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• W1441172009 abstract "Biochem. J. (1971) 123, 465--469 Printed in <keat Britain The Synthesis and Decay of Histone Fractions and of Deoxyribonucleic Acid in the Developina Avian Brain BY STEPHEN C. BONDY Diviaion of Neurology, University of Oolorado Me,dical Genter, Denver, Oolo. 80220, U.S.A. (Receive,d 8 January 1971) 1. The turnover of cerebral histones and DNA after injection of [4,5- 3 H]leucine or [methyl-3- 3 H]thymidine, respectively, was studied in the developing chick. 2. Chroma.tin was prepared from chick nuclei that had been purified by centrifuga- tion through l.9x-sucrose. 3. Nuclear proteins were fractionated into three major histone classes, Fl (lysine-rich). F2(b) (slightly lysine-rich) and [F3+F2(a.)] (arginine-rich), and a non-histone protein residue. 4. The proportions of the histone classes remained constant throughout the period of development studied. 5. All histone fractions decayed at a similar rate, initially with a half-life of a.round 5 days, later with a half-life of 19 days. 6. N on-histone proteins from chroma.tin decayed in a heterogeneous manner with a wide range of half-lives. 7. Short-term la.belling studies showed that all histone fractions were synthesized at the same rate. 8. Some non-histone proteins were very rapidly synthesized relative to histones. 9. DNA had a longer half-life than any histone fraction studied. A biphasic exponential decay curve with half-lives of 23 and 50 days was found. 10. It was concluded that the turnover ofhistones can occur independently of that of DNA and that different histone classes have similar rates of synthesis and decay. Regulation of gene expression in higher organisms may largely be achieved by template restriction of the DNA genome by histones (Bonner et al. 1968). Although neither DNA nor histones a.re clearly tissue-specific (Neidle & Wa.elsch, 1964; McCarthy & Hoyer, 1964; DeLange, Fambrough, Smith & Bonner, 1969), they interact with other nuclear components so as to result in a tissue-specific chroma.tin (Paul & Gilmour, 1968). The exceptional stability of cerebral DNA (Ada.ms, 1966), in conjunction with a considerable ca.pa.city of cerebral chroma.tin as a template in RNA synthesis (Bondy & Roberts, 1969), makes the relation between cerebral DNA and histone metabolism of considerable interest. The relative metabolic inertness of cerebral histones was demonstrated by Piha., Cuenod & Wa.elsch (1966) who found the turnover of radio- actively la.belled histones in the rat bra.in to be slower than that of any other protein class studied. Histone decay followed a biphasic exponential course. This suggested either heterogeneity in the stability of histone classes or that the histones of two cell types within the bra.in were turning over at differing rates. This pa.per is an attempt to resolve this question by a study of the synthesis and decay of various classes of cerebral histones. This process has been examined in relation to the stability of cerebral DNA. The system used was the developing avian bra.in. Injection of radioisotopes into fertile chick eggs very shortly after the start of embryo- genesis permitted virtually all molecular species to be radioactively la.belled. This approach avoids problems related to the placental barrier and wastage of isotopes that occur with labelling of mammalian foetuses by maternal injection. EXPERIMENTAL Administration of label. Fertile chick eggs of White Leghorn strain Kl37 were incubated at 37.5°C in a forced-draught incubator. After 2 days, the tops of the eggs were washed with ethanol and a small hole was bored in the shell. A I in hypodermic needle (no. 26 gauge) was vertically placed into the egg and 100µ.Ci of either [4,5- 3 H)leucine (19.7Ci/mmol) or [methyZ-3- 3 .H]thymidine (0.36Ci/mmol) in O.lml of water were injected into the yolk. The hole was then sealed with wax and the eggs were replaced in the incubator. Hatched chicks were injected intracranially at the base of the skull with 20µ.Ci of [ 3 H]leucine in 0.02ml of iso-osmotic (0.14M) NaCl. Radiochemicals were obtained from Schwarz BioResearch Inc., Orangeburg, N.Y., U.S.A. Preparation of cerebral chromatin and hiBtone fractiona- tion. At various times after injection, brains were dissected out from six to ten decapitated chicks or chick embryos. These brains were then homogenized in 0.32M-sucrose and chromatin was prepared as previously described (Bondy & Roberts, 1969). The resulting pellet was used in the preparation of three major histone classes by a" @default.
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• W1441172009 title "The synthesis and decay of histone fractions and of DNA in the developing avian brain" @default.
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