The reacton centre I (RCI)-type photosystems from plants, cyano-, helio- an
d green sulphur bacteria are compared and the essential properties of an ar
chetypal RCI are deduced. Species containing RCI-type photosystems most pro
bably cluster together on a common branch of the phylogenetic tree. The pre
dicted branching order is green sulphur, helio- and cyanobacteria. Striking
similarities between RCI- and RCII-type photosystems recently became appar
ent in the three-dimensional structures of photosystem I (PSI), PSII and RC
II The phylogenetic relationship between all presently known photosystems i
s analysed suggesting (a) RCI as the ancestral photosystem and (b) the desc
endence of PSII from RCI via gene duplication and gene splitting. An evolut
ionary model trying to rationalise available data is presented. (C) 2001 El
sevier Science B.V. All rights reserved.