Iron deficiency induces the formation of an antenna ring around trimeric PSI in cyanobacteria: Nature 2001

Iron deficiency induces the formation of an antenna ring around trimericPSI



Iron deficiency induces the formation of an antenna ring around trimeric photosystem I in cyanobacteria
- adapted from: Thomas S. Bibby, Jon Nield & James Barber. Nature: 412:743-745 NB This structure was subsequently visualised in 3D

Although iron is the fourth most abundant element in the Earth's crust, its concentration in the aquatic ecosystems, particularly the open oceansÐis sufficiently low to limit photosynthetic activity and phytoplankton growth1,2. Cyanobacteria, a major class of phytoplankton, respond to iron deficiency by expressing the `iron-stress-induced' gene, isiA(ref. 3). The protein encoded by this gene has an amino-acid sequence that shows significant homology with one of the chlorophyll a-binding proteins (CP43) of photosystem II (PSII)4,5. The precise function of the CP43-like protein, here called CP43', has not been elucidated, although there have been many suggestions3,6. Here we show that CP43' associates with photosystem I (PSI) to form a complex that consists of a ring of 18 CP43' molecules around a-PSI trimer. This significantly increases the size of the light-harvesting system of-PSI. The utilization of a-PSII-like protein as an extra antenna for-PSI emphasises the flexibility of cyanobacterial light-harvesting systems, and seems to be a strategy which compensates for the lowering of phycobilisome and-PSI levels in response to iron deficiency.
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