Physical organization of the ilvEDAC genes of Escherichia coli strain K-12

The redundant aminotransferases in lysine and arginine synthesis and the extent of aminotransferase redundancy in Escherichia coli

Aminotransferases can be redundant or promiscuous, but the extent and significance of these properties is not known in any organism, even in Escherichia coli. To determine the extent of redundancy, it was first necessary to identify the redundant aminotransferases in arginine and lysine synthesis, and then complement all aminotransferase-deficient mutants with genes for all aminotransferases. The enzymes with N-acetylornithine aminotransferase (ACOAT) activity in arginine synthesis were ArgD, AstC, GabT and PuuE; the major anaerobic ACOAT was ArgD. The major enzymes with N-succinyl-l,l-diaminopimelate aminotransferase (SDAP-AT) activity in lysine synthesis were ArgD, AstC, and SerC. Seven other aminotransferases, when overproduced, complemented the defect in a triple mutant. Lysine availability did not regulate synthesis of the major SDAP-ATs. Complementation analysis of mutants lacking aminotransferases showed that the SDAP-ATs and alanine aminotransferases were exceptionally redundant, and it is proposed that this redundancy may ensure peptidoglycan synthesis. An overview of all aminotransferase reactions indicates that redundancy and broad specificity are common properties of aminotransferases.

Structure and expression of a cyanobacterial ilvC gene encoding acetohydroxyacid isomeroreductase

Acetohydroxyacid isomeroreductase (AHAIR) is the shared second enzyme in the biosynthetic pathways leading to isoleucine and valine. AHAIR is encoded by the ilvC gene in bacteria. A 1,544-bp fragment of genomic DNA containing the ilvC gene was cloned from the cyanobacterium Synechocystis sp. strain PCC 6803, and the complete nucleotide sequence was determined. The identity of the gene was established by comparison of the nucleotide and derived peptide sequences with those of other ilvC genes. The highest degree of sequence similarity was found with the ilvC gene from Rhizobium meliloti. The isolated Synechocystis ilvC gene complemented an Escherichia coli ilvC mutant lacking AHAIR activity. The expressed Synechocystis gene encodes a protein that has a molecular mass of 35.7 kDa and that has AHAIR activity in an in vitro assay. Polyclonal antibodies raised against purified Synechocystis AHAIR produced a single band on a Western blot (immunoblot) of a Synechocystis cell extract and detected the protein in an extract of an E. coli ilvC mutant strain that was transformed with a plasmid containing the Synechocystis ilvC gene. The antibody did not react with an extract of an E. coli ilvC mutant strain that was transformed with a control plasmid lacking the Synechocystis ilvC gene or with an extract of an E. coli IlvC+ control strain.

IS2 activates the ilvA gene of Pseudomonas cepacia in Escherichia coli

The ilvA gene of Pseudomonas cepacia was expressed poorly in Escherichia coli. Insertion of IS2 upstream of the cloned gene dramatically increased its transcription, resulting in an 85-fold increase in threonine dehydratase (deaminase) specific activity. The results confirm earlier reports that IS2 promotes efficient expression of foreign genes in E. coli.

The ILV5 gene of Saccharomyces cerevisiae is highly expressed

The nucleotide sequence of the yeast ILV5 gene, which codes for the branched-chain amino acid biosynthesis enzyme acetohydroxyacid reductoisomerase, has been determined. The ILV5 coding region is 1,185 nucleotides, corresponding to a polypeptide with a molecular weight of 44,280. Transcription of the ILV5 mRNA initiates at position -81 upstream from the ATG translation start codon and terminates between 218 and 222 bases downstream from the stop codon. Consensus sequences have been identified for initiation and termination of transcription, and for general control of amino acid biosynthesis, as well as repression by leucine. The ILV5 gene is regulated slightly by general amino acid control. Codon usage of the ILV5 gene has the strong bias observed in yeast genes that are highly expressed. In agreement with this, the reductoisomerase monomer, with an apparent molecular weight of 40,000, has been identified in an SDS polyacrylamide gel pattern of total soluble yeast proteins as a gene dosage dependent band.

