THE PETF REGION OF THE CHLOROPLAST GENOME FROM THE DIATOM THALASSIOSIRA-WEISSFLOGII - SEQUENCE, ORGANIZATION AND PHYLOGENY

We report the cloning and characterization of a 1881 bp Thalassiosira weissflogii (Bacillariophyceae, Centrales) plastid genomic fragment co ntaining the ferredoxin gene (petF) and several other genes. The organ ization of this fragment is exactly the same as its equivalent region from the plastid genome of another centric diatom, Odontella sinensis, and the sequences share high homology in both coding and non-coding r egions. Traces of a probable vestigial protein-coding gene and of a pa rtial trn gene are found just after the trnR gene in both diatoms as w ell as in the plastid genome of the red alga Porphyra purpurea, illust rating the close relationship between red algal and heterokont algal p lastid genomes and arguing in favour of the origin of the latter by se condary endosymbiosis. In accordance with phylogenetic analyses of mos t plastid genes, phylogenetic trees inferred from 69 ferredoxin sequen ces show that both the green and the red algal plastid genes arose fro m cyanobacterial genes; our results furthermore suggest that each of t hese plastid lineages is related to different extant cyanobacterial ge nes, implying either that different endosymbiotic events were responsi ble for the origin of green and red plastids or that each lineage kept only one (and a different one) of several paralogous cyanobacterial g enes present in the ancestral cyanobacterial genome.