Histone III V. THE AMINO ACID SEQUENCE OF PEA EMBRYO HISTONE III

Abstract The amino acid sequence of pea embryo histone III has been determined and compared to that of calf thymus histone III. The sequence is identical in the two proteins except at four positions: residue 41 is phenylalanine in pea, tyrosine in calf; residue 53 is lysine in pea, arginine in calf; residue 90 is serine in pea, methionine in calf; and residue 96 is alanine (60%) or serine (40%) in pea, cysteine in calf. Studies on tryptic, chymotryptic, and thermolysin peptides of cyanogen bromide fragments of pea histone III revealed no other differences in the amino acid sequence, including the distribution of amides. As in calf and carp histone III, the e-amino groups of lysine residues 9 and 27 are methylated. Both sites contain one or two methyl groups, but unlike calf and carp histone III, pea histone III does not contain e-N-trimethyllysine. e-N-Acetyllysine was not detected. The microheterogeneity at residue 96 in pea embryo histone III is the first definitive evidence for two forms of this histone in this species. Microheterogeneity in calf thymus histone III had been suggested, but sequence work in this laboratory did not provide evidence to support this contention. This study on histone III indicates a high degree of conservation of sequence during the evolution of plant and animal life, with only four differences between these proteins of calf and pea. This is in accord with the even greater conservation of sequence in histone IV for the same two species, for which only two differences in sequence were reported earlier.