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J M Calvo

Publications and source records attributed to J M Calvo.

At least 91 records · Page 5Linked to original sources
PubMed1979

Effects of diphenylhydantoin on the spontaneous activity of Purkinje, nucleus interpositus, red nucleus and motor cortex cells.

(1) Extracellular multiunit recordings were made of the spontaneous activity in cerebellar Purkinje cells, nucleus interpositus, red nucleus and sensorimotor cortex in acute cat preparations. (2) Changes in this spontaneous neural activity produced by the administration of diphyenylhydantoin (DPH) were studied. DPH was infused i.v., generally at a concentration of 2.5 mg/ml and at a rate varying from 0.08 to 0.48 mg/kg/min. Two different patterns of infusion were used: fixed time, variable rate and variable time, fixed rate. Pulsed doses were also given at intervals of 5--10 min. (3) DPH at a level of 10--20 mg/kg produces a significative initial deceleration in all structures followed by a significative acceleration in the Purkinje cells, nucleus interpositus and red nucleus as a dose of 20--30 mg/kg is reached. Higher levels caused a profound depression of multiunit activity. (4) The activation produced by DPH is oscillatory (3--5/min) in character and is composed of 'trains' which occur at a rate of 20--30/sec with very rapid discharge frequencies (600--800 Hz). (5) A direct significant correlation was found between DPH serum levels and the intravenously administered dose. The activating DPH dose (20--30 mg/kg) corresponded to serum levels of 24--32 micrograms/ml. (6) The possibility is discussed whether the anticonvulsant action of DPH may be due in part to the production of rhythmic oscillatory activity in the cerebello-rubro-olivo-cerebellar ciruit and the depression of the cerebellothalamic-cortical pathway.

Animals↗
PubMed1979

leu operon of Salmonella typhimurium is controlled by an attenuation mechanism.

The nucleotide sequence of the control region of the leu operon of Salmonella typhimurium was determined. A prominent feature of this region is a signal for termination of transcription. In vitro, transcription does terminate at this site, yielding a leader RNA of about 160 nucleotides as a major product. This leader RNA is potentially translatable into a peptide containing 28 amino acids, 4 of which are adjacent leucine residues. Several regions of base complementarity exist within the leader, positioned such that pairing of one region precludes pairing of another. The position of the four leucine codons relative to two regions of base complementarity suggest a model for the regulation of the leu operon similar to that proposed by Yanofsky and coworkers for the trp operon. In addition, a third region of base complementarity was identified which, when incorporated into the model, explains why premature termination is the usual outcome when transcription is initiated in vitro by purified RNA polymerase.

Base Sequence↗
PubMed1979

Alternative secondary structures of leader RNAs and the regulation of the trp, phe, his, thr, and leu operons.

The trp, phe, his, thr, and leu operons of enteric bacteria are regulated by a transcriptional attenuation mechanism. Under conditions of amino acid sufficiency, transcription terminates at an attenuator site after a leader of about 150 nucleotides has been synthesized. Under conditions of limitation of a controlling amino acid, transcription continues past the attenuator into adjacent structural genes. As demonstrated by others, each of the five leader RNAs contains two regions of potential secondary structure which are partially overlapping. One of these regions occurs at the 3' terminus of the leader and is named the "terminator." The other region, which potentially can preclude the formation of the terminator, is named the "preemptor." Conditions that allow the preemptor to form result in derepression. We report here that the five published leader RNA sequences contain an additional potential region of secondary structure, which we call the "protector." The protector partially overlaps the preemptor in such a way that pairing of the former precludes pairing of the latter. For derepression to occur, a ribosome that is translating the leader must block the protector without blocking the preemptor, a condition that is met when the ribosome is arrested at the 3' end of a set of control codons. Including the protector in the model for attenuation explains why derepression of the operon does not result from the arrest of a ribosome at a codon preceding the control set. It also explains why termination is the outcome when transcription occurs in the absence of ribosomes. Finally, termination is the predicted outcome when unfettered translation of the leader RNA occurs, resulting in release of the ribosome at the translational stop signal.

Amino Acids↗
PubMed1977

[Phasic activity in rats].

