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B S Baliga

Publications and source records attributed to B S Baliga.

11 recordsLinked to original sources
PubMed1979

Release of in vitro-synthesized poly(A)-containing RNA from isolated rat liver nuclei: characterization of the ribonucleoprotein particles involved.

Nuclei isolated from rat liver were incubated under conditions in which RNA continued to be labeled with precursors for long periods. After 1 hr, during which the rate of RNA synthesis was constant, 25-30% of the newly synthesized RNA was recovered in the postnuclear supernatant. About 3-5% of this fraction was characterized as poly(A)-containing ribonucleoproteins by the following criteria: (i) characteristic elution profile in oligo(dT)-cellulose chromatography; (ii) size distribution of the molecules and their deproteinized RNAs; (iii) buoyant densities in CsCl gradients; (iv) presence of RNaseresistant fragments resembling poly(A)-protein complexes; and (v) identification of the protein components by sodium dodecyl sulfate/polyacrylamide gel electrophoresis. The sensitivity of labeling of the RNA synthesized and released from the nuclei to low doses of alpha-amanitin suggests the presence of polymerase II products in the particles. Comparison of the sizes of proteins in these particles with those of free and polysomal messenger ribonucleoproteins as well as with heterogenous nuclear ribonucleoproteins indicates that the released particles contain a protein of 78,000 daltons, which is also present in the other three classes of ribonucleoproteins, presumably at the 3'-poly(A) segments. In addition, a few other proteins, similar in size to those found in the cytoplasmic ribonucleoproteins, were also present in the released particles. It is suggested that proteins associated with heterogenous nuclear RNA are mostly exchanged before or at the time of release of mRNA from the nucleus; the remaining mRNA-associated proteins are added in the cytoplasm, possibly in relation to cytoskeleton attachment, followed by the removal of most of these proteins during polysome formation.

Amanitins↗
PubMed1977

High turnover rate of transfer RNA in tumor tissue.

Cancer patients and tumor-bearing animals excrete high levels of modified purines and pyrimidines some of which, e.g., N2,N2-dimethylguanosine, can originate only from transfer RNA (tRNA). Until recently, it could not be ascertained whether the high level of excretion of such compounds is due to cell death or specific tRNA turnover. However, an approach to this problem became feasible, with beta-aminoisobutyric acid as a probe. This compound is a terminal degradation product of thymine which is present in both DNA and tRNA. Since the pathway of synthesis of thymine is different in the two macromolecules, it and its end product, beta-aminoisobutyric acid can be differentially labeled with [14C]formate and [3H3]methylmethionine as precursors. Therefore the ratio of the two labels in the excreted beta-aminoisobutyric acid is a measure of the macromolecular origin of the degradation product. We have found from such analysis that tRNA's are not homogeneous in their turnover rate. There is a subpopulation that turns over much faster than the rest. The turnover rate of a subpopulation of tRNA's in tumor tissue exceeds the turnover rate of tRNA's in normal tissue. Such rapid degradation of tRNA's must be the source of the massive excretion of modified nucleosides by cancer patients which can be 10-fold higher than in normal subjects.

Aminoisobutyric Acids↗
PubMed1976

Mechanism of D-amphetamine inhibition of protein synthesis.

At 1 h after intraperitoneal administration of D-amphetamine sulphate (15 mg/kg), rat brain polyribosomes show disaggregation accompanied by reduced capacity for in vitro peptide chain elongation. The direct action of amphetamine on cell-fine protein-synthesizing systems was therefore explored. When brain or liver polyribosomes from untreated rats were incubated with pH 5 enzyme, peptide chain elongation was not inhibited by the addition 4 mM amphetamine to the medium. On the other hand, an initiation-dependent system consisting of rat liver of brain mRNA and wheat germ S-30 fraction showed inhibition of [3H]leucine incorporation by 50% when 4 mM amphetamine were added. The metabolites of amphetamine, p-hydroxyamphetamine and p-hydroxynorephedrine, had no inhibitory action in either system, but the potent neurotoxin p-chloroamphetamine was a more powerful inhibitor of initiation than amphetamine. By using [3H]amphetamine, it was shown that amphetamine binds to the 80-S ribosomes of the wheat germ system. This binding depended on the presence in the system of natural liver or brain mRNA or several synthetic mRNAs, but was not promoted by polyuridylic acid as the messenger. Significantly, polyuridylic acid-dependent polyphenylalanine synthesis by the wheat germ system was not inhibited by amphetamine or p-chloroamphetamine. Therefore, it was concluded that amphetamine inhibits protein synthesis by interfering with initiation through a step related to formation of the mRNA ribosome complex.