DNA replication termination in Escherichia coli parB (a dnaG allele), parA, and gyrB mutants affected in DNA distribution

We investigated the Escherichia coli mutants carrying the parB, parA, and gyrB mutations, all of which display faulty chromosome partitioning at the nonpermissive temperature, to see whether their phenotype reflected a defect in the termination of DNA replication. In the parB strain DNA synthesis slowed down at 42 degrees C and the SOS response was induced, whereas in the parA strain DNA synthesis continued normally for 120 min and there was no SOS induction. To see whether replication forks accumulated in the vicinity of terC at the nonpermissive temperature, the mutants were incubated for 60 min at 42 degrees C and then returned to low temperature and pulse-labeled with [3H]thymidine. In all cases the restriction pattern of the labeled DNA was incompatible with that of the terC region, suggesting that replication termination was normal. In the parA mutant no DNA sequences were preferentially labeled, whereas in the parB and gyrB strains there was specific labeling of sequences whose restriction pattern resembled that of oriC. In the case of parB this was confirmed by DNA-DNA hybridization with appropriate probes. This test further revealed that the parB mutant over initiates at oriC after the return to the permissive temperature. Like dna(Ts) strains, the parB mutant formed filaments at 42 degrees C in the absence of SOS-associated division inhibition, accompanied by the appearance of anucleate cells of nearly normal size (28% of the population after 3 h), as revealed by autoradiography. The DNA in the filaments was either centrally located or distributed throughout. The parB mutation lies at 67 min, and the ParB- phenotype is corrected by a cloned dnaG gene or by a plasmid primase, strongly suggesting that parB is an allele of dnaG, the structural gene of the E. coli primase. It is thus likely that the parB mutant possesses an altered primase which does not affect replication termination but causes a partial defect in replication initiation and elongation and in chromosome distribution.

Examination of the internal promoter, PE, in the ilvGMEDA operon of E. coli K-12

The ilvGMEDA operon of Escherichia coli K-12 contains an internal promoter, PE, in the distal portion of the ilvM gene immediately upstream from the ilvE gene. The location of this promoter was determined using S1 nuclease protection analyses of in vivo and in vitro transcripts. The transcriptional activity of the internal promoter was compared to the transcriptional activity of the operon-proximal promoter P1P2 using transcriptional fusion vectors and plasmid copy number determinations. These measurements showed that the P1P2 promoter is 52-fold stronger than the internal PE promoter. Estimates of the transcriptional role of the internal promoter on ilvE gene expression during growth conditions in excess and limiting branch chain amino acids is presented.

Regulation of Salmonella typhimurium ilvYC genes

The Salmonella typhimurium LT2 ilvYC genes were studied by fusion of each gene to the Escherichia coli K-12 galK gene. The expression of ilvY and ilvC could then be determined by measurement of the galK-encoded galactokinase enzyme. The promoter for ilvC, pC, was located by this technique to a 0.42-kilobase BglII-EcoRI fragment of the S. typhimurium ilvGEDAYC gene cluster. This sequence was completely sufficient for alpha-acetohydroxyacid-inducible galK expression. The ilvY gene was located within a 1.0-kilobase XhoI-SalI fragment. ilvY gene expression was constitutive with respect to ilv-specific control signals. The ilvY gene was transcribed in the same direction as the other two transcriptional units in the ilvGEDAYC gene cluster, ilvGEDA and ilvC. Transcription of the ilvC gene was completely dependent upon the activity of its own promoter, pC, and independent from transcription of the ilvY gene. The role of the intervening region between ilvY and ilvC in regulation of ilvC expression was explored.

Formation of type II F-primes from unstable Hfrs and their recA-independent conversion to other F-prime types

Four E. coli Hfr strains, representing stable (Hfr Cavalli), moderately stable (AB312) and unstable (Ra-1, Ra-2) Hfr states, were used in the isolation of a series of F' plasmids. Type II F's were found to be the most prevalent F' plasmid formed from all of the Hfrs, while the percentages of delta tra F's increased as the stability of the Hfr increased. Two observations suggested that F' formation in unstable Hfrs like Ra-2 may proceed through a type II F' precursor. First, the major F' products of Ra-2 are tra+ type II F's and, second, other F' types (I, II) and classes (tra+, delta tra) from Ra-2 appeared to be deletion derivatives of a larger F' progenitor. By monitoring the molecular changes that occur when the Ra-2 derived type II F' pWS200 is transferred from one recA host to another, we have found that all F' types and classes can be generated from pWS200 in a recA-independent manner. F sequences involved in the genetic conversions of pWS200 include the oriT locus and the directly repeated gamma delta junctions of F and chromosomal DNA. A model for the formation of F's in unstable Hfrs is postulated in which a tra+ type II F' primary excision product is seen to be modified, through recA-independent processes, to other F' types and classes. This model differs from the current model of F' formation in that independent excision events from the Hfr chromosome are not seen as the source of type I and type II F's. These studies have also shown that the formation of delta tra F's is a recA-independent process that can occur from the F' and Hfr states, that gamma delta-mediated deletions in pWS200 often demonstrate regional specificity in having endpoints near the ilv operon and that genetic alterations in either replication origin of pWS200 (F oriV, chromosomal oriC) stabilize the replication of this "mini-Hfr" cointegrate.

