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1.  Effect of traY amber mutations on F-plasmid traY promoter activity in vivo. 
Journal of Bacteriology  1996;178(19):5787-5789.
We have examined the effect of the F plasmid TraY protein on tra gene expression in vivo. Expression was assayed as alkaline phosphatase activity in cells containing a traY phi(traA'-'phoA)hyb operon under traY promoter control. Amber mutations in traY significantly reduced alkaline phosphatase activity. Since nonsense polarity effects were minimal, if they occurred at all, these data provide the first direct evidence that TraY regulates tra gene expression.
PMCID: PMC178421  PMID: 8824627
2.  Accumulation of the F plasmid TraJ protein in cpx mutants of Escherichia coli. 
Journal of Bacteriology  1993;175(4):921-925.
We report here studies of the cellular control of F plasmid TraJ protein levels, focusing on the effects of chromosomal cpx mutations. The principal conclusion from our results is that the cpx mutations impair accumulation of the TraJ protein, thereby reducing tra gene expression. We measured TraJ activity in vivo by expression of a traY'-'lacZ fusion gene and TraJ protein by immuno-overlay blot. In strains with normal TraJ levels, traY expression and donor-related functions were reduced in cells carrying any of four cpxA mutations. In the strain background used to isolate cpx mutants, these reductions were especially evident in cells grown to high density, when traY expression and donor activity both increased in cpx+ cells. In each of the four cpxA mutants tested, TraJ levels were lower than in the otherwise isogenic cpxA+ strain. In cells grown to high density, the differences ranged from 4-fold in the cpxA6 strain to > 10-fold in the cpxA2, cpxA5, and cpxA9 strains. The cpxA2 mutation had little or no effect on traY expression or on donor-related functions when TraJ was present in excess of its limiting level in F' or Hfr cells or on a mutant traY promoter whose expression in vivo was independent of TraJ.
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PMCID: PMC193002  PMID: 8432716
3.  Arc and Sfr functions of the Escherichia coli K-12 arcA gene product are genetically and physiologically separable. 
Journal of Bacteriology  1991;173(18):5648-5652.
The Escherichia coli arcA gene product regulates chromosomal gene expression in response to deprivation of oxygen (Arc function; Arc stands for aerobic respiration control) and is required for expression of the F plasmid DNA transfer (tra) genes (Sfr function; Sfr stands for sex factor regulation). Using appropriate lacZ fusions, we have examined the relationship between these two genetic regulatory functions. Arc function in vivo was measured by anaerobic repression of a chromosomal sdh-lacZ operon fusion (sdh stands for succinate dehydrogenase). Sfr function was measured by activation of a plasmid traY-lacZ gene fusion. An eight-codon insertion near the 5' terminus of arcA, designated arcA1, abolished Arc function, as previously reported by S. Iuchi and E.C.C. Lin (Proc. Natl. Acad. Sci. USA 85:1888-1892, 1988), but left Sfr function largely (greater than or equal to 60%) intact. Similarly, the arcB1 mutation, which depressed sdh expression and is thought to act by abolishing the signal input that elicits ArcA function, had little effect (less than or equal to 20%) on the Sfr function of the arcA+ gene product. Conversely, a valine-to-methionine mutation at codon 203 (the sfrA5 allele) essentially abolished Sfr activity without detectably altering Arc activity. These data indicate that Sfr and Arc functions are separately expressed and regulated properties of the same protein.
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PMCID: PMC208293  PMID: 1885542
4.  The Cpx proteins of Escherichia coli K-12: evidence that cpxA, ecfB, ssd, and eup mutations all identify the same gene. 
Journal of Bacteriology  1990;172(5):2456-2461.
An existing cpxA(Ts) mutant was resistant to amikacin at levels that inhibited completely the growth of a cpxA+ and a cpxA deletion strain and failed to grow as efficiently on exogenous proline. These properties are similar to those of mutants altered in a gene mapped to the cpxA locus and variously designated as ecfB, ssd, and eup. The amikacin resistance phenotype of the cpxA mutant was inseparable by recombination from the cpxA mutant phenotype (inability to grow at 41 degrees C without exogenous isoleucine and valine) and was recessive to the cpxA+ allele of a recombinant plasmid. Using methods that ensured independent mutations in the cpxA region of the chromosome, we isolated six new amikacin-resistant mutants following nitrosoguanidine mutagenesis. Three-factor crosses mapped the mutations to the cpxA locus. When transferred by P1 transduction to a cpxB11 Hfr strain, each of the mutations conferred the Tra- and Ilv- phenotypes characteristic of earlier cpxA mutants. Two of the new mutations led to a significantly impaired ability to utilize exogenous proline, and four led to partial resistance to colicin A. Two of the new cpxA alleles were recessive to the cpxA+ allele, and four were dominant, albeit to different degrees. On the basis of these data, we argue that cpxA, ecfB, eup, and ssd are all the same gene. We discuss the cellular function of the cpxA gene product in that light.
