Biochimica et Biophysica Acta (BBA) - Specialized Section on Nucleic Acids and Related Subjects最新文献

The study by the molecular orbital method of the electronic structure of the biological pyrimidine bases undergoing photodimerization leads to the conclusion that this reaction involves the triplet state of these molecules. It is shown that in this state the greatest concentration of the lone electrons is generally at C-5 on one of the half-filled orbitals and on C-6 on the other such orbital. When this is the situation, the rate of photodimerization parallels the total concentration of the uncoupled electrons on the C-5–C-6 bond.

Uridine kinase was found to be the rate-limiting step in the anabolism of uridine to its successive nucleotide derivatives. This enzymatic activity was susceptible to feedback inhibition by the pyrimidine nucleoside triphosphate end-products; effective inhibition was exerted by UTP and even more strikingly by CTP. The analog nucleoside triphosphates, azauridine triphosphate and fluorouridine triphosphate, mimicked the normal compound in exerting inhibition. The inhibition could be partially reversed by higher levels of either uridine or ATP-Mg²⁺. The phosphorylation of cytidine to its nucleotide derivatives was similarly inhibited by both CTP and UTP. Thus, feedback control over salvage pathways for pyrimidine ribonucleotide biosynthesis has been demonstrated.

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  1. During the first several days of ACTH administration the activities of the 105 000 × g supernatant and microsomes for supporting incorporation of [¹⁴C]glycine in vitro into rat adrenal protein are increased. Concomitantly, there are progressive increases in adrenal weight, protein content and RNA content, with little or no change in DNA content.

• 2.

  2. Over the next several days of ACTH administration a period of DNA replication occurs accompanied by the following changes: (a) a decrease in total adrenal-RNA content; (b) a decrease in microsomal-RNA concentration; (c) an interruption of the effect of ACTH on adrenal weight and net protein synthesis; and (d) a decrease in activities of the 105 × 000 g supernatant and microsomes for supporting [¹⁴C]glycine incorporation. These inhibitory changes are subsequently reversed with further ACTH administration and completion of the DNA-replication period.

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  3. It is suggested that the soluble (? transfer enzyme) and microsomal (increased ribosomal and messenger RNA) factors responsible for the effects of ACTH on adrenal protein synthesis are metabolically unstable, and that the induction of these factors by ACTH is mediated through (or requires) DNA-directed RNA synthesis.

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  1. The levels of the uracil-degrading enzymes have been determined in livers from young rats and from rats with different capacity to incorporate uracil into liver RNA. It was found that liver growth or increased ability of the liver to incorporate uracil into the nucleic acids was not necessarily associated with decreased levels of the uracil-degrading enzymes.

• 2.

  2. The rate of synthesis of dihydrouracil dehydrogenase (4,5 dihydrouracil: NADP oxidoreductase, EC 1.3.1.2) has been calculated from growth data and enzyme values from studies of regenerating rat liver, and it has been shown that the decrease which occurs in dihydrouracil dehydrogenase activity during liver regeneration can be adequately explained by assuming a 24-h delay of the synthesis of the enzyme after initiation of liver growth. It is concluded that a decrease in uracil-degrading activity is not involved in the initiation of liver growth.

• 3.

  3. Some factors which could possibly control the synthesis of dihydrouracil dehydrogenase in regenerating rat liver have been examined.

TMV-RNA has been shown to form two types of complexes with acridine orange depending on whether the ratio of RNA-P: acridine orange is close to 1:1 (Complex I) or 20:1 (Complex II). Cr³⁺ and Cu²⁺ completely suppressed the formation of Complex I at levels around 5·10⁻⁴ M. Partial inhibition was observed with Ni²⁺, Co²⁺, Zn²⁺ and Mg²⁺ at 2.5·10⁻³–10·10⁻³ M, and with Hg⁺² in the range 2.5·10⁻⁴–10·10⁻⁴ M. The inhibitory effectiveness of these metal ions followed the order Cr³⁺, Cu²⁺ > Ni²⁺, Co²⁺ > Zn²⁺ > Mg²⁺ > Hg²⁺. On the other hand, the reaction between TMV-RNA-P and acridine orange to form Complex II was modified by 1·10⁻⁵–10·10⁻⁵ M Cr³⁺, Cu²⁺, Ni²⁺, Mg²⁺ and Hg²⁺ in a different manner indicative of the formation of a new species with altered absorption and fluorescence characteristics. The new species, Complex III, consists of a RNA-acridine orange-metal ternary complex.

A method is described for the purification of protamine sulfate by chromatography on DEAE-cellulose. The precipitation of soluble RNA and ribosomal RNA by purified protamine is quantitative, while oligonucleotides of low molecular weight are not precipitated. The procedure allows the quantitative removal of soluble RNA from solutions and the preparation of soluble RNA samples of high purity.

Synchronized cultures of HeLa cells were caused during a single cycle of DNA synthesis to replicate given fractions of their DNA with 5-bromodeoxyuridine (BdeU) instead of thymidine. It was observed that the introduction of 5-bromodeoxyuridine into the early replicating DNA complexes resulted in a striking decline in the cloning efficiency whereas the introduction of 5-bromodeoxyuridine into the late replicating DNA was without effect. The data suggest that the early replicating DNA of the HeLa nucleus plays an important role in survival.

The rate of uptake of [³H]uridine into RNA has been studied in synchronous cultures of Physarum polycephalum. RNA synthesis was reduced at mitosis and again in mid-interphase, and sharply increased following each of these periods. The maximum increase early in interphase coincided with the time of nucleolar reconstruction and DNA synthesis, while the second maximum occured at the time of nucleolar swelling. Such swelling is the first cytological indication of the approaching mitosis. Pretreatment with actinomycin D reduced the postmitotic synthesis one half and completely eliminated premitotic synthesis. These results suggest a different physical or chemical nature of some of the RNA's made during the two periods of synthesis.