Evolutionary Origin of the Protozoan Parasites Histone-like Proteins (HU)

The histone-like proteins (HU) belong to a family of DNA architectural proteins that stabilize nucleoprotein complexes. We found a putative HU protein (TgGlmHMM_3045) in Toxoplasma gondii genome that was homologous to the bacterial HU protein. This putative sequence was located in the scaffold TGG_995361 of the chromosome 10. The sequence included the prokaryotic bacterial histone-like domain, KFGSLGlRRRGERVARNPRT (ID number PS00045). HU protein sequences were also found in Plasmodium falciparum, Neospora caninum, Theileria parva and Theileria annulata. We found that the homology of the putative HU protein in Apicomplexa was greater with bacterial histone-like proteins than with eukaryotic histone proteins. The phylogenetic tree indicated that the putative HU protein genes were acquired in Apicomplexa by means of a secondary endosymbiotic event from red algae and later they were transferred from the apicoplast organelle to the nuclear genome.