PMCID: PMC208883  PMID: 2185221
5.  Structure and function of conjugative pili: monoclonal antibodies as probes for structural variants of F pili. 
Journal of Bacteriology  1990;172(3):1174-1179.
The lac-tra operon fusion plasmid pTG801 contains the known F plasmid DNA transfer (tra) genes required by Escherichia coli to elaborate functional F pili (T. Grossman and P. M. Silverman, J. Bacteriol. 171:650-656, 1989). Here, we show that these pili are actually structural variants of normal F pili and that the F plasmid must contain additional genes that affect pilus structure and function. We confirmed a previous report that two monoclonal antibodies that recognize epitopes at and near the amino terminus of F pilin do not decorate the sides of normal F pili, as determined by immunogold electron microscopy. However, both antibodies laterally decorated pTG801 pili. The epitope for one of the antibodies has been shown to include the amino-terminal acetyl group of F pilin, which must therefore also be present on pTG801 pilin. Normal antibody staining was restored to pTG801 pili when cells contained, in addition to pTG801, the compatible plasmid pRS31, which must therefore include at least one gene affecting F-pilus structure. One candidate, traD, was excluded as the sole such gene, since traD+ derivatives of a pTG801 strain still elaborated pili that could be laterally decorated with antibody. Moreover, although traD alone restored RNA bacteriophage R17 infectivity to pTG801 cells, as expected, it did not mimic pRS31 in restoring to pTG801 pili other characteristics of normal F pili. We conclude that pRS31 contains as yet uncharacterized genes required for elaboration of structurally normal F pili. Finally, we identified vesicular material, especially abundant in cultures of pTG801 transformants, that stained heavily with the anti-F-pilin monoclonal antibodies. This material may reflect the inner membrane pool of F pilin.
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PMCID: PMC208581  PMID: 1689713
6.  Tumor necrosis factor alpha and the anemia associated with murine malaria. 
Infection and Immunity  1989;57(5):1542-1546.
The anemia associated with malaria is complex, and multiple factors contribute to its severity. An increased destruction and a decreased production of erythrocytes are involved; however, the mechanisms responsible remain unclear. Tumor necrosis factor alpha (TNF-alpha), released by macrophages in response to infection, is thought to play a role through its ability to inhibit erythropoiesis. In these studies we have examined erythropoiesis in mice infected with Plasmodium berghei and in mice infused with recombinant TNF-alpha via implanted osmotic pumps. In both groups of mice there was (i) a reduction of pluripotent stem cells in the bone marrow and a concomitant increase in the spleen, (ii) a reduction of erythroid progenitor cells, and (iii) a reduced incorporation of 59Fe into erythrocytes. When P. berghei-infected mice were given antiserum against recombinant murine TNF, erythropoiesis was partially restored. There was a significant increase in bone marrow stem cells, erythroid progenitor cells, and 59Fe incorporation into erythrocytes in P. berghei-infected mice that had been treated with anti-TNF. How TNF may act, directly or indirectly, to inhibit erythropoiesis is not yet clear. These results demonstrate that TNF mediates, in part, the anemia associated with malaria.
PMCID: PMC313311  PMID: 2707858
7.  Structure and function of conjugative pili: inducible synthesis of functional F pili by Escherichia coli K-12 containing a lac-tra operon fusion. 
Journal of Bacteriology  1989;171(2):650-656.
In vivo and in vitro recombination methods were used to construct the recombinant plasmid pTG801, in which the F-plasmid DNA transfer (tra) genes required for the formation of functional F pili were placed under the lac transcriptional control sequences of pUC19. The 20 kilobases of cloned F DNA includes genes traA through the 5'-terminal part of traG; the plasmid lacks the positive regulatory gene traJ and all of the known tra genes required for the DNA transfer stage of conjugation. pTG801 transformants were sensitive to the donor-specific bacteriophages Q beta and f1, as measured by the formation of infectious centers. They were relatively insensitive to bacteriophage R17, as expected from the absence of traD. In the presence of a lacIq allele, sensitivity of pTG801 transformants to f1 and Q beta depended on the concentration of inducer (isopropyl-beta-D-thiogalactopyranoside [IPTG]). Viewed by electron microscopy, pTG801 transformants elaborated 7- to 10-nm-diameter filaments that could be laterally decorated with RNA bacteriophage particles, consistent with the formation of F pili. In stationary-phase cultures, these filaments formed massive aggregates and could be seen to adhere lengthwise to the cell surface; few pili accumulated in the medium as single filaments.
