Identification and expression analysis of rice histone genes

Histones, acting as the core of nucleosome, are the chief protein component of chromatin. They play an important role in gene regulation by covalent modification at several sites and histone variants replacement. Five major families of histones exist: H1, H2A, H2B, H3 and H4. The protein sequences within each family appear to be highly conserved. In this paper, we identified 60 histone proteins in rice (Oryza sativa) including 14 H2A, 15 H2B, 16 H3, 11 H4 and 4 H1. Sequence analysis indicates that histone protein sequences in plant are more variable than in animal. Interestingly, we found a rice-specific H4 variant which showed several amino acid substitutions with canonical protein and was expressed in different tissues in a low level. Expression analysis indicates that a subset of histone genes were expressed in a similar pattern and many of them responded to stress conditions. Specifically, we found that two H2A.Z genes were down-regulated by stress in leaves but not in roots suggesting that they might be involved in stress response.