In vivo and in vitro methylation of lysine residues of Euglena gracilis histone H1.

We have earlier identified and purified two protein-lysine N-methyltransferases (Protein methylase III) from Euglena gracilis [J. Biol. Chem., 260, 7114 (1985)]. The enzymes were highly specific toward histone H1 (lysine-rich), and the enzymatic products were identified as epsilon-N-mono-, di- and trimethyllysines. These earlier studies, however, were carried out with rat liver histone H1 as the in vitro substrate. Presently, histone H1 has been purified from Euglena gracilis through Bio-Rex 70 and Bio-Gel P-100 column chromatography. The Euglena histone H1 showed a single band on SDS-polyacrylamide gel electrophoresis and behaved like other histone H1 of higher animals, whereas it had a much higher Rf value than the other histones H1 in acid/urea gel electrophoresis. When the Euglena histone H1 was [methyl-3H]-labeled in vitro by a homologous enzyme (one of the two Euglena protein methylase III) and analyzed on two-dimensional gel electrophoresis, three distinctive subtypes of histone H1 were shown to be radiolabeled, whereas five subtypes of rat liver histone H1 were found to be labeled. Finally, by the combined use of a strong cation exchange and reversed-phase Resolve C18 columns on HPLC, we demonstrated that Euglena histone H1 contains approximately 9 mol% of epsilon-N-methyllysines (1.40, 1.66, and 5.62 mol% for epsilon-N-mono-, di- and trimethyllysines, respectively). This is the first demonstration of the natural occurrence of epsilon-N-methyllysines in histone H1.