2004; Loll et al. 2005; Guskov et al. 2009; Umena et al. 2011). Most work on the structure and function click here of CyanoQ has come from studies of the mesophilic cyanobacterium Synechocystis sp. PCC 6803, hereafter Synechocystis, where it is known to be a subunit of oxygen-evolving PSII complexes (Roose et al. 2007). Synechocystis cells lacking CyanoQ grow photoautotrophically as well as WT under optimal growth conditions but do show some growth YH25448 inhibition when exposed to nutrient stress such as by depleting the medium of calcium and chloride (Thornton et al. 2004) and iron (Summerfield et al. 2005). Analysis of isolated PSII

complexes lacking CyanoQ from Synechocystis suggests that CyanoQ stabilises binding of PsbV and helps protect the oxygen-evolving Mn4CaO5 complex from reduction in the dark (Kashino et al. 2006). The crystal structure of Escherichia coli-expressed Synechocystis CyanoQ, determined to

a resolution of 1.8 Å, is similar to that of PsbQ from spinach with a root mean PX-478 square deviation (RMSD) for the Cα atoms of 1.4 Å despite only 17 % identity in primary structure (Jackson et al. 2010). Both crystallised proteins consist of a four-helix bundle and contain bound Zn2+, although a metal-free structure has also been determined for Synechocystis CyanoQ

(Jackson et al. 2010); the physiological relevance of these metal-binding sites is currently unknown. In contrast, much less is known about CyanoQ in the thermophilic cyanobacteria used for structural studies of PSII. Indeed the association of CyanoQ with PSII in either T. elongatus or T. vulcanus has yet to be demonstrated. Here, we describe the crystal structure of E. coli-expressed CyanoQ from T. elongatus and provide evidence that CyanoQ co-purifies with isolated PSII and strikingly is still present in samples used to generate PSII crystals lacking CyanoQ. Materials and methods Thermosynechococcus elongatus BP1 strains A His-tagged CP43 until strain (CP43-His) of Thermosynechococcus elongatus (Sugiura and Inoue 1999) was kindly provided by Dr Miwa Sugiura, and a His6-tagged derivative of CP47 (CP47-His) by Dr Diana Kirilovsky. The WT strain was the same as that used by Ferreira et al. (2004). Construction of plasmid for over-expression of CyanoQ The DNA sequence corresponding to the CyanoQ homologue of T. elongatus (tll2057) without the sequence encoding the predicted signal peptide and lipid-binding Cys24 residue was cloned into a pRSET-A vector modified as described in Bialek et al. (2013). The corresponding PCR fragment was amplified from T.