The choice between two distinct T cell determinants within a 23-amino acid region of lysozyme depends on their structural context

The specificity of C57BL/6 T cells reactive to peptide aa 74-96 of hen egg-white lysozyme (HEL) was analyzed by using a panel of synthetic peptides of varying lengths from this region. It was found that peptide 74-96-reactive T cells induced by native HEL (aa 1-129) or its denatured fragment L2 (aa 13-105) recognized two distinct but overlapping determinants contained within aa 74-90 or aa 81-96, respectively. Peptide 74-96 itself induced both peptide 74-90-and peptide 81-96-specific T cells. Thus, a choice was made between these two potential T cell determinants on peptide 74-96, depending on which immunogen was used. Interestingly, the ability of both peptide determinants aa 74-90 and aa 81-96 to stimulate peptide 74-96-reactive T cells was partly dependent on the presence of residues within the overlap region (aa 81-90), suggesting that this region may play an important role in Iab-restricted T cell activation. This was further supported by the poor immunogenicity of shorter peptides 74-86 or 85-96, lacking residues from the overlap region in B6 mice. These two short peptides were nevertheless capable of eliciting T cell responses in B10.A mice, suggesting that the importance of this overlap region in obtaining a response to peptide 74-96 is related to the MHC haplotype.     

Regional Kinetic Constants for Blood–Brain Barrier Pyruvic Acid Transport in Conscious Rats by the Monocarboxylic Acid Carrier

The present investigation using labeled pyruvate describes the regional distribution and kinetics of the monocarboxylic acid carrier at the blood-brain barrier of conscious rats. The experimental procedure involved the arterial injection of a single bolus of 200 microliter containing [1-14C]pyruvate, [3H]water, and varying concentrations of unlabeled pyruvate into the common carotid via an indwelling externalized catheter. The hemisphere ipsi-lateral to the injection and rostral to the midbrain was removed and dissected into five regions. A kinetic analysis revealed no significant regional differences in Km values with an overall average of 1.37 mM. However, there was regional variation in the density of the monocarboxylic acid carrier as indicated by varied levels of the kinetic constant Vmax. The cortex showed the highest Vmax value of 0.42 +/- 0.08 mumol/min/g whereas values for the caudate/putamen, thalamus/hypothalamus, and remaining portion of hemisphere ranged significantly lower at 0.22-0.27 mumol/min/g. The Vmax for the hippocampus was intermediate at 0.37 +/- 0.12 mumol/min/g. The nonsaturable carrier described kinetically by KD had an overall average of 0.034 ml/min/g. The present study confirms quantitatively previous results suggesting a variable regional distribution of the monocarboxylic acid carrier.     

Deletion mutants that affect expression of Epstein-Barr virus nuclear antigen in COS-1 cells after gene transfer with simian virus 40 vectors containing portions of the BamHI K fragment

We have identified sequences that affect the efficient expression of Epstein-Barr virus nuclear antigen (EBNA 1) when the structural portion of its gene, found within the 2.9-kilobase-pair BamHI/HindIII fragment called Ilf, is expressed from a simian virus 40 vector. A set of nested deletions at the BamHI end of the fragment was constructed by using BAL 31 digestion, the addition of linkers, and ligation into pSVOd. The mutants were tested for their ability to express antigen in COS-1 monkey cells by using indirect immunofluorescence and immunoblotting. Deletion endpoints were determined by DNA sequencing of the 5' ends of the mutants. The deletion mutants could be subclassified into four groups based on their ability to express EBNA polypeptide. Mutants that retain more than 106 base pairs upstream from the start of the open reading frame in Ilf exhibit antigen expression indistinguishable from that of wild type. Mutants that invade the structural gene by 1,115 or more bases destroy antigen expression. Mutants that alter the splice acceptor site or invade the open reading frame by a short distance make antigen at a markedly lower frequency. There are three mutants, whose deletions map at -78, -70, and -44 base pairs upstream of the open reading frame, that make reduced levels of EBNA. Since these three mutants differ in the extent to which EBNA expression is impaired, the data suggest that there are several critical regions upstream of the open reading frame that regulate EBNA expression in COS-1 cells. It is not known whether these regulatory sequences, which would be located in an intron in the intact genome, play any role in the expression of EBNA in infected lymphocytes.     

Remobilization of labelled nitrogen from vegetative tissues to pods of mungbean

Summary Remobilization of¹⁵N from vegetative tissue of mungbean (Vigna radiata (L.) Wilczek) into pods was measured during the reproductive phase of growth. Plant tissue was labelled with¹⁵N during vegetative development. Experiments were conducted in the field at two sites. At one site the soil provided cowpeas with most of their N but at the other site N fixation provided most of the N. Remobilized N from vegetative tissue to pods occurred soon after they began to develop. The quantity of the labelled N ultimately remobilized to the pods amounted to 50% for one cultivar (Tx33) at the high soil N site and 70% at the low N site. For the other cultivar (Tx13) the values were 25% and 30%, respectively. The two cultivars performed very differently with respect to partitioning of N into pods and the rate of N fixation. Even though more N was accumulated in the shoots of the high N fixing cultivar (Tx13) less total N was contained in the pods.     

