Nucleoprotein subunit structure in an unusual prokaryotic organism: Thermoplasma acidophilum

The freeliving thermophilic mycoplasma, Thermoplasma acidophilum, has a small acid-soluble protein tightly bound to its DNA. This protein is similar to eukaryotic histones in both size and amino acid composition. Here we report that the protein condenses DNA into globular particles that are about half the size of eukaryotic nucleosomes. Our conclusions are based primarily upon the following observations: (1) Nuclease digestion produced DNA fragments of 40 base-pairs. (2) The ratio of protein to DNA was such that 4--5 molecules of protein were associated with each 40 base-pair segment of DNA. (3) Protein crosslinking reagents produced tetramers of the histone-like protein. (4) Electron microscopy revealed globular particles 5--6 nm in diameter. (5) Each globular particle reduced the apparent contour length of the DNA by 40 base-pairs. Thus, each nucleoprotein particle is apparently composed of 40 base-pairs of DNA coiled around four molecules of proteins.