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Studies on the acetylation of trout testis histones Candido, Edward Peter Mario


Histones are basic chromosomal proteins which are complexed with DNA in eukaryotic organisms. During the sexual maturation of the rainbow trout (Salmo gairdnerii), there is extensive acetylation of the testis histones. Four of the five major histone fractions were found to be acetylated, and the acetate incorporated was shown to be largely present as ε-N-acetyl groups attached to internal lysyl residues in these proteins. Purified histones were seen to exhibit considerable heterogeneity when electrophoresed on starch gels, and this was shown to be largely due to the presence of acetylated components in the preparations. Histone Ilb₁ was shown to contain a single major acetylation site at lysyl residue 5; histone IIb₂ contains acetylated lysyl residues at positions 5, 10, 13 and 18, histone III at positions 9, 14, 18 and 23, and histone IV at positions 5, 8, 12 and 16. All of these modifications are found in the very basic amino-terminal regions of the proteins, which may be DNA binding sites. It is postulated that the function of these modifications is to modulate the positive charge density of histones, and hence their binding to DNA. The acetylation of histones in different trout testis cell types was investigated. All cell types incorporated acetate into histones, but whereas histone acetylation was accompanied by synthesis of these proteins in spermatocytes, the modification in spermatid cells occurred in the absence of significant histone synthesis. It is suggested that histone acetylation in spermatids may be involved in the process by which these proteins are replaced by protamine, a smaller, arginine-rich protein, during the maturation of trout testis spermatids. Histone acetylation in cell types which are actively synthesizing these proteins may be involved in achieving the correct binding of newly synthesized histones to DNA. An enzyme activity which transfers acetyl groups from acetyl coenzyme A to histones was isolated from trout testis chromatin, and some properties of this preparation have been noted.

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