Structure and expression of a cDNA encoding a histone H2A from Euglena gracilis.

Screening of a lambda gt11 cDNA expression library of Euglena gracilis with antibodies directed against histones H2 from maize resulted in the isolation of a full-length cDNA for a histone H2A. The open-reading frame of 408 bp corresponded to a protein of 136 amino acid residues (14 kDa). Despite the presence of a poly(A) tail, which is typical of plant histone mRNA but not of animal histone mRNA, the size of the deduced protein and its percentage of homology were closer to animal histone H2As than to plant or lower eukaryotic histone H2A. Sequence alignment revealed that the Euglena H2A protein was characterized by a shorter C-terminus and a N-terminus which extended 10 residues past the animal H2A. In contrast to other organisms studied, the expression of the Euglena H2A gene appeared to be almost constant during an entire life-cycle and presented no cell-stage-specific expression during development. Similar results are obtained for another histone gene, H3, and for beta-tubulin. Regulation of gene expression at a post-transcriptional level seems to be a general feature of Euglena.