MBE Advance Access published online on August 10, 2005
Molecular Biology and Evolution, doi:10.1093/molbev/msi230
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1 University of British Columbia, Department of Botany, 6270 University Blvd., Vancouver BC, V6T1Z4, Canada; Institute of Parasitology, Academy of Sciences of Czech Republic & University of South Bohemia, Faculty of Biological Sciences, Brani
* To whom correspondence should be addressed. Heme biosynthesis represents one of the most essential metabolic pathways in living organisms, providing the precursors for cytochrome prosthetic groups, photosynthetic pigments, and vitamin B12. Using genomic data, we have compared the heme pathway in the diatom Thalassiosira pseudonana and the red alga Cyanidioschyzon merolae to those of green algae and higher plants, as well as to those of heterotrophic eukaryotes (fungi, apicomplexans and animals). Phylogenetic analyses showed the mosaic character of this pathway in photosynthetic eukaryotes. Although most of the algal and plant enzymes showed the expected plastid (cyanobacterial) origin, at least one of them (porphobilinogen deaminase) appears to have a mitochondrial (
Accepted July 18, 2005
Research Article
Mosaic Origin of the Heme Biosynthesis Pathway in Photosynthetic Eukaryotes
ovská 31, 370 05 
eské Bud
jovice, Czech Republic
2 University of British Columbia, Department of Botany, 6270 University Blvd., Vancouver BC, V6T1Z4, Canada
Beverley R. Green, E-mail: brgreen{at}interchange.ubc.ca
Abstract 
-proteobacterial) origin. Another enzyme, glutamyl-tRNA synthase (GluRS), obviously originated in the eukaryotic nucleus. Since all the plastid-targeted sequences consistently form a well-supported cluster, this suggests that genes were either transferred from the primary endosymbiont (cyanobacteria) to the primary host nucleus shortly after the primary endosymbiotic event, or were replaced with genes from other sources at an equally early time, i.e. before the formation of three primary plastid lineages. The one striking exception to this pattern is ferrochelatase, the enzyme catalyzing the first committed step to heme and bilin pigments. In this case, two red algal sequences do not cluster either with the other plastid sequences or with cyanobacterial sequences, and appear to have a proteobacterial origin like that of the apicomplexan parasites Plasmodium and Toxoplasma. Although the heterokonts also acquired their plastid via secondary endosymbiosis from a red alga, the diatom has a typical plastid-cyanobacterial ferrochelatase. We have not found any remnants of the plastid-like heme pathway in the non-photosynthetic heterokonts Phytophthora ramorum and P. sojae.
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