Partial sequences for the hypothetical chloroplast frame 24 (ycf24) were determined for 13 species of algae representing the classes Bacillariophyceae, Chrysophyceae, Pelagophyceae, Pinguiophyceae, Prymnesiophyceae, Raphidophyceae, and Xanthophyceae. In many databases ycf24 is considered an ATP-binding cassette (ABC) transporter. Our analyses of translated amino acid sequences demonstrate that these sequences lack Walker A and B boxes, a conserved ABC signature sequence motif, and transmembrane domains that characterize ABC transporters. Our data, as well as those of others, convincingly indicate that ycf24 is not an ABC transporter and is instead homologous to the sufB gene first described from procaryotes. The function of the sufB gene product in eucaryotic plastids is not known with certainty but it likely plays a key role in iron-sulfur (2Fe-2S) cluster formation. Our data, unpublished results and database searches imply that sufB is plastid encoded in the "red" plastid lineage, but has been transferred to the nucleus or lost within the streptophyte lineage. Phylogenetic analyses of all available eucaryotic sufB sequences indicate that this gene is a useful marker for tracing the ancestry of plastids and their hosts. For example, our sufB analyses indicate that the 35-Kb circular plastid genome of Plasmodium falciparum is a member of the "red", not "green", plastid lineage and was acquired via secondary endosymbiosis. Our results also support the hypothesis that, despite their similarities, the plastids of prymnesiophytes and heterokont algae were independently obtained by separate secondary endosymbiotic events.

Key words: algae, chloroplasts, phylogeny, Plasmodium, Prymnesiophyceae, sufB