The rep region of pR plasmid regulates the expression of SOS system

Summary By using an artificial hybrid between phage and the pR plasmid, we have shown that the rep region of the pR plasmid encodes a function which regulates the expression of the muc genes (plasmid genes that are under the negative control of lexA and responsible for an increased rate of spontaneous mutagenesis and resistance to UV and chemicals). Expression of the muc genes was monitored by a fusion between the muc promoter and the lacZ structural gene. When E. coli cells containing such a fusion are infected by the hybrid pR phasmid, -galactosidase activity is enhanced, indicating that pR encodes an antagonist of lexA. By deletion mapping we have located the gene encoding the antagonist of lexA (bat) in the rep region of the plasmid. The bat gene product can also antagonize the cI repressor as shown by the observation that pR phasmids are virulent on a homoimmune lysogen. We have exploited this latter property to carry out genetic and functional analysis of the bat region. This region is organized as a classical operon where the expression of the bat structural gene is negatively regulated by a repressor gene that encodes a proteic product.     

Structure function studies on the lipoate-acetyltransferase--component-X-core assembly of the ox heart pyruvate dehydrogenase complex

Component X, the recently recognised subunit of mammalian pyruvate dehydrogenase complex, was shown by immune blotting to be present in all of nine tissues dissected from rat. This finding indicated that component X was not an isoenzyme of the lipoate acetyltransferase (E2) associated with one or a limited number of tissues. Native pyruvate dehydrogenase complex was shown to bind IgG raised to isolated component X, indicating that there were at least some regions of the X subunit exposed at the periphery of the complex. Lipoyl groups of ox heart pyruvate dehydrogenase complex were specifically cross-linked by reaction with phenylene-o-bismaleimide in the presence of pyruvate and the subunits contributing to the products of cross-linking were identified by immune blotting. Species with very high Mr containing both E2 and component X, were formed in high yield, as well as apparent E2/E2 and E2/X dimers and trimers and an X/X dimer. These results showed that acetylated lipoyl groups of different E2 and X subunits were able to interact in all possible combinations. The types of cross-linked E2 products formed suggested that two thiols, reactible with phenylene-o-bismaleimide, were rapidly generated in the presence of pyruvate. The results were most easily explained by the presence of two acetylatable lipoyl groups on each E2 polypeptide.     

The Mr-50 000 polypeptide of mammalian pyruvate dehydrogenase complex participates in the acetylation reactions

The mammalian pyruvate dehydrogenase complex, Mr 8.5 X 10(6), contains an additional tightly bound 50 000-Mr polypeptide, component X, which copurifies with the intact assembly. Small amounts of the individual E2 and X polypeptides were obtained by elution of the protein bands from SDS/polyacrylamide gels. One-dimensional peptide mapping studies with 125I-labelled lipoyl acetyltransferase (E2) and component X subunits indicate that these two proteins are structurally distinct entities. Similar analysis of purified subunits, initially radiolabelled in the intact complex in the presence of [2-14C]pyruvate and N-ethyl-[2,3-14C]maleimide confirm that distinct 14C-labelled peptides are generated from these two species. These protein-chemical data supplement recent immunological findings, which demonstrate that component X is not a proteolytic fragment of the larger lipoyl acetyltransferase (Mr 70 000) subunit. Incubation of the native PDC in the presence of [2-14C]pyruvate leads to rapid uptake of radiolabel, presumably as acetyl groups, into both E2 and protein X. Specific incorporation of acetyl groups declines to a similar extent on both polypeptides after inhibiting pyruvate dehydrogenase (E1) activity by phosphorylation or omitting thiamine diphosphate (TPP) from the assay mixture. Addition of CoASH promotes the parallel deacetylation of both lipoyl acetyltransferase and protein X in a reaction which displays sensitivity to N-ethylmaleimide.     

