Chromatin and histories from Giardia lamblia: A new puzzle in primitive eukaryotes

The three deepest eukaryote lineages in small subunit ribosomal RNA phyloge nies are the amitochondriate Microsporidia, Metamonada, and Parabasalia. Th ey are followed by either the Euglenozoa (e.g., Euglena and Trypanosoma) or the Percolozoa as the first mitochondria-containing eukaryotes. Considerin g the great divergence of histone proteins in protozoa we have extended our studies of histones from Trypanosomes (Trypanosoma cruzi, Crithidia fascic ulata and Leishmania mexicana) to the Metamonada Giardia lamblia, since Gia rdia is thought to be one of the most primitive eukaryotes. In the present work, the structure of G. lamblia chromatin and the histone content of the soluble chromatin were investigated and compared with that of higher eukary otes, represented by calf thymus. The chromatin is present as nucleosome fi laments which resemble the calf thymus array in that they show a more regul ar arrangement than those described for Trypanosoma. SDS-polyacrylamide gel electrophoresis and protein characterization revealed that the four core h istones described in Giardia are in the same range of divergence with the h istones from other lower eukaryotes. In addition, G. lamblia presented an H 1 histone with electrophoretic mobility resembling the H1 of higher eukaryo tes, in spite of the fact that H1 has a different molecular mass in calf th ymus. Giardia also presents a basic protein which was identified as an HU-l ike DNA-binding protein usually present in eubacteria, indicating a chimaer ic composition for the DNA-binding protein set in this species. Finally, th e phylogenetic analysis of selected core histone protein sequences place Gi ardia divergence before Trypanosoma, despite the fact that Trypanosoma bran ch shows an acceleration in the evolutionary rate pointing to an unusual ev olutionary behavior in this lineage. (C) 2001 Wiley-Liss, Inc.