A new unit of DNA-binding in protein different from α-helix, β-sheet and Zn-finger, was revealed by the analysis of primary structures of sea urchin histones. The primary structure of this unit was identified as Ser-Pro- Lys(Arg)-Lys(Arg) residues. Its secondary structure is probably the turn stabilized by two hydrogen bondings.
All the DNA-binding arms of sea urchin spermatogenous H1 and H2B have the repeat of the tetrapeptide, SPKK. The location of H1 and H2B in the nucleosome and their domain structures indicate that SPKK residues are bound to the linker DNA.