Component X. An immunologically distinct polypeptide associated with mammalian pyruvate dehydrogenase multi-enzyme complex

The mammalian pyruvate dehydrogenase multi-enzyme complex contains a tightly-associated 50 000-Mr polypeptide of unknown function (component X) in addition to its three constituent enzymes, pyruvate dehydrogenase (E1), lipoate acetyltransferase (E2) and lipoamide dehydrogenase (E3) which are jointly responsible for production of CoASAc and NADH. The presence of component X is apparent on sodium dodecyl sulphate/polyacrylamide gel analysis of the complex, performed in Tris-glycine buffers although it co-migrates with the E3 subunit on standard phosphate gels run under denaturing conditions. Refined immunological techniques, employing subunit-specific antisera to individual components of the pyruvate dehydrogenase complex, have demonstrated that protein X is not a proteolytic fragment of E2 (or E3) as suggested previously. In addition, anti-X serum elicits no cross-reaction with either subunit of the intrinsic kinase of the pyruvate dehydrogenase complex. Immune-blotting analysis of SDS extracts of bovine, rat and pig cell lines and derived subcellular fractions have indicated that protein X is a normal cellular component with a specific mitochondrial location. It remains tightly-associated with the 'core' enzyme, E2, on dissociation of the complex at pH 9.5 or by treatment with 0.25 M MgCl2. This polypeptide is not released to any significant extent from E2 by p-hydroxymercuriphenyl sulphonate, a reagent which promotes dissociation of the specific kinase of the complex from the 'core' enzyme. Incubation of the complex with [2-14C]pyruvate in the absence of CoASH promotes the incorporation of radio-label, probably in the form of acetyl groups, into both E2 and component X.     

Possible change in noradrenergic receptor sensitivity following methylphenidate treatment: Growth hormone and MHPG response to clonidine challenge in children with attention deficit disorder and hyperactivity

Growth hormone (GH) and 3-methoxy-4-hydroxyphenelethylene glycol (MHPG) response was measured hourly for 4 hours in 8 children with Attention Deficit Disorder with Hyperactivity (ADD+H) following an acute single-dose of clonidine. The clonidine challenge was repeated before, during, and one day after 12 weeks of treatment with methylphenidate (MPH). Before MPH treatment, the plasma growth hormone (GH) rose to 31.3 ± 4.6 (Mean ± SE) ng/ml; during MPH treatment, the GH peak was only 14.8 ± 3.2 ng/ml; one day after discontinuation of MPH, GH rose to only 20.8 ± 3.9 ng/ml. MHPG release was inhibited by clonidine in all treatment conditions but tended to be more decreased during MPH treatment. Some children with ADD+H may have hypersensitivity of the post-synaptic alpha-1 noradrenergic receptor which is diminished by MPH treatment. The extent to which these effects are pharmacological or represent a change in receptor sensitivity requires further study.     

Stabilization of purified potato virus X by dextran T-10 during lyophilization

Purification and preservation of potato virus X from leaf sap of tobacco plants before lyophilization was carried out by two methods: 1) precipitation by polyethylene glycol and ultracentifugation, and 2) precipitation by ammonium sulphate, chromatography on Sephadex G-50 and ultracentrifugation. The first method is preferable to the second because the final preparation contains more virus antigen. Both preparations were strongly infectious and maintained antigenic properties after lyophilization. To achieve a more gentle course of lyophilization of virus preparations, addition of urotropine and dextran T-10 to the virus suspension, purified by the precipitation by polyethylene glycol-6000, was examined. Addition of urotropine was proved unsatisfactory, because only antigenic properties were maintained after lyophilization while the infectivity disappeared. But we can recommend addition of dextran T-10 up to a concentration of 6% to the preparation of virus antigen before lyophilization. The course of lyophilization is much rapider, the lyophilized product can be very easily dissolved in water and is not hygroscopic. The product is strongly infectious and gives the serological precipitation reaction in a dilution four times that of X virus antigen lyophilized without addition of dextran T-10.     

