Typically 5-L Erlenmeyer flasks were used to grow five 3.5-l cultures to give a total culture volume of about 17.5 l. Cells were harvested at an optical density of about 1 at 750 nm using a Sartocon cross flow filtration system (Sartorius) followed by centrifugation at 10,000 rpm (JA14 rotor, Beckman Coulter Ltd.) for 5 min at

room temperature. The cell pellet was re-suspended in RSB buffer (40 mM MES–NaOH pH 6.5, 15 mM MgCl2, 15 mM CaCl2, 1.2 M betaine and 10 % (v/v) glycerol) to a volume of 50–75 ml and disrupted by 2 passes at 25,000 psi using a T5 cell disruptor set to 4 °C (Constant Systems Ltd). Unbroken cells were removed by centrifugation at 1,000×g (JA14 rotor, Beckman Coulter Ltd.) for 5 min at 4 °C, and membranes were pelleted and washed three times with the same buffer PSI-7977 cost by centrifugation at 184,000×g (Ti45

rotor, Beckman Coulter Ltd.) for 20 min at 4 °C. Membranes were then resuspended in 20 mM MES–NaOH pH 6.5, 10 mM MgCl2, 20 mM CaCl2, 25 % (v/v) glycerol and stored at −0 °C. These membranes were then used to isolate PSII oxygen-evolving complexes from WT T. elongatus using the two-step anion-exchange chromatography procedure described by Kern et al. (2005). Dimeric His-tagged oxygen-evolving complexes were isolated from a His-tagged CP47 strain of T. elongatus by VX-765 concentration Ni-affinity purification followed by anion-exchange chromatography as described by Nowaczyk et al. (2006) except for the following modifications: freshly grown cells were broken in 20 mM MES–NaOH pH 6.5, 2.5 mM CaCl2, 2.5 mM MgCl2, 10 % (v/v) glycerol and 1.2 M betaine, and unbroken cells BLZ945 cost were removed by centrifuging at 1,000 g (JA14 rotor, Beckman Coulter Ltd.) for 5 min at 4 °C; the resulting supernatant was diluted to a Chl concentration

of 1 mg/ml and the thylakoid membranes SSR128129E were solubilised for 10 min at 4 °C with 1 % (w/v) n-dodecyl-β-D-maltoside (β-DDM) at a detergent to Chl ratio of 18:1 followed by a 30-min spin at 4 °C and 184,000 g (Ti70 rotor, Beckman Coulter Ltd.); the extract was incubated for 45 min with Ni-affinity resin (Probond Resin, Invitrogen) equilibrated in buffer E (20 mM MES–NaOH pH 6.5, 2.5 mM CaCl2, 2.5 mM MgCl2, 0.5 M D-mannitol and 0.03 % (w/v) β-DDM); after loading, the Ni-affinity column was washed with 6 column volumes of buffer E + 5-mM histidine; His-tagged PSII complexes were eluted by application of a 100-mM histidine isocratic step gradient in buffer E and loaded directly onto a Bio-Rad UNO Q-12 column using a AKTA Purifier 10 system (GE Healthcare Life Sciences); PSII complexes were eluted through the application of a 5–200-mM MgSO4 gradient in buffer E (at 2 mM/min and 4 ml/min). The third peak containing active PSII dimeric complexes (Nowaczyk et al. 2006) was concentrated using Vivaspin centrifugal concentrators (100,000 MWCO) before storing at −80 °C.