Physical and genetic localization of ilv regulatory sites in lambda ilv bacteriophages

A set of nine lambda dilv phages were used to transduce bacterial recipients containing point mutations or deletions in the ilv genes located at 84 min on the Escherichia coli K-12 chromosome. This genetic analysis indicated that two phages carry the entire ilvGEDAC cluster; others carry the complete ilvC gene and, in addition, bacterial DNA that extends to a termination point between ilvA and ilvC, within ilvD, within ilvE, or within ilvG. DNA extracted from the lambda dilv phages was digested with EcoRI, HindIII, KpnI, PstI, SalI, and SmaI. The restriction maps revealed that these phages were generated after insertion at four distinct insertion sites downstream (clockwise) of ilvC. The physical relationships between the various phages were further examined by electron microscopic heteroduplex analysis. The physical maps of the phages thus generated were straightforward and in complete accord with the genetic data. No evidence for genetic rearrangements of ilv DNA in the phage was obtained, thus validating conclusions based on the use of these phages in previous and ongoing research projects. Bacterial cells with deletions of the ilv genes were made lysogenic with lambda dilv phage to examine the regulation of ilv genes present in the phage. The results confirm previous studies showing that one site for control by repression and derepression is upstream (counterclockwise) of ilvG. It was shown, in addition, that the activities of dihydroxy acid dehydrase and threonine deaminase were increased when the prototrophic lysogens were grown with 20 mM leucine. Since this increase was exhibited even when the ilvG-linked control region was not carried by the lambda dilv phage, additional control sites must be located within the ilvEDA region of the ilvGEDA transcription unit.

Chromosomal locations of the genes for rRNA in Escherichia coli K-12

Chromosomal locations of the seven rRNA operons in Escherichia coli K-12 were studied by digesting DNA from various merodiploid strains with SalI restriction enzyme followed by Southern gel analysis with 32P-labeled 23S rRNA as a probe. The seven unique SalI DNA fragments revealed in the autoradiograms were first correlated to the seven rRNA operons previously isolated as hybrid plasmids or transducing phages. The chromosomal locations of six (rrnA, B, C, D, E, and G) of the seven isolated operons were confirmed by increased gene dosage demonstrated in autoradiograms after Southern gel analysis of DNA from relevant merodiploid strains. The gene dosage analysis showed that the location of the remaining operon (now called rrnH) is between metD and proA. No evidence was obtained for the presence of rrnF, which was previously reported to map between aroB and malA. The chromosomal location of rrnH was confirmed by P1 transduction in the following way: a DNA fragment adjacent to rrnH was cloned into pBR322; the resulting hybrid plasmid was integrated at the homologous region of the chromosome of a polA mutant; and the ampicillin resistance marker originally carried by pBR322 was then used for mapping of the nearby rrnH by P1 transduction. A close linkage of rrnH to metD (about 60% cotransduction) was observed, and the data were consistent with the order metD-rrnH-proA. Thus, mapping of all seven rRNA operons has been completed. The present study has also determined the orientation of rrnG and rrnH and demonstrated that the direction of transcription of all the rRNA operons is identical to that of DNA replication.

DNA sequence fine-structure analysis of ilvG (IlvG+) mutations of Escherichia coli K-12

Six ilvG (IlvG+) mutations of Escherichia coli K-12 were transferred to recombinant plasmids, and the DNA sequence of each mutation was determined. This analysis confirmed that expression of the ilvG gene product (acetohydroxy acid synthase II) requires the deletion of a single base pair or the addition of two base pairs within ilvG to displace a frameshift site present in wild-type E. coli K-12. This system should be useful in the analysis of potential frameshift mutagens.

