Cell-specific expression of plant histone H2A genes.

Histone H2A is a component of eukaryotic chromatin whose expression has not been studied in plants. We isolated and characterized a tomato and a pea cDNA encoding histone H2A. We found that in tomato H2A is encoded by a small gene family and that both the pea and the tomato mRNAs are polyadenylated. Tomato H2A has 82% amino acid residue identity to pea H2A, 83% to wheat, and 65% to human and yeast H2A. Plant H2As differ from fungal and animal H2As in their amino-terminal and carboxy-terminal regions. Carboxy-terminal plant H2A regions contain the motif SPKK, a peptide implicated in binding of A/T-rich DNA regions. By using RNA gel blot analysis, we determined that the steady-state mRNA level of these genes was abundant in apices and early developing fruit and very low in mature tissues. In situ RNA hybridization showed strong spatial regulation because the mRNA was abundant in some cells and not detectable in others. In tomato shoot tips, H2A-expressing cells were distributed irregularly in or near meristems. In tomato or pea root tips, expressing cells were concentrated near the apex, and their distribution was consistent with that expected of cycling cells. Other H2A transcripts were found in nondividing cortical cells that are known to undergo endoduplication during the late maturation phase of primary development.