Histone Acetylation Associated with Gastrulation in Rana pipiens

Abstract

Differentiation of the embryo into a multicellular organism can be viewed as a result of selective repression of certain genes and activation of others. Since histone acetylation has been correlated with gene activation, it was of interest to determine if the activation of the embryonic genome at the beginning of gastrulation is associated with an increase in the rate of histone acetylation. Acetate incorporation into acid-soluble crude chromatin protein was studied in Rana pipiens embryonic cells dissociated from mid-blastula, late blastula, and early gastrula embryos. [3H] acetate incorporation increased two—three fold between the mid-blastula and early gastula stage. The increased rate of acetylation closely paralleled an increase in uridine incorporation observed at the onset of gastrulation. Acetate uptake, as well as acetate incorporation into total TCA-precipitable protein, remained constant during this period of development. Electrophoretic analysis of acetate incorporation into gastrula acid-soluble crude chromatin protein showed that acetate was almost completely incorporated into histones and not into other proteins found in the acid extract of crude chromatin. Inhibition of histone synthesis with cycloheximide completely abolished acetate incorporation into lysine-rich histone H1 while it produced only 50–60% inhibition of acetate incorporation into histones H3, H2A, and H4. This finding is consistent with other studies that have shown that H1 and H2A are acetylated at the serine N-terminus, H3 is acetylated at internal lysine residues, and H4 is acetylated N-terminally and internally. Histone acetylase activity was detected in whole embryonic homogenates. Determination of acetylase activity in embryos prior to and at the onset of gastrulation showed no change in acetylase activity. Therefore, the increase in histone acetylation at the onset of gastrulation cannot be attributed to an increase in acetylase activity. Since activation of RNA synthesis at the onset of gastrulation has been correlated with histone acetylation it is possible that a cause and effect relationship exists between these two biochemical events. Activation of RNA synthesis has been shown to be necessary for initiation of cellular differentiation at gastrulation. Thus, histone acetylation may be part of the underlying biochemical mechanism of embryonic differentiation.