Barber, James, Nield, Jon, Duncan, James and Bibby, Thomas S.
Accessory chlorophyll proteins in cyanobacterial photosystem I.
Golbeck, John H. (eds.)
Photosystem I: The Light-Driven Plastocyanin: Ferredoxin Oxidoreductase.
Dordrecht, The Netherlands,
(Advances in Photosynthesis and Respiration 24).
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The IsiA protein accumulates in many cyanobacteria under conditions of iron starvation. It is a Chl a-binding protein, which is closely related to the six-transmembrane ?-helical antenna family typified by CP43 of PS II. One of its functions is to provide an efficient accessory light harvesting system for Photosystem I (PS I) by forming a ring of 18 IsiA subunits around the trimeric reaction center core. This response is probably to compensate for a drop in the PS I level relative to Photosystem II (PS II) and, the level of phycobiliproteins, in response to iron deficiency. A similar accessory light harvesting system for PS I has been shown to occur in cyanobacteria that do not contain phycobiliproteins, often termed prochlorophytes. This light harvesting system is composed of Pcb proteins that are closely related to IsiA but bind Chl b as well as Chl a. Unlike IsiA, Pcb proteins can also provide accessory light harvesting systems for PS II. Some cyanobacteria contain phycobiliproteins and Pcb proteins, where both are able to function as light harvesting systems. In one case the Pcb protein seems to bind only Chl a (Fischerella muscicola) while in another they bind Chl a (Acaryochloris marina). Sequence analysis indicates that the IsiA/Pcb proteins have very similar pigment binding sites to those of CP43 and to a lesser extent to the other reaction center proteins of this family and have the capacity to accommodate several different forms of Chl. The six-transmembrane ?-helical Chl-binding proteins, of which IsiA and Pcb are members, seem to have evolved from a basic evolutionary building block providing both internal and accessory light harvesting systems for a wide range of photosynthetic organisms.
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