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 sufficiently low to limit photosynthetic activity and phytoplankton growth. 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). The precise function of the CP43-like protein, here called CP43', has not been elucidated, although there have been many suggestions. 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.
Bibby
T S
TS
Wolfson Laboratories, Department of Biological Sciences, Imperial College of Science, Technology & Medicine, London, UK.
Nield
J
J
Barber
J
J
D001426:Bacterial Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism / Q000502:physiology* / Q000648:ultrastructure; D002352:Carrier Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism / Q000502:physiology* / Q000648:ultrastructure; D018360:Crystallography, X-Ray; D000458:Cyanobacteria / Q000378:metabolism / Q000502:physiology* / Q000648:ultrastructure; D007501:Iron / Q000502:physiology*; D045342:Light-Harvesting Protein Complexes; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry / Q000378:metabolism* / Q000648:ultrastructure; D045524:Phycobilisomes; D010839:Phytoplankton / Q000737:chemistry / Q000502:physiology / Q000648:ultrastructure; D011487:Protein Conformation
D001426:Bacterial Proteins; D002352:Carrier Proteins; D045342:Light-Harvesting Protein Complexes; D045322:Photosynthetic Reaction Center Complex Proteins; D045524:Phycobilisomes; C076726:chlorophyll A binding protein CP43, Cyanobacteria; D007501:Iron