Isolation and characterization of lambdadargECBH transducing phages and heteroduplex analysis of the argECBH cluster

Biosynthesis of Arginine and Polyamines

Early investigations on arginine biosynthesis brought to light basic features of metabolic regulation. The most significant advances of the last 10 to 15 years concern the arginine repressor, its structure and mode of action in both E. coli and Salmonella typhimurium, the sequence analysis of all arg structural genes in E. coli and Salmonella typhimurium, the resulting evolutionary inferences, and the dual regulation of the carAB operon. This review provides an overall picture of the pathways, their interconnections, the regulatory circuits involved, and the resulting interferences between arginine and polyamine biosynthesis. Carbamoylphosphate is a precursor common to arginine and the pyrimidines. In both Escherichia coli and Salmonella enterica serovar Typhimurium, it is produced by a single synthetase, carbamoylphosphate synthetase (CPSase), with glutamine as the physiological amino group donor. This situation contrasts with the existence of separate enzymes specific for arginine and pyrimidine biosynthesis in Bacillus subtilis and fungi. Polyamine biosynthesis has been particularly well studied in E. coli, and the cognate genes have been identified in the Salmonella genome as well, including those involved in transport functions. The review summarizes what is known about the enzymes involved in the arginine pathway of E. coli and S. enterica serovar Typhimurium; homologous genes were identified in both organisms, except argF (encoding a supplementary OTCase), which is lacking in Salmonella. Several examples of putative enzyme recruitment (homologous enzymes performing analogous functions) are also presented.

The murI gene of Escherichia coli is an essential gene that encodes a glutamate racemase activity

The murI gene of Escherichia coli was recently identified on the basis of its ability to complement the only mutant requiring D-glutamic acid for growth that had been described to date: strain WM335 of E. coli B/r (P. Doublet, J. van Heijenoort, and D. Mengin-Lecreulx, J. Bacteriol. 174:5772-5779, 1992). We report experiments of insertional mutagenesis of the murI gene which demonstrate that this gene is essential for the biosynthesis of D-glutamic acid, one of the specific components of cell wall peptidoglycan. A special strategy was used for the construction of strains with a disrupted copy of murI, because of a limited capability of E. coli strains grown in rich medium to internalize D-glutamic acid. The murI gene product was overproduced and identified as a glutamate racemase activity. UDP-N-acetylmuramoyl-L-alanine (UDP-MurNAc-L-Ala), which is the nucleotide substrate of the D-glutamic-acid-adding enzyme (the murD gene product) catalyzing the subsequent step in the pathway for peptidoglycan synthesis, appears to be an effector of the racemase activity.

Detection of minimal amounts of DNA by electron microscopy using simplified spreading procedures

Electron microscopic examination of nucleic acids requires the use of special spreading techniques. The classical method was developed by Kleinschmidt and Zahn in 1959. Modifications of this method increased sensitivity to allow detection of a total amount of about 1 x 10(-3) micrograms of single-stranded DNA and 2 x 10(-5) micrograms of double-stranded DNA. Here we describe two rapid and simple procedures increasing sensitivity by 1-2 orders of magnitude to visualize at least 1 x 10(-5) micrograms of single- and/or double-stranded DNA.

Cloning and characterization of the genes for the two homocysteine transmethylases of Escherichia coli

We have cloned the genes for the two homocysteine transmethylases of Escherichia coli K12. The vitamin B12-independent enzyme is encoded by the metE gene while the metH gene codes for the vitamin B12-requiring enzyme. Overexpression of the gene products and Tn1000 mutagenesis have enabled the metE and metH gene products to be identified as 99 kDa and 130 kDa polypeptides, respectively. The truncated polypeptides generated by Tn1000 insertion were used to determine the direction of transcription of the metE and metH genes. Negative complementation suggests that the MetH enzyme exists as an oligomer. Investigation of the expression of the chromosomal- and plasmid-encoded gene products confirms that metE is subject to negative control by vitamin B12 and methionine, and that metH is under positive control by the cofactor and negative control by methionine. For vitamin B12 and methionine to act as regulatory effectors in metE control, functional metH and metJ genes are required, respectively. The use of stable Tn1000-generated fragments of the metE product as electrophoretic markers for the plasmid-encoded metE gene product demonstrated that the two regulatory proteins involved in negative control of metE are present in excess. Under conditions whereby both forms of negative metE control are non-functional, the metE gene product represented about 90% of the total protein, and cell growth was severely impaired.

Separate regulatory systems for the repression of metE and btuB by vitamin B12 in Escherichia coli

Synthesis of the btuB-encoded outer membrane receptor for vitamin B12 and the metE-encoded homocysteine methyltransferase is repressed by growth of Escherichia coli in the presence of vitamin B12. The regulation by vitamin B12 of the production of beta-galactosidase in strains carrying btuB-lac or metE-lac operon fusions indicated that repression of both genes operates at the transcriptional level. Selection for expression of these fusions under repressive conditions allowed isolation of second-site mutations in which repressibility by vitamin B12 had been lost. Mutations in metH and metF prevented vitamin B12-dependent regulation of metE, but not that of btuB. Mutations in btuB and other genes involved in uptake of the vitamin eliminated or reduced repression. Mutations in the newly identified gene, btuR, controlled the repressibility of btuB, but had no effect on metE regulation. The btuR gene resides at 27.9 min on the genetic map in the gene order cysB-topA-btuR-trp; it acts in a trans-dominant manner and appears to encode a repressor of btuB transcription.