Identification of the protein products of the rrnC, ilv, rho region of the Escherichia coli K-12 chromosome

Two methods have been used to identify the protein products of the Escherichia coli K-12 ilv region at 84 min and the flanking rrnC (counterclockwise) and rho (clockwise) loci. First, a set of lambda dilv specialized transducing phages, including some phages that carry rho and others that carry part of rrnC, was used to infect UV irradiated cells. The proteins produced by the infecting lambda dilv phage were selectively labelled with radioactivity amino acids and identified by SDS gel electrophoresis and autoradiography. Second, restriction enzyme fragments were cloned from the lambda dilv phage into pBR322 and the plasmid specific gene products produced in maxicells were similarly identified by SDS gel electrophoresis and autoradiography. The proteins produced were correlated with specific genes and restriction enzyme fragments present in the lambda dilv phage and the pBR322 derivatives. Several ilv gene products that have previously been refractory to protein purification attempts have been identified for the first time by this technique. The presence of mutations at the ilvO site is shown to activate the cryptic ilvG gene and to result in the production of a 62,000 dalton protein. A 15,000 dalton protein of unknown function is synthesized from a DNA segment between ilv and rrnC. The rho gene was cloned from lambda dilv phage into pBR322 and shown to be dominant to a rho mutation on the host chromosome. The rho gene product and four additional proteins coded by genes near or between rho and ilv have been detected.

Molecular cloning and expression of the ilvGEDAY genes from Salmonella typhimurium

The ilvGEDAY genes of Salmonella typhimurium were cloned in Escherichia coli K-12 by in vitro recombination techniques. A single species of recombinant plasmid, designated pDU1, was obtained by selecting for Valr Ampr transformants of strain SK1592. pDU1 was shown to contain a 14-kilobase EcoRI partial digestion product of the S. typhimurium chromosome inserted into the EcoRI site of the pVH2124 cloning vector. The ilvGEDAY genes were found to occupy a maximum length of 7.5 kilobases. Restriction endonuclease analysis of the S. typhimurium ilv gene cluster provided another demonstration of the gene order as well as established the location of ilv Y between ilvA and ilvC. The presence of a ribosomal ribonucleic acid operon on the pDU1 insert, about 3 kilobases from the 5' end of ilvG, was shown by Southern hybridization. The expression of the ilvGEDA operon from pDU1 was found to be elevated, reflecting the increased gene dosage of the multicopy plasmid. A polarity was observed with respect to ilvEDA expression which is discussed in terms of the possible translational effects of the two internal promoter sequences, one located proximal to ilvE and the other located proximal to ilvD.

Genetical and structural analysis of a group of lambda ilv and lambda rho transducing phages

Eight lambda ilv C transducing phages generated from E. coli K12 secondary site lysogens have been analysed genetically and physically. Two of them carry, in addition, the rho gene and its promotor region, but not the cya gene. The ilv O 603 mutation has been located between ilv G and ilv E. Electrophoretic analysis of the proteins synthesized by these phages in a system of UV irradiated cells allowed us to assign molecular weights of 55000 and 66000 daltons to the ilv C and the ilv D gene products, respectively, and to show that an ilv G-encoded polypeptide of 60000 daltons is made from an ilv O- but not from an ilv O+ phage. The expression of the ilv G gene is discussed in the light of the recent finding of a promoter-attenuator region lying upstream to ilv G. Finally, we have found that one of the lambda ilv phages does not have the classical structure of a transducing phage.

In vitro formation of beta-galactosidase with a template containing the lac genes fused to gene ilvD

