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1. The effects of alkylating agents and disulphides on the thiol-containing proteins of nuclei from rat thymus and liver were studied. Three protein fractions were examined: histones extracted with 50mm- and 250mm-hydrochloric acid and the residual protein. None of the reagents selectively reacted with any one of the protein fractions. 2. Amino acid uptake in vitro into the histones of nuclei from rat thymus was analysed by preparative electrophoresis of the proteins extracted with 50mm- and 250mm-hydrochloric acid. After 1hr. at 37 degrees the greater incorporation was into the proteins extracted with 50mm-hydrochloric acid. 3. Preparative electrophoresis was used to study the relative thiol contents of the proteins of the 50mm-hydrochloric acid extract from thymus nuclei by labelling the histones in vitro with (14)C-labelled N-ethylmaleimide. 4. The capacity of the proteins extracted from rat thymus with 50mm- and 250mm-hydrochloric acid, and of the components from these extracts separated by preparative electrophoresis, to combine with DNA and to depress DNA-dependent RNA synthesis was studied. The histones extracted with 50mm-hydrochloric acid were more lysine-rich than those extracted with 250mm-hydrochloric acid. Wide variations were found in the abilities of the separated components to depress RNA synthesis.

1. In regenerating rat liver the phosphate content of the lysine-rich histone F1, but not that of the more arginine-rich histone F3-1, increases during the period of DNA synthesis. 2. The phosphorylation of histone F1 in this ;S period' is decreased by gamma-irradiation, but, like phosphate uptake into DNA, is affected to an even greater extent if the irradiation is given in the presynthetic period. 3. Histones from three species of sea-urchin eggs show similarities to the F2 and F3 groups of histones from mammalian thymus gland. 4. The proportion of thiol to total thiol plus disulphide in acid extracts from sea-urchin eggs varies from less than 20% in mature unfertilized eggs to 59% just before cleavage. 5. The phosphorylated forms of histones F1 and F3 are less effective in decreasing DNA synthesis by DNA polymerase than the non-phosphorylated forms. 6. Oxidation of thiol groups on histone F3-1 does not affect its capacity to decrease DNA synthesis in vitro.

1. The proportion of thiol groups in the total thiol+disulphide of histone extracts from fertilized eggs from Echinus and Psammechinus was increased during periods of structural alterations in the nucleus. 2. The probable start of DNA synthesis in the fertilized eggs coincided with periods of maximum thiol content. 3. Histone extracts from rat liver and regenerating liver were predominantly in the thiol form and no significant variations could be detected during the first 30hr. after partial hepatectomy. 4. An assay system was developed to follow the phosphorylation believed to be associated with the arginine-rich histone F3. Phosphorylation increased by 50% at 1 and 2hr. after partial hepatectomy. The phosphate content also increased during the period of DNA synthesis. 5. The increased phosphorylation found 1hr. after partial hepatectomy was not prevented by actinomycin or prior irradiation. 6. The phosphate content of histone F1 was very high in livers from foetal rats and declined in neonatal rats similarly to the decline in DNA synthesis.

1. At 0.5-1.0h after partial hepatectomy the intracellular acid-soluble fraction of rat liver took up twice as much radioactivity from [(3)H]orotic acid, [(3)H]uridine and [(3)H]thymidine as did similar fractions from sham-operated animals. This increase in penetration was not prevented by adrenalectomy or actinomycin, both of which decreased precursor uptake into nuclear RNA at this time. The increase in entry was still shown by thymidine at 22h after operation, at the height of the S period. Adenine penetration was not increased 1h after partial hepatectomy. 2. Plasma concentrations of ornithine and possibly methionine, tyrosine and lysine were raised 1.5h after partial hepatectomy. [(3)H]Lysine entry into regenerating liver at this time was increased by 60%; [(3)H]valine uptake was unaffected. Intracellular amounts of tyrosine, phenylalanine and ornithine in the liver were also increased. 3. The relation of these events to the start of liver regeneration is discussed.

1. The activity of the soluble phosphokinase for histone F1 increases in regenerating rat liver during the first period of DNA synthesis after partial hepatectomy. The increase probably represents new enzyme synthesis. 2. A dose of 500rd of gamma-irradiation given early in G1 decreases the amount of histone F1 phosphokinase found 22h after partial hepatectomy by 60-70%. 3. The enzyme preparations also contained a histone F1 phosphatase; the presence together of the kinase and phosphatase caused a disproportion between net (31)P uptake and (32)P incorporation into histone F1. 4. All four subclasses of histone F1 could accept phosphate from ATP. 5. Crude enzyme preparations transferred more (31)P into histone F1 with an initially low phosphate content than into one with a high phosphate content; conversely, more (32)P was transferred into the latter.

