Molecular characterization of core histones in the silkworm, Bombyx mori

In eukaryotes, genomic DNA is wrapped around an octamer of the core histone proteins H2A, H2B, H3 and H4. Silkworms have been known to bear holocentric chromosomes and their regulatory mechanisms of chroma-tin remodeling remain unclear. We have cloned the silkworm canonical core histones and their variants. The H2A variants, H2AX and H2AZ, were highly conserved in the silkworm, whereas the fy has only one H2A variant, H2Av, which is a chimera of H2AX and H2AZ with the function of both molecules. In the silkworm, all histones except for H2AX were ubiquitously expressed in all tested tissues, and H2AX was expressed only in the genital organs. A subcellular localization analysis of the cloned histones using an EGFP fusion construction demonstrated that the behaviors of the histones are the same as those of genomic DNA throughout the cell cycle. The fuo-rescence intensities of H2AX and H3.3 were weaker than those of the other histones. The incorporation of these histones into chromatin might be restricted due to their specifc function in DNA repair and transcription activity. There were two H3 variants, H3.2 and H3.3, in the silkworm; H3.1 was not present. Lysine 9 on H3 was methylated and acetylated in silkworms bearing holocentric chromosomes.