MJ1647, an open reading frame in the genome of the hyperthermophile Methanococcus jannaschii, encodes a very thermostable archaeal histone with a C-terminal extension

All archaeal histones studied to date have similar lengths, 66 to 69 amino acid residues that form three alpha-helices separated by two beta-strand lo op regions which together constitute a histone fold. In contrast, the eukar yal nucleosome core histones are larger, 102 to 135 residues in length, wit h N-terminal and C-terminal extensions flanking the histone fold that parti cipate in gene regulation and higher-order chromatin assembly. In the Metha nococcus jannaschii genome, MJ1647 was annotated as an open reading frame p redicted to encode an archaeal histone with an approximately 27-amino-acid C-terminal extension, and we here document the DNA binding and assembly pro perties and thermodynamic stability parameters of the recombinant product o f MJ1647 synthesized in Escherichia coli with (rMJ1647) and without (rMJ164 7 Delta) the C-terminal extension. The presence of the C-terminal extension did not prevent homodimer formation or inhibit DNA binding, but the comple xes formed by rMJ1637, presumably archaeal nucleosomes containing a (rMJ163 7)(4) tetramer, were apparently less stable than these formed by (rMJ1647 D elta)(4). The presence of the C-terminal extension increased the thermostab ility of rMJ1647 when compared with rMJ1647 Delta in 0.2 M KCl at pH 4 but not in the absence of KCl at pH 1. Based on thermal unfolding transitions, rMJ1617 and rHAfB generated by expression of AF0337 cloned from the genome of the related hyperthermophile Archaeoglobus fulgidus in E. coli were foun d to have higher thermodynamic stabilities than all previously studied arch aeal histones.