Part 1 - Introduction

Iron is the most abundant transition metal in the crust of the earth and is an absolute requirement for photosynthetic organisms such as cyanobacteria, because it is needed for many of the redox reactions of the photosynthetic electron transport system. However, in most aquatic ecosystems it can be sufficiently low to limit photosynthetic activity (1, 2). This finding is attributed mainly to the low solubility of Fe3+ above neutral pH in oxygenic ecosystems (3). As a result, cyanobacteria and other microorganisms have evolved a number of responses to cope with frequently occurring conditions of iron deficiency (4). One such response is to express two "iron stress-induced" genes, isiA and isiB (5, 6), which are located on the same operon. The isiB gene encodes for flavodoxin, which can functionally replace the iron containing ferredoxin (7). The isiA gene encodes for a protein often called CP43', because it has an amino acid sequence homologous to that of the chlorophyll a-binding protein, CP43 of photosystem II (PSII)1 (8, 9). Like CP43, CP43' is predicted to have six transmembrane helices, and judged by the conservation of histidine residues, it is likely to bind the same number of chlorophyll a molecules. The major difference is that CP43' lacks the large hydrophilic loop that joins the luminal ends of helices V and VI of CP43. For this reason, it has 342 amino acids rather than 472 (see Fig. 1).