This study investigates the structural changes in histone H1 (H1) induced by nucleotides and calcium ions. Using circular dichroism (CD) analysis, the researchers found that ATP and calcium ions induce structural changes in H1, particularly increasing its α-helicity. These changes occur even when H1 is interacting with DNA, suggesting that nucleotide and calcium binding may be important for H1-mediated chromatin changes. The study also shows that ATP and calcium ions can induce distinct changes in the CD spectrum of DNA, indicating changes in DNA structure. H1 is a 220-amino-acid protein consisting of a structured globular domain (gH1) and mobile N- and C-terminal extensions. The structures of gH1 and gH5 have been determined using NMR and X-ray diffraction, revealing three α-helices and a β-hairpin on the C-terminal side. The β-hairpin portion of gH1 resembles the phosphate-binding motifs of β-actin and the catalytic subunit of cAMP-dependent protein kinase, which are involved in the recognition of ATP. There is also limited data supporting specific binding of nucleotides to H1. The study used CD spectroscopy to analyze the effect of ATP and other nucleotides on the structure of H1. They found that ATP can induce a high level of α-helicity in H1, even in the presence of DNA and/or calcium ions. They also showed that in the presence of ATP or Ca²⁺-ATP, H1 induces distinct changes in the CD spectrum of DNA, indicative of changes in the DNA structure. The results suggest that nucleotides and calcium ions play a role in the structural changes of H1, which may be important for chromatin remodeling. The study also indicates that the effect of ATP on H1 is distinct from the effect of inorganic phosphate and DNA. The findings support the idea that ATP is involved in chromatin remodeling upon activation, as it can induce structural changes in H1 and DNA. The study also suggests that the conformational changes in H1 may alter its interaction with DNA, which could be important for gene regulation.This study investigates the structural changes in histone H1 (H1) induced by nucleotides and calcium ions. Using circular dichroism (CD) analysis, the researchers found that ATP and calcium ions induce structural changes in H1, particularly increasing its α-helicity. These changes occur even when H1 is interacting with DNA, suggesting that nucleotide and calcium binding may be important for H1-mediated chromatin changes. The study also shows that ATP and calcium ions can induce distinct changes in the CD spectrum of DNA, indicating changes in DNA structure. H1 is a 220-amino-acid protein consisting of a structured globular domain (gH1) and mobile N- and C-terminal extensions. The structures of gH1 and gH5 have been determined using NMR and X-ray diffraction, revealing three α-helices and a β-hairpin on the C-terminal side. The β-hairpin portion of gH1 resembles the phosphate-binding motifs of β-actin and the catalytic subunit of cAMP-dependent protein kinase, which are involved in the recognition of ATP. There is also limited data supporting specific binding of nucleotides to H1. The study used CD spectroscopy to analyze the effect of ATP and other nucleotides on the structure of H1. They found that ATP can induce a high level of α-helicity in H1, even in the presence of DNA and/or calcium ions. They also showed that in the presence of ATP or Ca²⁺-ATP, H1 induces distinct changes in the CD spectrum of DNA, indicative of changes in the DNA structure. The results suggest that nucleotides and calcium ions play a role in the structural changes of H1, which may be important for chromatin remodeling. The study also indicates that the effect of ATP on H1 is distinct from the effect of inorganic phosphate and DNA. The findings support the idea that ATP is involved in chromatin remodeling upon activation, as it can induce structural changes in H1 and DNA. The study also suggests that the conformational changes in H1 may alter its interaction with DNA, which could be important for gene regulation.