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PMCID: PMC209646  PMID: 2563366
8.  Alkylation of acetohydroxyacid synthase I from Escherichia coli K-12 by 3-bromopyruvate: evidence for a single active site catalyzing acetolactate and acetohydroxybutyrate synthesis. 
Journal of Bacteriology  1987;169(6):2494-2499.
Acetohydroxyacid synthase I (AHAS I) purified from Escherichia coli K-12 was irreversibly inactivated by incubation with 3-bromopyruvate. Inactivation was specific, insofar as bromoacetate and iodoacetate were much less effective than bromopyruvate. Inactivation was accompanied by incorporation of radioactivity from 3-bromo[2-14C]pyruvate into acid-insoluble material. More than 95% of the incorporated radioactivity coelectrophoresed with the 60-kilodalton IlvB subunit of the enzyme through a sodium dodecyl sulfate-polyacrylamide gel; less than 5% coelectrophoresed with the 11.2-kilodalton IlvN subunit. The stoichiometry of incorporation at nearly complete inactivation was 1 mol of 14C per mol of IlvB polypeptide. These data indicate that bromopyruvate inactivates AHAS I by alkylating an amino acid at or near a single active site located in the IlvB subunit of the enzyme. We confirmed that this alkylation inactivated both AHAS reactions normally catalyzed by AHAS I. These results provide the first direct evidence that AHAS I catalyzes both acetohydroxybutyrate and acetolactate synthesis from the same active site.
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PMCID: PMC212102  PMID: 3294793
9.  Role of small subunit (IlvN polypeptide) of acetohydroxyacid synthase I from Escherichia coli K-12 in sensitivity of the enzyme to valine inhibition. 
Journal of Bacteriology  1986;166(3):901-904.
Most of the coding sequence for the IlvN polypeptide subunit of acetohydroxyacid synthase I was deleted from the ilvB+ ilvN+ plasmid pTCN12 by in vitro methods. Several ilvB+ delta ilvN derivatives of pTCN12 were identified among transformants of a strain otherwise lacking any acetohydroxyacid synthase. Deletion derivatives produced an enzymatically active IlvB polypeptide, as shown by the Ilv+ phenotype of transformed cells and by immunologic and enzymatic assays. However, whereas the growth of pTCN12 transformants was sensitive to valine inhibition, growth of the ilvB+ delta ilvN transformants was relatively resistant. Moreover, in vitro analyses confirmed that both acetolactate and acetohydroxybutyrate synthesis in extracts of the ilvB+ delta ilvN transformants was resistant to valine inhibition, in comparison with that in extracts of pTCN12 transformants or with that catalyzed by purified acetohydroxyacid synthase I. The IlvN polypeptide had a minimal effect, if any, on IlvB polypeptide accumulation as measured by immunoprecipitation, but its absence resulted in a greater than 10-fold reduction in enzyme specific activity.
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PMCID: PMC215211  PMID: 3011751
10.  Purification and subunit composition of acetohydroxyacid synthase I from Escherichia coli K-12. 
Journal of Bacteriology  1984;157(1):184-189.
Acetohydroxyacid synthase I from Escherichia coli K-12 has been purified to near homogeneity. Analysis of the purified enzyme by sodium dodecyl sulfate-polyacrylamide gel electrophoresis showed the presence of two polypeptides, one with a molecular weight of 60,000 and one with a molecular weight of 9,500. These two polypeptides were present in constant proportion to each other and to enzyme activity. The molar ratio of the two polypeptides (Mr 9,500:60,000), estimated from stained polyacrylamide gels, was 1. Antisera raised against the 60,000 Mr polypeptide precipitated both the 60,000 and the 9,500 Mr polypeptides from extracts of cells labeled with [35S]methionine. The addition of sodium dodecyl sulfate before immunoprecipitation eliminated the smaller polypeptide, and only the larger one was recovered. The hydrodynamic properties of the native enzyme confirmed a previous report that the largest enzymatically active species has a molecular weight of about 200,000; this species contains both the 60,000- and 9,500-molecular-weight polypeptides.
