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R L Somerville

Publications and source records attributed to R L Somerville.

69 records · Page 4Linked to original sources
PubMed1976

Fluorine-containing analogues of intermediates in the Shikimate pathway.

The phosphoenolpyruvate analogue (Z)-phosphoenol-3-fluoropyruvate is a substrate for phenylalanine-inhibitable 3-deoxy-D-arabino-heptulosonic acid-7-phosphate synthase from Escherichia coli. In the presence of excess erythrose 4-phosphate, apparent KM values of 65 and 38 muM were observed for phosphoenol-3-fluoropyruvate and phosphoenolpyruvate, respectively. Because the apparent Vmax for phosphoenol-3-fluoropyruvate is only 1.17% of that for phosphoenolpyruvate, one can study the former as an inhibitor of 3-deoxy-arabino-heptulosonic acid-7-phosphate synthase. Kinetic experiments showed phosphoenol-3-fluoropyruvate to be competitive with respect to phosphoenolpyruvate. Two distinguishable Ki values of 8 and 48 muM were obtained. The product (3S)-3-deoxy--3-fluoro-arabino-heptulosonic acid 7-phosphate was purified, characterized, and shown to act as a substrate for 5-dehydroquinate synthase. 3-Deoxy-3-fluoro-arabino-heptulosonic acid 7-phosphate, in contrast to 3-deoxy-arabino-heptulosonic acid 7-phosphate reacts slowly or not at all with reagents specific for 2-keto-3-deoxy sugars and is relatively resistant to oxidative cleavage by sodium periodate. The expected product of periodate oxidation, 3-fluoro-3-formylpyruvate, cannot be detected. This observation was clarified by studies with model compounds.

3-Deoxy-7-Phosphoheptulonate Synthase↗
PubMed1976

Purification and properties of a new aminopeptidase from Escherichia COLI K12.

An aminopeptidase (EC 3.4.11.-) capmable of hydrolyzing L-alanyl-beta-naphthyl-amide and certain other aminoacyl beta-naphthylamides was purified to homogeneity from extracts of Exherichia coli K-12. The enzyme, designated aminopeptidase II, is a monomeric protein of mol. wt. 100 000. It exhibits a broad pH optimum in the range pH 7.0--9.0. Although Zn2+, Fe3+ and Cr3+ are strong inhibitors of enzyme activity, a metal requirement for catalysis could not be firmly established. Neither sulfhydryl reagents nor serine protease inhibitors affected enzyme activity.

Aminopeptidases↗
PubMed1976

Isolation and characterization of mutations creating high-efficiency transcription initiation signals within the trp operon of Escherichia coli.

Two different mutational events generate promoter-active deoxyribonucleic acid sequences within the trp operon of Escherichia coli, probably through single base-pair changes. The mutations, obtained after ethyl methane sulfonate mutagenesis by selecting for elevated lac gene expression in a trp-lac fusion, are cis dominant and trans recessive with respect to their effects on the synthesis of downstream enzymes. One of the mutants (trpD11), obtained repeatedly under the selective conditions employed, prevents the formation of active phosphoribosyltransferase. The second mutation, trp3B, has no effects on any trp enzyme. By deletion mapping, trpD11 was localized near the operator-distal end of trpD, outside the segment of deoxyribonucleic acid that contains the low-efficiency internal promoter trpP2. Reversion to prototrophy of trpD11 was greatly stimulated by 2-aminopurine and ethyl methane sulfonate. Tests with suppressors indicated that trpD11 is a UAA (ochre) nonsense mutation. Under repression conditions, strains harboring either lesion in a normal trp operon synthesize the tryptophan biosynthetic enzymes in a noncoordinate fashion. The products of the operator-distal structural genes trpC, trpB, and trpA are formed at rates approximately 15-fold higher than those of wild type. The enzymes encoded by operator-proximal genes trpE and trpD are low or not detectable. Under derepression conditions, coordinate expression of the operon was observed.

Alleles↗
PubMed1972

Mutant strains of Escherichia coli K-12 exhibiting enhanced sensitivity to 5-methyltryptophan.

Eighteen mutants (designated MT(s)), isolated in Escherichia coli K-12, showed increased sensitivity to inhibition of growth by 5-methyltryptophan. All mutants were also much more sensitive to 4-methyltryptophan and 7-azatryptophan but exhibited near normal sensitivity to 5-fluorotryptophan and 6-fluorotryptophan. All of the mutations were linked to the trp operon. Their locations within the trp operon were established by deletion mapping. There was good agreement between the map position of an MT(s) mutation and a lowered activity of one of the tryptophan pathway enzymes. Three mutants, one of which contained a mutation that mapped within the trpE gene, were deficient in their ability to use glutamine as an amino donor in the formation of anthranilic acid. Another trpE mutation led to the production of an anthranilate synthetase with an increased sensitivity to feedback inhibition by tryptophan.