Animals↗
PubMed1976

Synthesis of human placental lactogen and human chorionic gonadotropin by polyribosomes and messenger RNA's from early and full term placentas.

Synthesis of human placental lactogen (hPL) and of human chorionic gonadotropin (hCG) by membrane-bound and free polyribosomes from early and from full term human placentas was investigated by in vitro release of the nascent hormone peptides, followed by immunoprecipitation and electrophoresis in sodium dodecyl sulfate gels, and by specific binding of 125I-labeled hPL antibody to nascent peptide chains. In addition, messenger RNA'S Were extracted from total, free, and membrane-bound placental polyribosomes and their capacities for hPL and hCG synthesis were measured in a heterologous cell-free system prepared from wheat germ. Membrane-bound polyribosomes from full term placentas were several times more active in the synthesis of both peptide hormones than were free polyribosomes. By binding 125I-labeled hPL antibody to nascent chains on the polyribosomes, it was determined that hPL is made by clusters of seven to nine ribosomes. About 8% of the nascent peptide chains released by incubation of polyribosomes from full term placentas was accounted for by hPL, and 2% by hCG. In contrast, no chains of hPL were released by polyribosomes from 10-week placentas, whereas 11% of the total released chains were accounted for by hCG. When messenger RNAs prepared from the polyribosomes of 20- and 40-week placentas were used to stimulate protein synthesis in a wheat germ system, hPL accounted for 0.4 and 2%, respectively, of total protein synthesis, while hCG was 8 and 2%, respectively. This confirmed the relative proportions observed for nascent chains on the polyribosomes of early and late placentas. Unexpectedly, translation of mRNA from free polyribosomes yielded as much hPL and two-thirds as much hCG as did translation of mRNA from bound polyribosomes. We conclude, that the decreased blood levels of hCG and increased blood levels of hPL with advancing gestational age reflect the relative in vitro rates of synthesis of these hormones by placental polyribosomes, the abundance of which is determined by availability of their respective messenger RNAs at different times in gestation.

Animals↗
PubMed1976

Status of tRNA charging, trinucleotide acceptor sequence and tRNA nucleotidyltransferase activity in the human placenta.

Samples of tRNA isolated from the cell sap of full-term human placenta were found to have a low capacity for accepting amino acids in the presence of partially purified synthetase preparations made from placental or rat liver cell sap. Gel electrophoresis of placental tRNA showed that part of this could be accounted for by gross degradation. The proportion of chargeable tRNA carrying amino acids was estimated by periodate oxidation followed by stripping and then charging with labeled amino acids. Only 50% of chargeable placental tRNA was in the charged state when isolated, whereas 87% of freshly isolated rat liver tRNA was found to be charged with amino acids. A fraction from placental cell sap was shown to have tRNA nucleotidyltransferase activity. When placental tRNA was incubated with this fraction and [3H]ATP or [3H]CTP, ATP was incorporated into about 12% of the tRNA molecules and CTP into 5-7%. When rat liver tRNA was used in place of placental tRNA, [3H]ATP was incorporated into less than 5% of the tRNA molecules. By using snake-venom diesterase over short periods of incubation, it was confirmed that the ATP had been incorporated terminally as AMP into the placental tRNA. These observations show that, in contrast to rat liver tRNA, tRNA prepared from human placenta is poorly charged with amino acids, many of the molecules lack the acceptor trinucleotide and there is extensive degradation beyond this stage.

Amino Acyl-tRNA Synthetases↗