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PMCID: PMC215150  PMID: 6360995
11.  Synthesis of outer membrane proteins in cpxA cpxB mutants of Escherichia coli K-12. 
Journal of Bacteriology  1983;154(1):375-382.
Two major proteins, the murein lipoprotein and the OmpF matrix porin, are deficient in the outer membrane of cpxA cpxB mutants of Escherichia coli K-12. We present evidence that the cpx mutations prevent or retard the translocation of these proteins to the outer membrane. The mutations had no effect on the rate of lipoprotein synthesis. Mutant cells labeled for 5 min with radioactive arginine accumulated as much lipoprotein as otherwise isogenic cpxA+ cpxB+ cells. This lipoprotein accumulated as such; no material synthesized in mutant cells and reactive with antilipoprotein antibodies had the electrophoretic mobility of prolipoprotein. Hence, the initial stages of prolipoprotein insertion into the inner membrane leading to its cleavage to lipoprotein appeared normal. However, after a long labeling interval, mutant cells were deficient in free lipoprotein and lacked lipoprotein covalently bound to peptidoglycan, suggesting that little if any of the lipoprotein synthesized in mutant cells reaches the outer membrane. Immunoreactive OmpF protein could also be detected in extracts of mutant cells labeled for 5 min, but the amount that accumulated was severalfold less in mutant cells than in cpxA+ cpxB+ cells. Analysis of beta-galactosidase synthesis from ompF-lacZ fusion genes showed this difference to be the result of a reduced rate of ompF transcription in mutant cells. Even so, little or none of the ompF protein synthesized in mutant cells was incorporated into the outer membrane.
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PMCID: PMC217469  PMID: 6339479
12.  Mutations in genes cpxA and cpxB alter the protein composition of Escherichia coli inner and outer membranes. 
Journal of Bacteriology  1982;151(3):1553-1559.
Mutations in chromosomal genes cpxA and cpxB altered the protein composition of the inner and outer bacterial membranes. Electrophoretic analyses of membrane proteins from isogenic strains differing only at their cpx loci and of spontaneous cpxA+ revertants of a cpxA cpxB double mutant showed that the alterations define a pattern that is uniquely attributable to the cpx mutations. Two major outer membrane proteins, the OmpF matrix porin and the murein lipoprotein, were deficient or absent from the outer membrane of mutant cells, whereas the quantities of two other major outer membrane proteins, the OmpC matrix porin and the OmpA protein, were not significantly altered. The cpx mutations did not generally alter the functional or chemical properties of the cell envelope. In the electron microscope, mutant cells appeared ovoid, but individual cells showed no surface irregularities to suggest gross defects in the cell envelope. These observations suggest that the primary effect of the mutations is to alter selectively the synthesis or translocation of certain envelope proteins.
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PMCID: PMC220437  PMID: 7050092
13.  Mutations in genes cpxA and cpxB of Escherichia coli K-12 cause a defect in acetohydroxyacid synthase I function in vivo. 
Journal of Bacteriology  1982;151(2):976-982.
Mutations in Escherichia coli genes cpxA and cpxB together cause a temperature-sensitive defect in isoleucine and valine syntheses that is related specifically to acetohydroxyacid synthase I. This enzyme catalyzes the first pair of homologous reactions required for the synthesis of these two amino acids. At both permissive and nonpermissive temperatures, mutant cells containing ilvB (the structural gene for acetohydroxyacid synthase I) cloned in a derivative of plasmid pBR322 synthesized comparable amounts of ilvB mRNA and contained several times the enzyme activity normally required to sustain exponential growth, yet these cells remained temperature sensitive for growth in the absence of isoleucine and valine. These observations suggest that the primary effect of the cpx mutations is to block enzyme function in vivo. The enzyme was unstable in mutant cells at growth temperatures above 37 degrees C, but this instability appeared to be a secondary effect on the cpx mutations.
PMCID: PMC220350  PMID: 7047501
14.  Gene cpxA is a new addition to the linkage map of Escherichia coli K-12. 
Journal of Bacteriology  1982;150(1):425-428.
The cpxA gene of E. coli K-12 lies between genes glpK and tpi, closely linked to the latter at 87.8 min on the linkage map. Since no other gene has been mapped in this interval, cpxA is a new addition to the linkage map.
PMCID: PMC220135  PMID: 7037752
15.  Mutations in genes cpxA and cpxB of Escherichia coli K-12 cause a defect in isoleucine and valine syntheses. 
Journal of Bacteriology  1980;144(1):68-73.
