Supplemental Figure S1: Appearance of sucrose gradients for isolation of the Chl-protein complexes from cells grown in Fe-replete (Fe+) or Fe-depleted (Fe–) growth media. The numbered fractions were collected for further purification of the Chl-proteins by ion-exchange chromatography and/or clear-native gel electrophoresis. JSC-1 (Fe+) JSC-1 (Fe–) 4 5 1 2 3

Supplemental Figure S2: Protein composition analyses of the PSI trimer complexes (lanes 1, JSC1 Fe+_T4) isolated from iron-replete cells and the Chl-protein complexes (lanes 2, JSC1 Fe–_T5 (fractions from sucrose gradients shown in Supplemental Figure S1) isolated from iron-starved cells of Leptolyngbya JSC-1. A. Immunoblotting with anti-IsiA4 antibodies. B. SDS-PAGE followed by silver staining. The antibodies detect both IsiA and IsiL proteins as indicated in panel A. It is possible that some proteolysis of IsiA4 may have occurred during the isolation, which could explain the two bands of PsaL and IsiA that were observed in panels A and B, lane 2. It should be noted that the JSC-1 genome contains three genes for PsaL. A kDa 55 40 35 25 15 1 2 IsiA4 IsiA (1,2,3, 5) PsaL B kDa 50 40 30 25 15 1 2 20 70 60 85 10 PsaA1/PsaB1 IsiA4 IsiA(1, 2, 3, 5) PsaD PsaL1 PsaF1 PsaE1 PsaK1 PsaC PsaJ1/X/I1

Supplemental Figure S3: Clear Native-PAGE analysis of trimeric PSI complexes (from fraction Fe+_T4) from cells grown in Fe-replete conditions and Chl-protein complexes in fractions 5-Fe–-E40 and 5-Fe–-E80 from cells grown Fe-depleted growth media. The three boxed green bands (GB1, GB2 and GB3) were subjected to in-gel tryptic digestion and LC-MS-MS analysis (see Table 2). Fe–_E40 Fe+_T4 GB1 GB2 GB3 Fe–_E80

Supplemental Figure S4: Comparison of the TEM images for trimeric PS I complexes isolated from cells grown in iron-replete growth media (left panel) and for chlorophyll-protein complexes isolated from cells under iron-starvation conditions (right panel). Complexes were negatively stained with uranyl formate (1%, w/v).

Supplemental Figure S5: Alignment of the amino acid sequences of the five IsiA-family proteins of Leptolyngbya JSC-1 with three IsiA-like proteins of the low-light ecotype A14-63 of Synechococcus sp. and the single copy of IsiA protein from Synechococcus sp. PCC 7002. Transmembrane α–helices are shaded in green and possible Chl liganding histidine residues are shaded yellow.

IsiA4 (side view) IsiA4 (top view) IsiA1 IsiA2 IsiA3 IsiA5 Supplemental Figure S6: Predicted structures of the five IsiA-family proteins from Leptplyngbya JSC-1. The PsaL domain of IsiA4 is shown in aqua blue and its IsiA-like domain is shown in green. IsiA4 (top view) IsiA4 (side view) IsiA5 IsiA2 IsiA1 IsiA3