1. Differences in washing procedures and conditions of dialysis affected the type of histone f3 obtained from rat liver and thymus nuclei. 2. Sulphur-rich preparations of f3 contained phosphate and had SH+SS: phosphorus ratio 1:2. 3. Sulphur-rich f3 from livers of [(14)C]adenine-labelled rats gave counts indicating a phosphorus: adenine ratio 1:1. 4. Evidence was presented that histones f3 and f1 exist in nuclei as families of proteins, differing with respect to their sulphur or phosphorus content or both. 5. The significance of the different states of phosphorylation and oxidation of histones f1 and f3 was discussed.

1. Changes in the microstructure of histones from the pancreas were followed in rats given ethionine while on a protein-free diet. The lysine-rich histone F1 and the other histones extracted subsequently with 250mm-hydrochloric acid were isolated. When the ethionine-treated rats returned to the stock diet, regeneration of the pancreas was accompanied by an immediate increase in phosphate content of the total histone extract from which histone F1 had been removed. After 3-4 days there was a further increase in the phosphate content of this extract, and also in that of histone F1. 2. Similar changes in the phosphate and thiol+disulphide content of pancreas histones other than F1 were produced by starvation and re-feeding.

1. DNA synthesis in Echinus esculentus eggs kept at 10 degrees C takes place just after fusion, 0.75-1.5h after fertilization, and at telophase at about 2.67-3.33h after fertilization. 2. An increase in the thiol/thiol+disulphide ratio in acid extracts from washed nuclear fractions of the eggs is found at fusion, at early stages of mitosis and at telophase. When DNA is being synthesized, the relative amount of thiol in the extracts increases. 3. There are at least five thiol-containing histones in the acid extract together with a diffusible thiol peptide containing methyl-lysine and 3-methylhistidine and a thiol-containing acidic protein.

1. When [(3)H]thymidine was injected intravenously into rats in amounts up to 40mg/kg body wt. and the (3)H radioactivity in the livers measured at 30min, saturation kinetics for thymidine uptake were not found. If the animals were examined 3 min after intravenous injection, saturation could be attained in normal rats with 12mg of thymidine/kg and in partially hepatectomized rats with 4mg/kg. At concentrations of thymidine close to saturation, no differences were found in rate or amount of uptake/g of liver between normal and partially hepatectomized rats 1-2h after operation. 2. Perfusion techniques were used to compare thymidine uptakes in the two sets of rats at concentrations up to 40mum-thymidine. Uptakes with tracer amounts of thymidine after 30min were identical in vivo and in the perfusion studies and were twice as great in livers from partially hepatectomized rats with concentrations up to 40mum-thymidine. 3. At 1.5h after operation there was nearly twice as much beta-aminoisobutyrate present per g of liver from partially hepatectomized as compared with normal rats.

1. The incorporation of (14)C-labelled amino acids into acid-extractable proteins from rat-liver and -thymus nuclei confirmed the existence of a protein component with a higher uptake than that into the major histone components. 2. This rapidly labelled component appeared to contain the thiol groups detectable in the acid extracts. 3. Histone f1 contained 1mol. of serine phosphate/mol. of mol.wt. of 42000-43000. 4. Phosphate was present in other components of the 50mm-hydrochloric acid extract from liver and thymus nuclei, and was probably associated with the thiol-containing component. 5. The difference in amino acid uptake into the histones of diffuse and dense chromatin was confirmed. Dense chromatin was found to have a higher proportion of disulphide than did diffuse chromatin.

1. Liver cell fractions were prepared by non-aqueous procedures and nuclei were also obtained in a hyperosmotic sucrose medium. Histone phosphokinase activity, assayed with histone F1 as substrate, was present in the soluble fraction of the cytoplasm and also bound on to the chromatin fraction of the nucleus. 2. The activity of the enzyme increased sixfold in nuclei from regenerating livers 22h after partial hepatectomy. 3. The enzyme bound in the nucleus was only marginally activated by 1mum-3':5'-cyclic AMP which stimulated the cytoplasmic soluble enzyme fourfold. 4. Nuclei prepared by the non-aqueous technique were also able to phosphorylate histones F2a and F3 and showed histone phosphatase activity with histone F1 phosphate as substrate.