Analysis of the charge distribution in the C-terminal region of histone H1 as related to its interaction with DNA

We have studied the net positive charge distribution in the C-terminal region of histone H1. We find that it is not random, but rather uniform. In most histone H1 sequences, 4 +/- 1 positive charges are found in this region of the molecule in over 95% of all possible segments that are 10 amino acids long. Neither alternating sequences (basic-nonbasic) nor more complex repeating sequences are ever found. Clusters of three or more basic amino acids are seldom observed in somatic H1s, yet their presence increases in sperm histones and even more so in protamines. It is concluded that the C-terminal region of histone H1 has a remarkably uniform distribution of charge, in spite of its apparent variations in sequence in different proteins and within individual molecules. The functional significance of these findings is discussed, suggesting a purely electrostatic role for the C-terminal region of histone H1, which may be evenly wrapped around individual segments of DNA molecules, thus decreasing its net charge. A likely candidate for a long alpha-helical region in the C-terminal region of histone H1 from sea urchin spermatozoa also has been located. This region may contribute to the aggregating properties of this histone in sperm chromatin.