteins are conserved in all oxygenic organisms, a subset is present only in the cyanobacteria. These include the Synechocystis sp. PC...

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Advances in the Understanding of the Lifecycle of Photosystem II

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