The histone N tails correspond to conserved amino acid sequences that are peripherally located in the nucleosome and undergo a variety of post-synthetic modifications during cell cycle. These N tails have been recently recognized as directly interacting with transcription-related proteins. We show here, based on circular dichroic evidence, that the N tails of both tetrameric histones H3 and H4 are highly organized as DNA-bound polypeptide segments in the nucleosome core particle, with about half of their residues, taken together, being alpha-helical. In contrast, the N tails of both dimeric histones H2A and H2B are found essentially in a random-coil conformation. The implications of these findings on nucleosome structure and recognition are discussed.