Interrelationships of Leucine and Glutamate Metabolism in Cultured Astrocytes

Abstract: The aim was to study the extent to which leu-cine furnishes α-NH₂ groups for glutamate synthesis via branched-chain amino acid aminotransferase. The transfer of N from leucine to glutamate was determined by incubating astrocytes in a medium containing [¹⁵N]leucine and 15 unlabeled amino acids; isotopic abundance was measured with gas chromatography-mass spectrometry. The ratio of labeling in both [¹⁵N]glutamate/[¹⁵N]leucine and [2-¹⁵N]glutamine/[¹⁵N]leucine suggested that at least one-fifth of all glutamate N had been derived from leucine nitrogen. At the same time, enrichment in [¹⁵N]leucine declined, reflecting dilution of the ¹⁶N label by the unlabeled amino acids that were in the medium. Isotopic abundance in [¹⁶N]-isoleucine increased very quickly, suggesting the rapidity of transamination between these amino acids. The appearance of ¹⁵N in valine was more gradual. Measurement of branched-chain amino acid transaminase showed that the reaction from leucine to glutamate was approximately six times more active than from glutamate to leucine (8.72 vs. 1.46 nmol/min/mg of protein). However, when the medium was supplemented with α-ketoisocaproate (1 mM), the ketoacid of leucine, the reaction readily ran in the “reverse” direction and intraastrocytic [glutamate] was reduced by ～50% in only 5 min. Extracellular concentrations of α-ketoisocaproate as low as 0.05 mM significantly lowered intracellular [glutamate]. The relative efficiency of branched-chain amino acid transamination was studied by incubating astrocytes with 15 unlabeled amino acids (0.1 mM each) and [¹⁵N]glutamate. After 45 min, the most highly labeled amino acid was [¹⁵N]alanine, which was closely followed by [¹⁵N]leucine and [¹⁵N]isoleucine. Relatively little ¹⁵N was detected in any other amino acids, except for [¹⁵N]serine. The transamination of leucine was ～17 times greater than the rate of [1-¹⁴C]leucine oxidation. These data indicate that leucine is a major source of glutamate nitrogen. Conversely, reamination of a-ketoisocaproate, the ketoacid of leucine, affords a mechanism for the temporary “buffering” of intracellular glutamate.     

Identification of a locus involved in meningococcal lipopolysaccharide biosynthesis by deletion mutagenesis

A novel method for insertion/deletion mutagenesis in meningococci was devised. This consisted of ligating a digest of total chromosomal DNA to a 1.1 kb restriction fragment containing an erythromycin-resistance marker ( ermC ), and subsequent transformation of the ligation mixture into the homologous meningococcal strain H44/76. Southern blotting of a number of the resulting erythromycin-resistant transformants demonstrated that all carried the ermC gene inserted at different positions in the chromosome. Mutants with a specific phenotype were identified by screening with the anti-lipopolysaccharide (LPS) monoclonal antibody MN4A8B2, which is specific for immunotype L3. In this way, two independent L3-negative mutant strains were isolated. In transformation experiments with chromosomal DNA from these mutants, erythromycin-resistance and lack of MN4A8B2 reactivity were always linked, showing that the insertion/deletion was in a locus involved in LPS biosynthesis. On SDS–PAGE, the mutant LPS displayed an electrophoretic mobility intermediate between that produced by the previously isolated galE and rfaF mutant strains. Chemical analysis of the mutant LPS revealed that the structure was probably lipid A–(KDO)₂–(Hep)₂. Chromosomal DNA flanking the ermC insertion in these two mutant strains was cloned, and used as probe for the isolation of the corresponding region of the wild-type strain. From hybridization and polymerase chain reaction (PCR) analysis, it could be concluded that both mutations map to the same locus. The affected gene probably encodes the glycosyltransferase necessary for adding N -acetylglucosamine to heptose.     

几种红树植物幼苗中可溶性糖的含量

用Ｓｏｍｏｇｙｉ法测定了红海榄、木榄和秋茄３种红树植物幼苗的根、茎、叶和胚轴中可溶性糖的含量。同时．进行了加样回收率试验,回收率平均为１００．４８％,变异系数为２．１４％