| The fruit fly Drosophila melanogaster is an important model organism in research fields of genetics and developmental biology due to the advantages of easy to breed and to get enough embryos in the laboratory, short life cycle, having large polytene chromosomes, and etc. It has made great contributions to the development of modern life science. D. melanogaster genome has been completely sequenced and contains～14,000 genes. About 2/3 genes in human being which cause diseases including those associated with cancer, can be found in Drosophila as homologues.Hir/Hira (histone regulation) genes were initially identified in yeast as negative regulators of histone gene expression. It has been confirmed that HIRA, a histone chaperone, contains a conserved family of proteins found in various species including low eukaryotes, invertebrates and vertebrates. The HIRA family is characterized by containing seven WD repeat domains in its N-terminal half which was predicted to form aβ-propeller structure. HIRA may function in regulating chromatin structure, nucleosome reassembly and cell cycle progression by the interactions of its different domains with different proteins. The N-terminal half may participate in the regulation of histone gene transcription and the silence of heterochromatin via interacting with other histone chaperones, such as ASF1 and CAF-1. Furthermore the N-terminal half also may contribute to control of cell growth by modulating the transcription of cell cycle-related genes. The C-terminal half is responsible for its binding ability as to homeodomains, HIRA-interacting protein 3(HIRIP3), and core histones. HIRA is well conserved during the long evolutionary pathway which indicates that this protein may be essential for life development. Targeted mutation of Hira gene resulted in gastrulation defects and embryonic lethality by around E10. Kirov et al. and Llevadot et al. successively found a new member of the HIRA family, DHIRA from D. melanogaster. Northern analysis showed that hybridization is strong in female adults, unfertilized eggs and 0-3-h-old embryos, then diminishes, but is still detectable. These results indicate that the dHira transcripts are maternal and they may play very important roles during early embryogenesis. While the accurate function in development of Hira still remains unclear.In order to study the function of dHira in Drosophila development, the dHira gene was cloned into UASp vector and overexpressed by UAS-Gal4 system under the control of maternal Gal4 driver in the Drosophila embryogenesis. The overexpression of dHira caused significantly higher lethality of embryos than that in wild type groups, and the embryonic lethality increased with the increasing of transgenic copies. The results suggest that dHira gene may have important functions in embryogenesis of Drosophila. The result of PI staining of early embryos displayed that the nuclear division in gain-of-function mutant embryos is abnormal. The number of nuclei in synchronous blastoderm is much fewer than that in wild type group, and the distribution of nuclei was uneven, which indicates that the nuclear division in mutated early embryos was asynchronous and maybe some nuclei undergoes degeneration. RT-PCR analysis showed that the gene expressions of core histone H2A, H2B, H3 and H4 had no obvious differences in early embryos between dHira overexpressing groups and wild types. Whereas the transcripts of cell cycle-related and apoptosis-related genes: CycA, CycE, Cdc25 and debcl increased in dHira overexpressing groups comparing with wild types. These results suggest that overexpression of DHIRA may affect the rapid and synchronous nuclear division in Drosophila early embryos and cause the high embryonic lethality by disturbing the normal expression of cell- cycle-related and apoptosis associated genes.
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