Calf and Pea Histone IV

The complete amino acid sequence of pea seedling histone IV has been determined with the tryptic peptides from the maleylated protein and compared to the sequence of calf thymus histone IV. Both proteins contain 102 amino acid residues and have identical sequences except at two genetically determined positions: Residue 60 is valine in the calf but isoleucine in the pea, and Residue 77 is lysine in the calf but arginine in the pea. In addition to these changes, lysyl Residue 20 is methylated in the calf histone but not in the pea histone, and the amount and distribution of e-N-acetyllysine is different in the two histones. In calf histone IV, Residue 16 is acetylated to the extent of 50% and is the only lysyl residue so modified. Only about 6% of the pea histone IV molecules are acetylated, and the acetyl groups are distributed between Residue 16 and at least one of three other lysyl residues: 5, 8, or 12. Evidence obtained by peptide mapping suggests that Residue 8 is the other acetylated residue. These similarities and differences are discussed in the context of possible structure-function relationships of histone IV.