Sequence homology and role of cysteine in plant and animal arginine-rich histones.

The two arginine-rich histones III and IV, isolated from pea buds and calf thymus glands, were separated by preparative disc electrophoresis and each was shown to be homogeneous by amino-terminal and carboxyl-terminal analysis. Homologous arginine-rich components from calf thymus and pea bud were found to be extremely similar or identical in their aminoand carboxyl-terminal amino acids, amino acid compositions, electrophoretic mobilities, and tryptic peptide maps. The complex electrophoretic behavior of calf thymus histone III was shown to be artifactual and due to the formation of disulfide bridges between electrophoretically identical monomer units. Each monomer was found to contain 2 moles of cysteinyl residues per 20,000 g of protein (presumably 2 residues per molecule). Pea bud histone III was shown to differ from calf thymus histone III in that the former contains only 1 cysteinyl residue per molecule; thus only dimers can be formed by oxidative polymerization. It is proposed on the basis of the extensive sequence homology between calf and pea histone III that histone III performs the same function in pea and calf and that consequently no more than 1 cysteinyl residue per molecule may be required in histone III function. Thus calf thymus histone III polymers are artifacts of preparation. The question is raised: are histone III-nonhistone protein complexes, linked by disulfide bridges, biologically meaningful?