Two Chemically and Metabolically Distinct Forms of Calf Thymus Histone F3

The F3 histones of duck erythrocytes and calf thymus have been purified and compared by a variety of techniques. The results of analytical gel electrophoresis, gel filtration, cysteine content determinations, and amino acid analysis support the conclusion that there exist two chemically and metabolically distinct forms of F3 histone. The duck erythrocyte has a single form of F3 histone, which contains 1 cysteine residue. The calf thymus, however, has two forms of F3 histone. The minor form present as approximately 20% of the total F3 histone fraction closely resembles the duck erythrocyte F3 in that it contains a single cysteine residue. The major form of calf thymus F3 histone, however, contains 2 half-cystine residues. These two forms of F3 histone differ metabolically in that they are acetylated at different rates in isolated calf thymus nuclei. The implications of these findings are discussed in terms of the possible evolutionary origins of histone F3.