Specialized transduction of Pseudomonas aeruginosa PAO by bacteriophage D3.

D3, a temperate bacteriophage of Pseudomonas aeruginosa PAO, was found to specifically transduce the alleles met-49 and met-117. Induction of established lysogens with UV light was necessary for the production of transducing lysates. Transduced cells were immune to superinfection by phage D3 and could give rise to high-frequency transducing lysates. Cotransduction of these two alleles could not be demonstrated. met-117 was mapped to 26 min on the PAO genetic map. Complementation studies using the generalized transducing phage F116L indicated that met-49 is an allele of met-9011 which maps at 55 min. The integrated D3 prophage was shown to be coinherited with met-117 and with met-49.     

Separation of cardiac plasmalemma into cell surface and T-tubular components. Distribution of saxitoxin- and nitrendipine-binding sites

To compare surface sarcolemmal with T-tubular distributions of [3H]saxitoxin (STX)- and [3H]nitrendipine (NTD)-binding sites, we centrifuged membrane vesicles from sheep and bovine ventricles on a 10-40% linear sucrose gradient from which fractions were assayed for STX and NTD binding; for markers of surface sarcolemma (ouabain-sensitive Na,K-ATPase activity, [3H]quinuclidinyl benzilate binding); and for markers of junctional sarcoplasmic reticulum known to be preferentially associated with T-tubules (ryanodine-sensitive Ca2+ uptake, calsequestrin, an Mr 300,000 putative phosphorylatable "foot" protein, and electron microscopically visible junctional sarcoplasmic reticulum-plasmalemma complexes). We identified three distinct peaks in the sucrose gradient, each characterized by significant high and low affinity STX- and high affinity NTD-binding: Peak I (approximately 19% sucrose), highly enriched in surface sarcolemma; Peak III (approximately 36% sucrose), enriched in junctional sarcoplasmic reticulum markers and hence in junctional sarcoplasmic reticulum complexes with T-tubule; and Peak II (approximately 27% sucrose), showing greatest specific STX binding and only moderate NTD binding, enriched in T-tubular membrane, unassociated with junctional sarcoplasmic reticulum. For ventricular myocytes, the ratio NTD sites/STX sites was 2.5 for surface sarcolemma, but only approximately 1.0 for T-tubules. Unlike data published for mammalian skeletal muscle, sheep and beef cardiac NTD receptors were not significantly more concentrated in T-tubular than in surface plasmalemma.     

Characterization of the pathways for phosphatidylethanolamine biosynthesis in Chinese hamster ovary mutant and parental cell lines

A tritium suicide procedure was devised to facilitate the isolation of Chinese hamster ovary cell mutants defective in phosphatidylethanolamine biosynthesis. One mutant with a 20-50% reduction in [3H]ethanolamine incorporation was chosen for further analysis and was shown to have reduced activity of CTP: phosphoethanolamine cytidylyltransferase. Levels of phosphatidylethanolamine and rates of its biosynthesis were compared in the mutant and parent cell lines. Despite the reduced activity of the CDP-ethanolamine pathway in the mutant, levels of phosphatidylethanolamine were the same in mutant and parent cells. Rates of phosphatidylethanolamine synthesis de novo, as measured by incorporation of 32PO4 into phosphatidylethanolamine, were also the same in mutant and parent cells, as was the rate of incorporation of [3H]serine into both phosphatidylserine and phosphatidylethanolamine. After a long term labeling with [3H]serine, the specific radioactivity of phosphatidylserine was the same as that of phosphatidylethanolamine, and there was no difference in the specific radioactivities of the two lipids between mutant and parent cells. These results implicate decarboxylation of phosphatidylserine as the sole route for synthesis of phosphatidylethanolamine under normal culture conditions.     

Stimulation of Escherichia coli adenylate cyclase activity by elongation factor Tu, a GTP-binding protein essential for protein synthesis

A unique feature of eucaryotic adenylate cyclases is their interaction with GTP-binding proteins that mediate hormonal responses. Until now, there has been no evidence for regulation of Escherichia coli adenylate cyclase by a GTP-binding protein. We describe here that the most abundant protein in E. coli, the GTP-binding protein EF-Tu, which is important as an elongation factor in protein synthesis, also serves as a stimulator of adenylate cyclase activity. Homogeneous EF-Tu specifically increased the activity of purified adenylate cyclase as much as 70%; other E. coli GTP-binding proteins had no effect on enzyme activity. A study of the guanine nucleotide specificity for EF-Tu-mediated stimulation of adenylate cyclase activity suggested that the preferred activator is EF-Tu X GDP. To account for the GTP-specific stimulation of adenylate cyclase activity observed in intact cells, we propose that the nucleotide specificity for EF-Tu-dependent activation of adenylate cyclase is governed by other factors in the cell.