## 2 Materials and Methods
### Generation and Growth of Synechocystis Strains  
_Synechocystis_ strains were grown photoautotrophically under \(\sim\)50 \(\upmu\)mol photons m\({}^{-2}\) s\({}^{-1}\) constant illumination at 30 \({}^{\circ}\)C in sterile BG11 medium [29] buffered with 10 mM TES (N-Tris(hydroxymethyl)methyl-2-aminoethanesulfonic acid)-KOH pH 8.2 (BG11-TES). For LAFG, cultures were grown in BG11-TES supplemented with 5 mM glucose in complete darkness apart from a 5-min pulse of white light (\(\sim\)40 \(\upmu\)mol photons m\({}^{-2}\) s\({}^{-1}\)) once every 24 h [30]. For growth on solid media, 1.5% (\(w\)/\(v\)) batoagar and 0.3% (\(w\)/\(v\)) sodium thiosulphate were added to BG11-TES. Antibiotics were included where appropriate, as detailed below. For attempted purification of FLAG-tagged Slr1599, cultures were grown either photoautotrophically (\(\sim\)50-100 \(\upmu\)mol photons m\({}^{-2}\) s\({}^{-1}\)) or under LAFG conditions (as above) in 8 L vessels mixed by bubbling with sterile air and maintained at 30 \({}^{\circ}\)C by a temperature coil connected to a thermostat-controlled circulating water bath. To make a Aslr1599 deletion mutant, a linear mutagenesis construct was generated from three individual PCR product templates by overlap-extension (OLE)-PCR. The first fragment consisted of 427 bp upstream of the slr1599 start codon (see Section 3.1), the first 80 bp of the slr1599 gene, and 26 bp of homology to the start of the erythromycin-resistance cassette from pERY [31] and was amplified with primers AH912 and AH913. The second fragment consisted of the erythromycin-resistance gene, which was amplified from pERY using primers AH595 and AH596. The third fragment consisted of 26 bp of homology to the end of the erythromycin-resistance cassette, the final 80 bp of slr1599, and 391 bp downstream of the slr1599 stop codon and was amplified with primers AH914 and AH915. The purified fragments were mixed in equimolar amounts and joined by OLE-PCR using primers AH912 and AH915, and the resulting construct was sequenced to confirm correct assembly (Eurofins). Following introduction of the OLE-PCR product into wild-type (WT) _Synchocystis_ by natural transformation, transformants were selected on BG11 agar containing 5 \(\upmu\)g mL\({}^{-1}\) erythromycin-cin and single colonies were picked and streaked onto BG11 agar containing a progressively doubled concentration of erythromycin-cin up to 40 \(\upmu\)g mL\({}^{-1}\). Full segregation of the Aslr1599 mutant was confirmed by PCR amplification with primers AH912 and AH915 and Sanger sequencing of the resultant purified PCR product (Eurofins). Complementation of Aslr1599 was performed by integrating slr1599 or the _Arabidopsis Z-ISO_ gene at the _psbAII_ locus using the pPD-NFLAG plasmid [32,33]. slr1599 was amplified from _Synchocystis_ WT genomic DNA using the primers AH955 and AH956 and cloned into the Notl and BgIII sites of pPD-NFLAG downstream of the sequence encoding an N-terminal 3\(\times\)FLAG tag. Attempts to insert slr1599 into the pPD-CFLAG plasmid [34] --such that when integrated in place of _psbAII_ in the _Synchocystis_ genome the construct would encode Z-ISO with a C-terminal 3\(\times\)FLAG tag--were unsuccessful, presumably due to toxicity of this fusion protein in the _E. coli_ JM109 cells used for cloning. Therefore, we PCR-amplified a linear DNA fragment consisting of slr1599 followed by the sequence adding the C-terminal FLAG-tag and the kanamycin-crisistance gene and flanked by the _psbAII_ homology arms from the ligation reaction using primers AH198 and AH199, which was verified by sequencing the PCR product. The _Arabidopsis Z-ISO_ gene (At1g10830) was synthesized (Integrated DNA Technologies) lacking bases 4-174 (the sequence encoding the N-terminal chloroplast transit peptide) and with the sequence encoding a Ser-Ala linker followed by the Strep-tag(r) II added to the 3' end of the gene, and then cloned into the NdeI and BgIII sites of pPD-NFLAG.