Modification of rat liver chromatin by N-methyl-N'-nitro-N-nitrosoguanidine or N-ethyl-N'-nitro-N-nitrosoguanidine and template activity for RNA synthesis by Escherichia coli RNA polymerase after reconstitution.

Rat liver chromatin was fractionated into DNA, histones and non-histone chromosomal proteins and each component was modified with N-methyl-l-N'-nitro-N-nitrosoguanidine of N-ethyl-N'-nitrosoguanidine. The radioactivity of 14C-labeled alkyl or guanidino moieties of both compounds bound significantly to both histones and non-histone chromosomal proteins and the binding of N-methyl-N'-nitro-N-nitrosoguanidine was higher than N-ethyl-N'-nitro-N-nitrosoguanidine. However the binding of both compounds to DNA was very low and its significance was hard to evaluate. All of the three components, one of which was modified, were reconstituted into chromatin, then, [3H]UMP incorporation into acid insoluble material using Escherichia coli RNA polymerase (EC 2.7.7.6) was measured. Only with the reconstituted chromatin containing histones modified either by N-methyl-N'-nitro-N-nitrosoguanidine or N-ethyl-N'-nitro-N-nitrosoguanidine, the template activity increased drastically; i.e., about 10 or 5 times higher than that with the unmodified reconstituted chromatin, respectively. However, any remarkable alteration in the electrophoretic pattern of protein fraction of the reconstituted chromatin could not be found. The results obtained in this study are discussed in the context that the modified histones could give rise to change in the mutual interaction of chromosomal components during the reconstitution of chromatin accompanied with the increase of chromatine template activity.     

Effects of hypothyroidism on RNA synthesis in the adult rat brain

In this study we investigated the effects of hypothyroidism on adult brain RNA synthesis. Our data show that in the cerebral hemispheres of hypothyroid rats there is a decrease in microsomal RNA content and microsomal [³H]uridine incorporation. Sucrose gradient analysis revealed that these changes are mainly associated with free ribosomes and subunits and reflect changes in rRNA. The above changes are accompanied by a decrease in RNA polymerase I activity. All of the above mentioned changes returned to normal after thyroxine (T₄) treatment. In contrast to RNA polymerase I, RNA polymerase II activity was not affected. However, electrophoretic analysis of the in vitro poly(A)⁺RNA translation products revealed that hypothyroidism affects a few mRNAs. These results indicate that thyroid hormones have a role in adult brain tissue metabolism.     

Activation of nuclear polypeptide synthesis in rat liver cells after inhibition of template protein synthesis. The role of nuclear polypeptide synthesis in the changes of the chromatin structure

The correlation between the rates of nuclear polypeptide synthesis (NPS) and matrix protein synthesis in rat liver cells was investigated. It was shown that NPS is activated under conditions of protein synthesis inhibition in the cytoplasm. Model experiments revealed that the NPS and ADP ribosylation systems compete for chromatin structure: ADP ribosylation induces condensation, while NPS--decondensation of chromatin.     

L-thyroxine and L-triiodothyronine in the cerebral tissues of the adult rat. Effect of hypothyroidism]

The L-thyroxine and L-triiodothyronine concentrations in several brain areas (cerebral cortex, brain stem, hypothalamus, total brain and hypophysis) in normal and hypothyroid rats have been studied. Results show that L-thyroxine values at tissue level are inferior in the hypothyroid group, although non-significant with respect to the control group, whereas L-triiodothyronine presents values similar to the hypothyroid group and its control in all the brain regions studied with the exception of hypophysis. These results show that in hypothyroid situations exist a compensatory mechanism for maintaining the adequate L-triiodothyronine levels in several brain areas, although the serum levels are strongly decreased in hypothyroid animals.