Histones of Drosophila Embryos

Abstract

Five main classes of histones and one minor component have been isolated from Drosophila melanogaster embryos and purified by means of a gel electrophoretic method in which the electrophoretic mobilities of proteins depend on their hydrophobicity in addition to their charge and mass. When the purified histones are compared with analogous vertebrate histones in amino acid composition, the extent of difference varies widely among the different histone classes, the order from most to least conserved being F2a1, F3, F2a2, F2b, and F1. The compositional differences between mammalian and Drosophila F1 histones are particularly large, exceeding 30% for all amino acids except lysine and arginine. The F1 histone of Drosophila was cleaved with CNBr, and the COOH-terminal peptide, representing 60% of the molecule, was isolated. It was found to be enriched in lysine, as in rabbit F1 histone. These results suggest that despite the high rate of evolutionary change of this histone, the general arrangement of positively charged residues is largely conserved, possibly to satisfy requirements for correct binding to DNA. However, this COOH-terminal peptide contains less than one-half of the number of prolyl residues present in the corresponding portion of rabbit F1, indicating that the lysine-, alanine-, and proline-rich portion of this region, believed to be very constant among mammalian F1 histones, can undergo striking evolutionary changes.