Daddy, where did (PS)I come from?

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.