An in vitro coupled transcription-translation system was used to synthesize transaminase B and beta-galactosidase in the presence of a deoxyribonucleic acid template containing lac deoxyribonucleic acid under normal lac-specific control and in the presence of several deoxyribonucleic acid templates containing lac deoxyribonucleic acid fused to the ilvD gene. Time course experiments revealed that transcription of the lacZ gene from the fusion template required a longer time than did that initiated at the lac promoter. With a phage template containing an intact ilvE gene but lacking the normal ilv-specific promoter, synthesis of ilvE message was completed before synthesis of lacZ message. A phage template that contained the normal ilv-specific promoter but from which part of ilvE had been deleted also allowed formation of beta-galactosidase. Three plasmids containing the ilv-lac fusion were also used as templates. Two plasmids that contained both an intact ilvE gene and the normal ilv-specific promoter required longer times for lacZ transcription but were more efficient templates than was a plasmid in which the ilv-lac fusion, the ilvE gene, and the contiguous non-specific ilvE promoter were inverted with respect to the normal ilv-specific promoter. beta-Galactosidase synthesis was stimulated by guanosine 3'-pyrophosphate-5'-pyrophosphate with all templates tested except that in which the ilv-lac fusion had been inverted. Presumptive evidence was obtained for the generation of a limiting isoleucine signal by incorporating inhibitors of isoleucyl transfer ribonucleic acid synthetase into the coupled transcription-translation system.

Regulation of ilvEDA expression occurs upstream of ilvG in Escherichia coli: additional evidence for an ilvGEDA operon

A low-copy-number plasmid was prepared that contained the entire ilv gene cluster of Escherichia coli. The introduction of an ilvO mutation allowed the ilvG gene of the plasmid to be expressed and imparted valine resistance to strains carrying it. Insertion of Tn10 into the ilvG gene of the plasmid resulted in a strong polar effect on ilv genes E, D, and A. Replacement of a region of ilv deoxyribonucleic acid between two KpnI sites on the high-copy-number plasmid carrying the entire ilv gene cluster with a KpnI fragment carrying an ilv-lac fusion but not extending into the ilv-specific control region resulted in a plasmid expressing the lacZ gene under ilv control when the fusion had been inserted in its normal orientation but not when it had been inserted in the opposite orientation. These experiments indicate that ilv-specific control over ilvE, ilvD, and ilvA expression is dependent on these genes being continguous with deoxyribonucleic acid that lies upstream of ilvG. The results also add further support to the concept of an ilvGEDA operon in E. coli.

Physical location of the ilvO determinant in Escherichia coli K-12 deoxyribonucleic acid

A plasmid carrying the 4,6-kilobase (kb) HindIII-derived fragment from an ilvO mutant derivative of lambda h80dilv imparted a valine-resistant phenotype on strains it carried. This fragment carries a small amount of the promoter-proximal end of ilvE, the ilvO determinant, and apparently the entire ilvG gene, which specifies the valine-insensitive acetohydroxy acid synthase. Comparable deoxyribonucleic acid (DNA) from the original lambda h80dilv did not carry the valine resistance marker. The valine-resistant phenotype was always correlated with the formation of the resistant enzymes. The ilvO determinant was shown to be carried within an approximately 600-based-pair region lying between the SalI and KpnI sites on the HindIII fragment and perhaps within the ilvG gene itself. Ribonucleic acid that hybridizes with the DNA corresponding to the ilvG gene is formed in wild-type K-12 cells. This fact, coupled with the fact that ilvG is transcribed from the same DNA strand as the ilvE, D, and A genes, led to the idea that transcription is normally initiated upstream from ilvG in both wild-type and ilvO strains. In wild-type strains either the formation or the translation of the transcript would be terminated with the ilvG gene, thus preventing expression of that gene.

Location of the multivalent control site for the ilvEDA operon of Escherichia coli

A strain of Escherichia coli K-12 containing a deletion extending from early in the ilvE gene toward the ilvG gene was shown to exhibit a higher expression of the downstream genes, ilvD and ilvA, than did an ilv+ strain. The elevated expression was under apparently normal ilv-specific control, however. The deletion was transferred to the ilv region of lamba h80dilv and shown by restriction endonuclease and heteroduplex analysis to extend through the deoxyribonucleic acid (DNA) shown, in the preceding paper (C. S. Subrahmanyam, G. M. McCorkle, and H. E. Umbarget, J. Bacteriol 142:547--555, 1980), to contain the ilvO determinant. The deletion was also transferred to an ilv-lac fusion strain and shown to cause an increase in beta-galactosidase formation while allowing retention of ilv-specific control. Transducing phages excised from these fusion strains with and without the ilvO determinant were compared. The phage carrying the ilvO+ determinant contained ilv DNA extending only into but not through the ilvG gene. It did not exhibit an ilv-specific control of beta-galactosidase formation. The phage carrying the deletion of ilvO but containing ilv DNA extending beyond the ilvG gene exhibited ilv-specific control of beta-galactosidase formation. It was concluded that the multivalently controlled ilv-specific promoter affecting ilv operon expression lies upstream from ilvG and that the ilvO region in the wild-type K-12 strain is a region of polarity preventing ilvG expression and reducing ilvEDA expression.