At the central level, in the rat, phasic activity has been recorded during paradoxical sleep and in acute conditions after injection of reserpine or parachlorophenylalanine. At the external level, during paradoxical sleep, the extraocular muscles lateral rectus, superior rectus and superior oblique are activated in both plastic and tonic manners. The muscles of the whiskers are also activated; these muscular activations are more often than not synchronous with the eye movements (80%). The time distribution of these ocular movements is homogenous. Reserpine induces phasic muscular activations of the extraocular muscles.

Animals↗
PubMed1977

Isolation and characterization of lambda pleu bacteriophages.

In the Escherichia coli lysogen HfrH73 described by Shimada et al. (1973), none of the enzymes coded for by the leucine operon is synthesized due to an insertion of phage lambda into cistron leuA. The orientation of lambda in the chromosome is ara leuDCB lambda JAN leuA. After heat induction of the lysogen, plaque-forming transducing phages of two types are formed at low frequency. One type (e.g., lambda pleu9) transduces leuD, leuC, and leuB strains to prototrophy. The other type (e.g., lambda pleu 13) transduces leuA strains to prototrophy. lambda pleu 13 forms lysogens at low frequency (about 0.2%) by integration into the leucine operon. These lysogens are unstable, segregating phage-sensitive clones at high frequency (about 1%). Phages carrying different portions of the leucine operon were formed by aberrant excision after heat induction of strain CV437 (leuA371 lambda pleu13). A phage carrying the entire leucine operon (lambda K2) was constructed by a cross between lambda pleu9 and lambda pleu13. An analysis of leucine-forming enzyme levels in strains lysogenized with lambdaK2 indicated that leuO and leuP are present and functional in lambda K2. leu-specific messenger ribonucleic acid from E. coli hybridizes to the heavy (r) strand of lambdaK2. The leucine operon of lambda G4 pleuABCD (an S7 derivative of lambda K2) exists intact on a 7.3 x 10(6)-dalton fragment (lambdaG4EcoRI-B) generated by cleavage with endonuclease EcoRI. Heteroduplexes formed between lambda G4 and lambda show a 5.4 x 10(6)-dalton piece of bacterial deoxyribonucleic acid (DNA) replacing a 4.5 x 10(6)-dalton piece of lambda DNA starting at 0.46 fractional unit on the map of lambda. Fragment lambda G4EcoRI-B has about 0.6 x 10(6) daltons of lambda DNA from the b2 region at one end and about 1.4 x 10(6) daltons of lambda DNA from the int region at the other end.

Coliphages↗
PubMed1977

Relationship between messenger ribonucleic acid and enzyme levels specified by the leucine operon of Escherichia coli K-12.

The levels of leucine-forming enzymes in Escherichia coli K-12 varied over a several thousand-fold range, depending upon conditions of growth. The highest levels were achieved by growing auxotrophs in a chemostat under conditions of leucine limitation. Under such conditions, enzyme levels were increased 45- to 90-fold relative to cells grown in minimal medium containing leucine (the latter values arbitrarily called 1). Leucine operon-specific messenger ribonucleic acid levels were elevated to about the same extent as enzyme levels in cells grown in a chemostat. Growth in media of greater complexity resulted in progressively lower levels of leucine-forming enzymes, reaching a value of less than 0.02 for growth in a medium containing tryptone broth and yeast extract. The levels of leucine operon-specified enzymes and messenger ribonucleic acid were also measured in strains containing about 25 copies of plasmid pCV1(ColE1-leu) per chromosome. For such strains grown in minimal medium, enzyme levels were proportional to the number of plasmids per cell. Furthermore, they followed the same trends as those described above upon derepression in a chemostat or upon repression following growth in rich media. Leucine messenger ribonucleic acid, measured both by pulse-labeling and hybridization-competition experiments, was roughly proportional to enzyme levels over this entire range. For a plasmid-containing strain grown in a chemostat under conditions of leucine limitation (about 100 plasmids per chromosome), about 27% of pulse-labeled ribonucleic acid was coded for by genes in or adjacent to the leucine operon, and 10% of the total protein was beta-isopropylmalate dehydrogenase.

2-Isopropylmalate Synthase↗
PubMed1976

Anatomical organization of the phasic activity produced by reserpine at the level of the oculomotor system.

