Core histone genes of Giardia intestinalis: genomic organization, promoter structure, and expression
1 Departments of Biology and Chemistry, Biochemistry Program, Trent University, Peterborough, Ontario, K9J 7B8, Canada
2 Departments of Immunobiology and Medicine, University of Arizona College of Medicine, Tucson, AZ 85724, USA
3 The Rockefeller University, 1230 York Avenue, New York, NY 10021, USA
4 Collegium Budapest, H 1012 Budapest, Hungary
5 Haskins Laboratories and Department of Chemistry and Physical Sciences, Pace University, 41 Park Row, New York, NY 10038, USA
BMC Molecular Biology 2007, 8:26 doi:10.1186/1471-2199-8-26Published: 10 April 2007
Giardia intestinalis is a protist found in freshwaters worldwide, and is the most common cause of parasitic diarrhea in humans. The phylogenetic position of this parasite is still much debated. Histones are small, highly conserved proteins that associate tightly with DNA to form chromatin within the nucleus. There are two classes of core histone genes in higher eukaryotes: DNA replication-independent histones and DNA replication-dependent ones.
We identified two copies each of the core histone H2a, H2b and H3 genes, and three copies of the H4 gene, at separate locations on chromosomes 3, 4 and 5 within the genome of Giardia intestinalis, but no gene encoding a H1 linker histone could be recognized. The copies of each gene share extensive DNA sequence identities throughout their coding and 5' noncoding regions, which suggests these copies have arisen from relatively recent gene duplications or gene conversions. The transcription start sites are at triplet A sequences 1–27 nucleotides upstream of the translation start codon for each gene. We determined that a 50 bp region upstream from the start of the histone H4 coding region is the minimal promoter, and a highly conserved 15 bp sequence called the histone motif (him) is essential for its activity. The Giardia core histone genes are constitutively expressed at approximately equivalent levels and their mRNAs are polyadenylated. Competition gel-shift experiments suggest that a factor within the protein complex that binds him may also be a part of the protein complexes that bind other promoter elements described previously in Giardia.
In contrast to other eukaryotes, the Giardia genome has only a single class of core histone genes that encode replication-independent histones. Our inability to locate a gene encoding the linker histone H1 leads us to speculate that the H1 protein may not be required for the compaction of Giardia's small and gene-rich genome.