Position effect on expression of dsd genes cloned onto multicopy plasmids

In the D-serine deaminase system of Escherichia coli, which is regulated by positive control, we have fouand a complete lack of trans activation in vivo with multicopy dsd hybrid plasmids. A PLASmid carrying the regulatory gene, dsdC+, did not promote expression of chromosomal dsdCO+A+ loci, nor did a chromosomal dsdC+ gene promote expression of plasmid-borne dsdC delta O+A+ (dsd regulatory gene negative) restriction fragments. However, hybrid plasmids that comprise the entire dsd system (dsdC+O+A+) are highly inducible for the enzyme. These dsd hybrid plasmid deoxyribonucleic acids functioned well as templates in the in vitro coupled transcription-translation system. In vitro-synthesized dsdC+ protein promoted expression of the dsdA+ operation efficiently. Exogenously purified dsdC+ protein also activated expression of several dsdC delta O+A+ plasmid deoxyribonucleic acid templates in vitro. An explanation that reconciles these results with previous dominance studies is presented.

The nucleotide sequence preceding and including the beginning of the ilvE gene of the ilvGEDA operon of Escherichia coli K12

The DNA sequence of the 570 base pairs that precedes and includes the beginning of the ilvE gene shows no evidence of a leader-attenuator region. Instead, the sequence shows that the ilvE gene is preceded by another structural gene, presumably the normally cryptic ilvG gene. In vitro transcription of the two plasmids (pLC26-3 and pRL5) containing the regulatory region of the ilvEDA operon results in the formation of two "leader" RNAs. These results are consistent with the suggestion that the ilvEDA operon is regulated by an attenuator mechanism and that the attenuator region lies before the ilvG gene.

Physical characterization of ilv-lac fusions

Electron microscopic heteroduplex analysis and comparative restriction digests have been used to characterize lambda p123(209), the complementary pair of phages used in the Casadaban technique of gene fusion. Derivatives of lambda 1(209) constructed to carry fusions of the lac genes to the control regions of the ilvC and ilvEDA operons were also analyzed. These physical maps have provided confirmation of the genetic models for these constructions and physical specifications important in interpreting the behavior of these ilv-lac fusions.

Transaminase B from Escherichia coli: quaternary structure, amino-terminal sequence, substrate specificity, and absence of a separate valine-alpha-ketoglutarate activity

Transaminase B (branched-chain amino acid aminotransferase, EC 2.6.1.42), the ilvE gene product, was purified to apparent homogeneity from an Escherichia coli K-12 strain which carries the ilvE gene both on the host chromosome and on a plasmid. The oligomeric structure of the enzyme, as determined by analytical ultracentrifugation and sodium dodecyl sulfate-polyacrylamide gel electrophoresis, was confirmed to be that of a hexamer with a molecular weight of about 182,000 and apparently identical subunits. Cross-linking with dimethylsuberimidate yielded trimers, dimers, and monomers, but essentially no species of higher molecular weight. These results are consistent with a double-trimer arrangement of the subunits in native enzyme. The amino-terminal sequence was found to be: Gly Thr Lys Lys Ala Asp Tyr Ile (Trp) Phe Asn Gly (Thr) (Met) Val. Purified transaminase B catalyzed transamination between alpha-ketoglutarate and l-isoleucine, l-leucine, l-valine, and, to a lesser extent, l-phenylalanine and l-tyrosine, the latter reacting very sluggishly. The enzyme was free of aspartate transaminase and of transaminase C. The apparent K(m) values for the branched-chain alpha-ketoacids were smaller than those for the corresponding amino acids. The lowest K(m) was recorded for dl-alpha-keto-beta-methyl-n-valerate, and the highest was recorded for l-valine. The ratio of the valine- and isoleucine-alpha-ketoglutarate activities did not change significantly during purification, and both activities were quantitatively removed from crude extract by antibody raised against purified transaminase B. These observations argue against the existence of a separate valine-alpha-ketoglutarate transaminase. Anti-E. coli transaminase B antibody cross-reacted with crude extract from Salmonella typhimurium, but not with extract obtained from Pseudomonas aeruginosa.