Mutations in two chromosomal genes of Escherichia coli, cpxA and cpxB, produced a temperature-sensitive growth defect that was remedied specifically by the addition of isoleucine and valine to the minimal medium. This auxotrophy was manifested only when the medium contained exogenous leucine, suggesting that mutant cells fail to elaborate active acetohydroxy acid synthase, isozyme I. In the presence of leucine, this enzyme was required to catalyze the first reaction common to the biosynthesis of isoleucine and valine. Measurements of enzyme activity in crude extracts showed that mutant cells were seven- to eightfold deficient in active isozyme I when the cells were grown in the presence of leucine. When grown in the absence of leucine, mutant cells contained more acetohydroxy acid synthase activity. We attribute this activity to isozyme III, the product of the ilvHI genes, which are derepressed in the absence of exogenous leucine. The cpxA and cpxB mutations appear to affect the production of active isozyme I, rather than its activity, since (i) neither the cpxA nor the cpxB gene mapped near the structural gene for isozyme I (ilvB), (ii) the growth of mutant cells shifted from the permissive (34 degrees C) to the nonpermissive (41 degrees C) temperature did not immediately cease, but declined gradually over a period corresponding to several normal generation times, and (iii) the enzyme from mutant cells grown at 34 degrees C was as stable at 41 degrees C as the enzyme from cpx+ cells.
PMCID: PMC294590  PMID: 6998970
16.  Genetic analysis of Escherichia coli K-12 chromosomal mutants defective in expression of F-plasmid functions: identification of genes cpxA and cpxB. 
Journal of Bacteriology  1980;144(1):60-67.
Two temperature-sensitive, chromosomal mutants of Escherichia coli were selected for their inability to express deoxyribonucleic acid donor activity and other activities associated with the conjugative plasmid F. These mutants were also auxotrophic for isoleucine and valine at 41 degrees C. Each mutant strain contained two altered genes: cpxA, located at 88 min on the E. coli K-12 genetic map, and cpxB, located at 41 min. Mutations in both genes were required for maximal expression of mutant phenotypes. The parent strain of mutants KN401 and KN312 already contained the cpxB mutation that is present in both mutants (cpxB1). This mutation by itself was cryptic. The cpxA mutations represent different mutant alleles since they are of independent origin. A cpxA mutation by itself significantly affected the expression of plasmid functions and growth at 41 degrees C in the absence of isoleucine and valine, but strains containing both a cpxA and cpxB mutation were more severely affected. Along with the observation that both cpxA mutations were revertable, the temperature sensitivity of cpxA cpxB+ cells suggests that both cpxA alleles contain point mutations that do not completely destroy the activity of the cpxA gene product.
PMCID: PMC294588  PMID: 6998969
17.  Selection of Escherichia coli K-12 chromosomal mutants that prevent expression of F-plasmid functions. 
Journal of Bacteriology  1980;143(3):1519-1523.
Chromosomal mutants of Escherichia coli deficient in the expression of F-plasmid functions were selected by mutagenizing F- cells, introducing an F' plasmid into the mutagenized cells by conjugation, and identifying transconjugants resistant to the donor-specific bacteriophage Q beta by a simple spray test. All but 1 of 25 mutants were defective in an extracellular stage of Q beta infection, suggesting that they fail to elaborate F-pili. At least six of these were also deficient as deoxyribonucleic acid donors. More than half of the mutants appear to be altered in peviously undetected chromosomal genes required for the expression of F-related cellular functions.
PMCID: PMC294549  PMID: 6106014
18.  Induction of cyclic AMP phosphodiesterase in Blastocladiella emersonii and its relation to cyclic AMP metabolism. 
Journal of Bacteriology  1978;135(3):968-975.
Extracts of vegetative cells of Blastocladiella emersonii contain 5% or less of the cyclic AMP phosphodiesterase activity in zoospore extracts. This difference in activity could be accounted for entirely by an increase in the differential rate of phosphodiesterase synthesis during sporulation, beginning after a lag period of about 60 min and extending for at least an additional 90 min into the 4-h sporulation process. To examine the relation between enzyme synthesis and cyclic nucleotide metabolicm, we determined the substrate specificity of phosphodiesterase synthesized during sporulation and partially purified from zoospores. Zoospore extracts contain two components, separable by gel filtration chromatography, with cyclic AMP phosphodiesterase activity. The larger component accounts for 20% of the total activity and the smaller component for 80%. Both components show essentially an absolute substrate specificity for cyclic AMP among several cyclic purine and cyclic pyrimidine nucleotides tested. Nevertheless, we found no change in the total cyclic AMP content of sporulating cells before, during, or after enzyme activity increased. We speculate that some other component of cyclic AMP metabolism or function limits the rate of cyclic AMP hydrolysis in sporulating cells.