The organization of the pathways responsible for the transmission of phasic electrical activity at the level of the oculomotor system was studied in the encéphale isolé cat which was injected with reserpine or exhibited spontaneous phases of paradoxical sleep. At the level of the VIth nuclei there are both ipsilateral and contralateral connections deriving from each generator. The ipsilateral pathway transmits an "inhibiting" potential eliciting electromyographic inhibition of the ipsilateral rectus muscle, and the contralateral pathway, an "activating" potential eliciting activation of the corresponding lateral rectus muscle. A medial saggital section from frontal plane APO, extending caudally to the VIth nuclei at frontal plan P9 is necessary to suppress the bilateral synchronization of phasic activities recorded from the VIth nuclei and the activation of the lateral rectus muscles. The areas responsible for phasic activity recorded at the level of the central visual and oculomotor systems have been delimited through brain transections. The pathways responsible for the transmission of phasic activity at the level of the IIIrd and IVth nuclei are contiguous with the ponto-geniculate pathways. This was demonstrated by electrocoagulation.

Abducens Nerve↗
PubMed1974

Distribution of the isopropylmalate pathway to leucine among diverse bacteria.

alpha-Isopropylmalate synthase and beta-isopropylmalate dehydrogenase activities were detected in extracts of the following organisms: Chromatium D, Rhodopseudomonas spheroides, Hydrogenomonas H16, Pseudomonas aeruginosa, Pseudomonas fluorescens, Vibrio extorquens, Rhizobium japonicum, Alcaligenes viscolactis, Escherichia coli B, Proteus vulgaris, Aerobacter aerogenes, Salmonella typhimurium, Micrococcus sp., Micrococcus lysodeikticus, Bacillus polymyxa, Bacillus subtilis, and Nocardia opaca. The alpha-isopropylmalate synthase activity in these extracts was inhibited by low concentrations of l-leucine. Taken together with other data, these results suggest that the isopropylmalate pathway is widespread among organisms that can synthesize leucine.

Alcohol Oxidoreductases↗
PubMed1974

flrB, a regulatory locus controlling branched-chain amino acid biosynthesis in Salmonella typhimurium.

Salmonella typhimurium strain CV123 (ara-9 gal-205 flrB1), isolated as a mutant resistant to trifluoroleucine, has derepressed and constitutive levels of enzymes forming branched-chain amino acids. This strain grows more slowly than the parent at several temperatures, both in minimal medium and nutrient broth. It overproduces and excretes sizeable amounts of leucine, valine, and isoleucine in comparison with the parental strain. Both leuS (coding for leucyl-transfer ribonucleic acid [tRNA]synthetase) and flrB are linked to lip (min 20 to 25) by P1 transduction, whereas only leuS is linked to lip by P22 transduction. Strain CV123 containing an F' lip(+) episome from Escherichia coli has repressed levels of leucine-forming enzymes, indicating that flrB(+) is dominant to flrB. Leucyl-tRNA synthetase from strain CV123 appears to be identical to the leucyl-tRNA synthetase in the parent. No differences were detected between strain CV123 and the parent with respect to tRNA acceptor activity for a number of amino acids. Furthermore, there was no large difference between the two strains in the patterns of leucine tRNA isoaccepting species after fractionation on several different columns. Several other flrB strains exhibited temperature-sensitive excretion of leucine, i.e., they excreted leucine at 37 C but not 25 C. In one such strain, excretion at 37 C was correlated with derepression of some enzymes specified by ilv and leu. These latter results suggest that flrB codes for a protein.

Amino Acids↗
PubMed1972

Leucyl-transfer ribonucleic acid synthetase from a wild-type and temperature-sensitive mutant of Salmonella typhimurium.

