Isolation and characterization of a Drosophila hydei histone DNA repeat unit

Histone genes in D. hydei are organized in tandemly repeated clusters., accomodating in total 120-140 repeat units. We cloned one of the repeat units and analysed the nucleotide sequence. The repeat unit has a size of 5.1 x 10(3) base-pairs and contains one copy of each of the genes coding for the core histones and one copy coding for the histone H1. In the promoter regions of the genes we identified the presumptive cap sites and TATA boxes. Two additional sequence elements are shared by all five Drosophila hydei histone genes in the cluster. The sequence CCCTCT/G1 is found in the region upstream of the presumptive CAP sites. The sequence element AGTGAA occurs downstream of the presumptive cap sites and is, in contrast to the promoter element, also seen in the histone genes of Drosophila melanogaster. Cell-cycle dependent regulation of transcription of the Drosophila histone genes may be different from that in other eukaryotes since sequence elements involved in the regulation of cell-cycle dependent transcription are absent. Also other regulatory elements for transcription differ from those of other genes. The highly conserved H1-specific promoter sequence AAACACA and the H2B specific promoter sequence ATTTGCAT, which are involved in the cell-cycle dependent transcription of those histone genes in eukaryotes, are missing in the Drosophila genes. However at the 3' end of the genes the palindrome and the purine-rich region, both conserved sequence elements in histone genes of eukaryotes, are present. The spacer regions show a simple sequence organization. The silent site substitution rate between the coding regions of the D. hydei and D. melanogaster histone genes is at least 1.5 times higher for Drosophila than for sea urchin histone genes.