PMCID: PMC222471  PMID: 211117
19.  Cyclic AMP binding proteins and cyclic AMP-dependent protein kinase from Blastocladiella emersonii. 
Journal of Bacteriology  1978;135(3):976-980.
The stoichiometry of cyclic AMP binding protein to cyclic AMP in sporulating cells of Blastocladiella emersonii and the resistance of protein-bound cyclic AMP to enzyme-catalyzed hydrolysis suggest that the distribution of cyclic AMP between free and protein-bound pools is an important factor in cyclic AMP metabolism. Most but not all of the cyclic AMP binding protein in sporulating cells is associated with a cyclic AMP-dependent protein kinase.
PMCID: PMC222472  PMID: 211118
20.  Long-term studies on rhesus monkeys (Macaca mulatta) immunized against Plasmodium knowlesi. 
Infection and Immunity  1977;15(2):461-465.
Studies carried out on four rhesus monkeys (Macaca mulatta) that had been vaccinated against Plasmodium knowlesi show that the immunized animals were protected against a challenge with a heterologous strain of P. knowlesi. This protection was shown to be present even 4 years after the immunization schedule has been completed. The effect could not be attributed toprevious infections with the parasite, since four control rhesus monkeys that had recovered from one to four challenges with P. knowlesi died when exposed to the heterologous strain. Data obtained from the lymphocyte transformation test and the radioimmunoassay are also presented.
PMCID: PMC421390  PMID: 403136
21.  Behaviour of rats exposed to trichloroethylene vapour. 
Rats were exposed to trichloroethylene (TCE) vapour for about five five-day weeks at concentrations from 100 to 1 000 ppm, and at 100 ppm for 12 1/2 weeks. The social behaviour of paired male rats was observed periodically in the home cage for five minutes after they had been exposed to TCE. The principal finding was a consistent reduction of up to 24% in the total acitivity. A single day's exposure to TCE was sufficient at the highest concentration. At 100 ppm, a similar decline in activity was significant after 1 1/2 weeks' exposure in one experiment and 8 1/2 weeks' in another. The decline in activity was fairly uniform and not usually because of specific reductions in particular kinds of behaviour. However, exploration of the cage and submission to, or escape from, the other rat were sometimes specifically reduced. In an 'exploration-thirst' test, rats were deprived of water overnight and placed on two or three occasions in a previously unfamiliar cage. Rats exposed to 100, 200, or 1 000 ppm TCE found water and began drinking sooner than their controls without altering the rate of movement about the cage. These results suggest lowered performance in a familiar situation where rats are usually very active and some loss of inhibitory control in an unfamiliar one. At the present threshold limit value, repeated exposure to TCE eventually had effects similar to those of one day's exposure to a higher concentration, but only after a widely variable delay.
PMCID: PMC1008081  PMID: 1238103
22.  Letter: Malaria vaccines. 
British Medical Journal  1975;2(5966):335-336.
PMCID: PMC1681890  PMID: 805622
23.  The ilvB locus of Escherichia coli K-12 is an operon encoding both subunits of acetohydroxyacid synthase I. 
Nucleic Acids Research  1985;13(11):3979-3993.
The ilvB locus of Escherichia coli K-12 encloses two open reading frames defining polypeptides of 60,000 and 11,200 molecular weight. The entire locus, about 2.3 kb, is co-transcribed as an operon. The molecular weights and amino acid compositions of the presumptive operon polypeptides agree with those of the large and small subunit polypeptides of acetohydroxyacid synthase (AHAS) I, for which ilvB is the structural locus. We reserve the designation ilvB for the promoter proximal (longer) cistron and designate the promoter distal cistron ilvN. The molecular weight and amino acid sequence of the ilvB polypeptide are strikingly similar to those of the I1vI (larger subunit of AHAS III) and I1vG (larger subunit of AHAS II) polypeptides. There is less size uniformity among the I1vN, I1vH (smaller subunit of AHAS III), and I1vM (smaller subunit of AHAS II) polypeptides. Nevertheless, there is significant amino acid sequence homology among the three small subunit polypeptides. Thus, all three AHAS isozymes of E. coli K-12 probably have a common evolutionary origin.
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PMCID: PMC341291  PMID: 2989781

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