Iron deficiency induces the formation of an antenna ring around trimeric photosystem I in cyanobacteria

Although iron is the fourth most abundant element in the Earth's crust, its concentration in the aquatic ecosystems-particularly the open oceans-is su fficiently low to limit photosynthetic activity and phytoplankton growth(1, 2). Cyanobacteria, a major class of phytoplankton, respond to iron deficien cy by expressing the 'iron-stress-induced' gene, isiA(ref. 3). The protein encoded by this gene has an amino-acid sequence that shows significant homo logy with one of the chlorophyll a-binding proteins (CP43) of photosystem I I (PSII)(4,5). The precise function of the CP43-like protein, here called C P43', has not been elucidated, although there have been many suggestions(3, 6). Here we show that CP43' associates with photosystem I (PSI) to form a c omplex that consists of a ring of 18 CP43' molecules around a PSI trimer. T his significantly increases the size of the light-harvesting system of PSI. The utilization of a PSII-like protein as an extra antenna for PSI emphasi ses the flexibility of cyanobacterial light-harvesting systems, and seems t o be a strategy which compensates for the lowering of phycobilisome and PSI levels in response to iron deficiency.