Leucyl-transfer ribonucleic acid (tRNA) synthetase was purified 100-fold from extracts of Salmonella typhimurium. The partially purified enzyme had the following K(m) values: leucine, 1.1 x 10(-5)m; adenosine triphosphate, 6.5 x 10(-4)m; tRNA(I) (Leu), 4.1 x 10(-8)m; tRNA(II) (Leu), 4.3 x 10(-8)m; tRNA(III) (Leu), 5.3 x 10(-8)m; and tRNA(IV) (Leu), 2.9 x 10(-8)m. The tRNA(Leu) fractions were isolated from Salmonella bulk tRNA by chromatography on reversed-phase columns and benzoylated diethylaminoethyl cellulose. The enzyme had a pH optimum of 8.5 and an activation energy of 10,400 cal per mole, and was inactivated exponentially at 49.5 C with a first-order rate constant of 0.064 min(-1). Strain CV356 (leuS3 leuABCD702 ara-9 gal-205) was isolated as a mutant resistant to dl-4-azaleucine and able to grow at 27 C but not at 37 C. Extracts of strain CV356 had no leucyl-tRNA synthetase activity (charging assay) when assayed at 27 or 37 C. Temperature sensitivity and enzyme deficiency were caused by mutation in the structural gene locus specifying leucyl-tRNA synthetase. A prototrophic derivative of strain CV356 (CV357) excreted branched-chain amino acids and had high pathway-specific enzyme levels when grown at temperatures where its doubling time was near normal. At growth-restricting temperatures, both amino acid excretion and enzyme levels were further elevated. The properties of strain CV357 indicate that there is only a single leucyl-tRNA synthetase in S. typhimurium.

Adenosine Triphosphate↗
PubMed1971

Mutants of Salmonella typhimurium with an altered leucyl-transfer ribonucleic acid synthetase.

Two trifluoroleucine-resistant mutants of Salmonella typhimurium, strains CV69 and CV117, had an altered leucyl-transfer ribonucleic acid (tRNA) synthetase. The mutant enzymes had higher apparent K(m) values for leucine (ca. 10-fold) and lower specific activities (ca. twofold) than the parent enzyme when tested in crude extracts. Preparations of synthetase purified ca. 60-fold from the parent and strain CV117 differed sixfold in their leucine K(m) values. In addition, the mutant enzyme was inactivated faster than the parent enzyme at 50 C. The growth rates of strains CV69 and CV117 at 37 C were not significantly different from that of the parent, whereas at 42 C strain CV69 grew more slowly than the parent. Leucine-, valine-, and isoleucine-forming enzymes were partially derepressed when the mutants were grown in minimal medium; the addition of leucine repressed these enzymes to wild-type levels. During growth in minimal medium, the proportion of leucine tRNA that was charged in the mutants was about 75% of that in the parent. The properties of strain CV117 were shown to result from a single mutation located near gal at minute 18 on the genetic map. These studies suggest that leucyl-tRNA synthetase is involved in repression of the enzymes required for the synthesis of branched-chain amino acids.

Adenosine Triphosphate↗
PubMed1971

Salmonella locus affecting phosphoenolpyruvate synthase activity identified by a deletion analysis.

Strain leu-4017, derived from Salmonella typhimurium LT2, cannot utilize acetate, pyruvate, or citric acid cycle intermediates as sole sources of carbon. The mutation in this strain extends from the A cistron of the leucine operon to some point between leu and azi, presumably deleting one or more loci involved in the utilization of these compounds. One of these loci is required for phosphoenolpyruvate synthase activity.

Acetates↗
PubMed1971

Effect of 4-azaleucine upon leucine metabolism in Salmonella typhimurium.

dl-4-Azaleucine (5 x 3(-3)m) added to exponentially growing cells of Salmonella typhimurium resulted in an abrupt cessation of growth lasting 4 to 8 hr followed by a resumption of division. The transitory nature of inhibition was not due to the instability or modification of the analogue or to a derepression of leucine-forming enzymes. Of many compounds tested, leucine served most efficiently to reverse 4-azaleucine-induced inhibition. Inhibition of growth can be explained by the fact that 4-azaleucine inhibits alpha-isopropylmalate synthase, the first enzyme unique to leucine biosynthesis. The analogue was a poor inhibitor of both the transamination of alpha-ketoisocaproate to leucine and the charging of leucine to transfer ribonucleic acid. With a leucine auxotroph starved for leucine, the analogue was incorporated into protein specifically in place of leucine. Such incorporation was accompanied by the death of almost all of the cells.

Amino Acids↗
PubMed1971

Recognition of an Escherichia operator by a Salmonella repressor.

Escherichia coli leu(+) episomes were transferred to Salmonella typhimurium. The levels of two leucine-forming enzymes were normal (low) in the resulting merodiploids, suggesting that the putative Samonella leucine repressor recognizes the Escherichia leucine operator.

DNA, Bacterial↗