Aspartate-Semialdehyde Dehydrogenase↗
PubMed1971

Mutant strains of Escherichia coli K12 that use D-amino acids.

A series of mutations has been isolated that confer upon amino-acid auxotrophs of Escherichia coli K-12 the ability to grow when fed various D-amino acids. Several distinct systems, mediating cellular use of the D-isomers of leucine, histidine, phenylalanine, tyrosine, tryptophan, isoleucine, and valine, can be mutationally activated. Mutations leading to D-tryptophan use (dadR) all map near purB. They result in high activities of an enzyme that deaminates D-amino acids. Neither the enzymes of the tryptophan biosynthetic pathway nor tryptophanase (EC 4.2.1.e) are involved in D-tryptophan utilization.

Amino Acids↗
PubMed1971

Repression of aromatic amino acid biosynthesis in Escherichia coli K-12.

Mutants of Escherichia coli K-12 were isolated in which the synthesis of the following, normally repressible enzymes of aromatic biosynthesis was constitutive: 3-deoxy-d-arabinoheptulosonic acid 7-phosphate (DAHP) synthetases (phe and tyr), chorismate mutase T-prephenate dehydrogenase, and transaminase A. In the wild type, DAHP synthetase (phe) was multivalently repressed by phenylalanine plus tryptophan, whereas DAHP synthetase (tyr), chorismate mutase T-prephenate dehydrogenase, and transaminase A were repressed by tyrosine. DAHP synthetase (tyr) and chorismate mutase T-prephenate dehydrogenase were also repressed by phenylalanine in high concentration (10(-3)m). Besides the constitutive synthesis of DAHP synthetase (phe), the mutants had the same phenotype as strains mutated in the tyrosine regulatory gene tyrR. The mutations causing this phenotype were cotransducible with trpA, trpE, cysB, and pyrF and mapped in the same region as tyrR at approximately 26 min on the chromosome. It is concluded that these mutations may be alleles of the tyrR gene and that synthesis of the enzymes listed above is controlled by this gene. Chorismate mutase P and prephenate dehydratase activities which are carried on a single protein were repressed by phenylalanine alone and were not controlled by tyrR. Formation of this protein is presumed to be controlled by a separate, unknown regulator gene. The heat-stable phenylalanine transaminase and two enzymes of the common aromatic pathway, 5-dehydroquinate synthetase and 5-dehydroquinase, were not repressible under the conditions studied and were not affected by tyrR. DAHP synthetase (trp) and tryptophan synthetase were repressed by tryptophan and have previously been shown to be under the control of the trpR regulatory gene. These enzymes also were unaffected by tyrR.

Aldehyde-Lyases↗
PubMed1966

Tryptophan operon of Escherichia coli: regulatory behavior in Salmonella typhimurium cytoplasm.

Hybrids hemizygous for the tryptophan genes were prepared by episomal transfer of an Escherichia coli element into Salmonella typhimurium. Regulation of enzyme production by hybrids carrying wild-type E. coli genes in response to changes in the growth medium occurs in precisely the same manner as in haploid E. coli wild type. Mutant alleles of the anthranilate synhetase gene of E. coli which prevent derepression in E. coli function identically in S. typhimurium. At least one Salmonella tryptophan regulatory gene unlinked to the structural genes is known. Any dijferences which may exist between the tryptophan regulatory genes of E. coli and Salmonella have little effect on the regulation of enzyme formation in hybrids.

Cytoplasm↗
PubMed1966

Comparison of the tryptophan synthetase alpha-subunits of several species of Enterobacteriaceae.

Creighton, T. E. (Stanford University, Stanford), D. R. Helinski, R. L. Somerville, and C. Yanofsky. Comparison of the tryptophan synthetase alpha subunits of several species of Enterobacteriaceae. J. Bacteriol. 91:1819-1826. 1966.-The tryptophan synthetase alpha subunits of Escherichia coli K-12, E. coli B, Shigella dysenteriae, Salmonella typhimurium, and Aerobacter aerogenes have been purified and their structures compared. Each of these alpha subunits exhibits a sedimentation coefficient of about 2.7S. Peptide patterns of trypsin plus chymotrypsin digests of the alpha subunits have indicated that all of the alpha subunits have peptide regions in common. The patterns of E. coli K-12, E. coli B, and S. dysenteriae alpha subunits appear to be nearly identical, whereas the alpha subunits from S. typhimurium and A. aerogenes differ from those of E. coli and from each other. It has also been shown that the E. coli structural gene for the alpha subunit is translated identically in E. coli and S. typhimurium.

Amino Acids↗