Mapping of ilvO loci of Escherichia coli K-12 with bacteriophage lambda dilv

A set of lambda dilv phage have been used in a deletion mapping procedure to determine the location of two previously characterized ilvO alleles. In contrast to earlier conclusions derived from three-factor crosses and episome-shortening techniques with phage P1, the order found is ilvG-ilvO-ilvEDA. A three-factor cross with phage P1 is described that is not consistent with this location for an ilvO allele. Further analysis of this particular three-factor cross revealed than an artifact attributable to a mutual syntrophism had skewed the apparent frequency of inheritance of the ilvO locus. The role of mutual syntrophism is discussed as a source of mapping errors for the ilvO locus. The value of this set of lambda dilv phage and this mapping procedure for obtaining comparatively unambiguous data on the locations of the ilv structural and regulatory genes is demonstrated.

Deletion mapping of the ilvGOEDAC genes of Escherichia coli K-12

A set of lambdadilv phage has been examined that carry overlapping segments of isoleucine-valine structural and regulatory genes derived from the ilv cluster at 83 min on the Escherichia coli K-12 chromosome. The ilv genes present in these phage, and their order, have been determined by transduction of auxotrophs, escape synthesis, and deletion mapping. The order of ilv genes in the phage, and hence the order in the host chromosome, was found to be ilvG-ilvO-ilvEDA-ilvC. Lysogens containing lambdadilv phage were constructed for dominance analysis of regulatory mutations in the ilvO and ilvA genes. The ilvO671 allele is cis-dominant to ilvO+, while the ilvA538 allele is trans-recessive to ilvA+. Thus, the ilvO gene, that is identified by cis-dominant regulatory mutations that result in increased ilvG and ilvEDA expression, is situated between and may be contiguous with ilvG and ilvEDA.

Map location of the Escherichia coli origin of replication

Working with restriction fragments obtained directly from the Escherichia coli K12 chromosome, the EcoRI-HindIII restriction map of the section of the chromosome containing the replication origin has been extended by 14 kilobase pairs (kb) to cover 56 kb. Within this newly mapped portion, the ilv and rrnC cistrons have been identified by (1) hybridization of individual restriction fragmanents to the ilv-transducing phage lambdadilv5 and (2) a comparison of the restriction map of this region with the EcoRI map of lambda dilv5 and the Hind III map of the plasmid pJC110, a ColEl-ilv hybrid. The replication origin is located approximately 30 kb from the ilvE gene and 20 kb from the rrnC 16S rRNA cistron. This places the origin near 82.7 min on the genetic map, close to uncA.

Suppressors of a genetic regulatory mutation affecting isoleucine-valine biosynthesis in Escherichia coli K-12

Escherichia coli K-12 mutant PS187 carries a mutation, ilvA538, in the structural gene for the biosynthetic L-threonine deaminase that leads to a leucine-sensitive growth phenotype, an isoleucine- and leucine-hypersensitive L-threonine deaminase, and pleiotropic effects resulting in abnormally low and invariant expression of some of the isoleucine-valine biosynthetic enzymes. Fifty-eight derivatives of strain PS187 were isolated as resistant to growth inhibition by leucine, by valine, or by valine plus glycly-valine and were biochemically, genetically, and physiologically characterized. All of these derivatives produced the feedback-hypersensitive L-threonine deaminase, and thus presumably possess the ilvA538 allele of the parent strain. Elevated synthesis of L-threonine deaminase was observed in 41 of the 58 isolates. Among 18 strains analyzed genetically, only those with mutations linked to the ilv gene clusters at 83 min produced elevated levels of L-threonine deaminase. One of the strains, MSR91, isolated as resistant to valine plus glycyl-valine, was chosen for more detailed study. The locus in strain MSR91 conferring resistance was located in four factor crosses between ilvE and rbs, and is in or near the ilvO gene postulated to be a site controlling the expression of the ilvEDA genes. Synthesis of the ilvEDA gene products in strain MSR91 is constitutive and derepressed approximately 200-fold relative to the parent strain, indicating that the genetic regulatory effects of the ilvA538 allele have been suppressed. Strain MSR91 should be suitable for use in purification of the ilvA538 gene product, since enzyme synthesis is fully derepressed and the suppressor mutation is clearly not located within the ilvA gene.