IS PROCHLOROTHRIX-HOLLANDICA THE BEST CHOICE AS A PROKARYOTIC MODEL FOR HIGHER-PLANT CHL-A B PHOTOSYNTHESIS/

We examine the issue of prochlorophyte origins and provide analyses wh ich highlight the limitations of inferring evolutionary trees from anc iently diverged sequences that have markedly different GC contents. Un der these conditions we have found that current tree reconstruction me thods strongly group together sequences with similar GC contents, whet her or not the sequences share a common ancestor. We provide 3' psbA t ermini sequence for Prochloron didemni and find it does not have the 7 amino acid deletion that occurs in Chl a/b chloroplasts and Prochloro thrix hollandica. This is consistent with the recent findings of a Chl c like pigment in the light harvesting system in other prochlorophyte s but apparently absent in P. hollandica. From these observations we s uggest that P. hollandica is the prochlorophyte most closely related t o Chl a/b containing chloroplasts and hence the most appropriate proka ryotic model for higher plant Chl a/b photosynthesis.