6 Å and the structure solved by molecular replacement using the crystal

structure of CyanoQ from Synechocystis (PDB:3LS0, for details see Table 1). The refined co-ordinates of the 3D model of CyanoQ from T. elongatus have been Selleck LDK378 deposited at the Protein Data Bank using the accession code 3ZSU. The first nine N-terminal residues as well as the last C-terminal residue of CyanoQ could not be detected in the BX-795 electron density map so only residues 34–151 were fitted. Topologically the protein belongs to four-helix bundle superfamily and its fold is classified as mainly alpha up-down bundle (CATH 1.20.120.290) with four α-helices, of which the first two are broken, and one 310 helix (Fig. 4a). The three-dimensional structure of CyanoQ from thermophilic T. elongatus showed a high level of similarity with the two structures of CyanoQ (with and without bound zinc) from the mesophilic Synechocystis LY2835219 datasheet (Jackson et al. 2010) with a RMSD of 1.6 Å for the C α atoms (Table 2 and Fig. S7). Table 1 Data collection and

refinement statistics for the CyanoQ crystal structure   CyanoQ data X-ray source Diamond I03 Data processing Mosflm/Scala Space group P 21 21 21 Unit-cell parameters a = 47.165 Å, b = 47.165 Å, c = 106.700 Å, α = β = 90°, γ = 120° Wavelength (Å) 1.0722 Resolution (Å) 53.4–1.6 (1.69–1.60) Measured reflections 130,767 (19,307) Unique reflections 18,728 (2707) Mn (I/sd) 10.8 (3.7) Completeness (%) 99.38 (100.0) Multiplicity 6.98 (7.13) R meas (%) 0.11 (0.62) Solvent content (%) 48.6 R work/R Sulfite dehydrogenase free (%) 16.7/19.0 Protein atoms 974 Solvent atoms 79 RMSD from ideal   Bond lengths (Å) 0.022 Bond angles

(°) 1.982 Average B factor (Å2) 18.2 Ramachandran favoured region (%) 100 Ramachandran allowed region (%) 0 \(R_\textmeas = \mathop \sum \limits_h (\fracn_hn_h – 1)\mathop \sum \limits_I I_hl – < I_h > /\mathop \sum \limits_h \mathop \sum \limits_I < I_h >\) Fig. 4 a Overall structure of CyanoQ from T. elongatus coloured according to DSSP (Kabsch and Sander 1983): α-helices (α1-α4, red), 310 helix (blue, η1), hydrogen-bonded turns (cyan) and bends (green). b top and c bottom view of the protein coloured according to sequence conservation in cyanobacteria with most conserved residues shown as sticks. Bottom view in c corresponds to the end of CyanoQ containing the N- and C-termini. d Consurf (Ashkenazy et al. 2010) analysis of two conserved cavities (H4-H1 in upper view and H2–H3 in lower view; see text for details) with most conserved residues shown in dark pink and magenta. The most divergent regions are coloured in cyan Table 2 Comparison of sequence identities and similarities (%, top) and structural RMSD (bottom) of CyanoQ from T. elongatus (3ZSU), Synechocystis with and without zinc (3LS1 and 3LS0) and PsbQ from spinach (1VYK and 1NZE)   3ZSU 3LS0 3LS1 1VYK 1NZE   T. elongatus Synechocystis S. oleracea 3ZSU   31/50 31/50 14/24 14/24 3LS0 1.6 Å   100/100 17/33 17/33 3LS1 2.0 Å 0.