Regulation of expression of the gene for vitamin B12 receptor cloned on a multicopy plasmid in Escherichia coli

The btuB gene of Escherichia coli codes for a protein (BtuB) located in the outer membrane. BtuB is the receptor for vitamin B12 (cyanocobalamin). We have cloned the btuB gene into pUC8 using transposon Tn5 as the marker to first isolate several parts of the relevant DNA fragment from the specialized transducing phage lambda darg13. After reconstitution of the gene, Tn5 was removed by selecting for spontaneous excision. The partial nucleotide sequence and transcriptional start of the btuB gene were determined. The BtuB+ plasmid allowed a large amplification of the synthesis of BtuB, resulting in a 65-fold increased level of vitamin B12 binding. The level of vitamin B12 binding was reduced by a factor of 22 when cells were grown in the presence of high concentrations of vitamin B12. The regulation of the gene was studied in more detail by the use of a protein fusion between the extreme amino-terminus of BtuB and beta-galactosidase of E. coli. The kinetics of repression and derepression were consistent with the presence in the cells of a large amount of a regulatory molecule exhibiting an apparent Km for vitamin B12 of 3 microM.

Structure-function relationship in allosteric aspartate carbamoyltransferase from Escherichia coli. I. Primary structure of a pyrI gene encoding a modified regulatory subunit

In a previous article, we have identified a lambda bacteriophage directing the synthesis of a modified aspartate carbamoyltransferase lacking substrate-co-operative interactions and insensitive to the feedback inhibitor CTP. These abnormal properties were ascribed to a mutation in the gene pyrI encoding the regulatory polypeptide chain of the enzyme. We now report the sequence of the mutated pyrI and show that, during the generation of this pyrBI-bearing phage, six codons from lambda DNA have been substituted for the eight terminal codons of the wild-type gene. A model is presented for the formation of this modified pyrI gene during the integrative recombination of the parental lambda phage with the Escherichia coli chromosome. An accompanying paper emphasizes the importance of the carboxy-terminal end of the regulatory chain for the homotropic and heterotropic interactions of aspartate carbamoyltransferase.

Cloning and expression of the metE gene in Escherichia coli

A lambda-transducing phage was isolated that contains the metE gene. This gene codes for N5-methyl-H4-folate:homocysteine methyltransferase (EC 2.1.1.14), an enzyme that catalyzes the terminal reaction in methionine biosynthesis. A 9.1-kb EcoR1 fragment of this phage, containing the metE gene, was then cloned into pBR325. This plasmid, pJ19, was used to transform Escherichia coli strain 2276, a metE mutant, and restore the MetE+ phenotype. Although the transformed cells produced large amounts of the metE protein in vivo, in vitro studies using pJ19 as template showed low synthesis of the metE protein.

Cloning and expression of the gene for the vitamin B12 receptor protein in the outer membrane of Escherichia coli

The transport of cyanocobalamin (vitamin B12) in cells of Escherichia coli is dependent on a receptor protein (BtuB protein) located in the outer membrane. A 9.1-kilobase pair BamHI fragment carrying the btuB gene was cloned from a specialized transducing phage into multicopy plasmids. Insertions of transposon Tn1000 which prevented production of the receptor localized btuB to a 2-kilobase pair region. Further subcloning allowed isolation of this region as a 2.3-kilobase pair Sau3A fragment. The BtuB+ plasmids were shown by maxicell analysis to encode a polypeptide with a molecular weight of 66,000 in the outer membrane. This polypeptide was missing in cells with Tn1000 insertions in btuB and was reduced in amount upon growth of plasmid-bearing cells in repressing concentrations of vitamin B12. Several Tn1000 insertions outside the 5' end of the coding region exhibited reduced production of receptor. A deletion at the 3' end of btuB resulted in formation of an altered receptor. Amplified production of this polypeptide was associated with increased levels of binding of the receptor's ligands (vitamin B12 and phage BF23), increased rates of vitamin B12 uptake, and altered susceptibility to the group E colicins. Deficiency in various major outer membrane proteins did not affect production of the btuB product, and the amplified levels of this protein partially reversed the tolerance to E colicins seen in these mutants.

Physical organization of the metJB component of the Escherichia coli K-12 metJBLF gene cluster

The structures of a series of plaque-forming metJB transducing phage were studied by restriction endonuclease mapping and enzyme activity assay. One of these phage, lambda pmet100, was inactivated by heat shock in the presence of EDTA, and deletion mutants were selected from the survivors. Two of these mutants, lambda pmet100 delta 1 and lambda pmet100 delta 2, were used to confirm the gene order metJ metB when moving clockwise on the linkage map of Escherichia coli K-12. Additional results indicate that the metB gene can be expressed independently of any other component of the met regulon and that the metJ gene also forms a separate transcription unit.

Turn-on of inactive genes by promoter recruitment in Escherichia coli: inverted repeats resulting in artificial divergent operons

We have characterized two rearrangements consisting of inverted repeats of the argE gene. The promoters (p) of argE and of argCBH face each other over an internal operator. The rearrangements were obtained as reactivations of argE in a strain harboring an argEp deletion on a lambda darg prophage. In both cases the repeat included argE and argCBHp on either side of a unique sequence; the result is a divergent operon in which each copy of argCBHp reads into the adjacent argE repeat. In one case, the pair of repeats adjoins the silent parental gene, forming a triplication (comes from leads to comes from). The other rearrangement consists of a single argE palindrome, but the whole prophage is rearranged into an inverted repeat, analogous to certain lambda dv's. Both structures could be explained by breakage of a replication fork passing argE and by inaccurate rejoining of strands. The lambda dv-like rearrangement would result from breakage at both replication forks of a phage or prophage replicating during transient release of immunity. The triplication would imply breaking of a chromosomal replication fork, formation of a cyclic intermediate by recombination between the daughter duplex molecules and reinsertion into the parental argE gene. Formation of a triplication by replication errors involving appropriate strand switchings and branch migrations can not be excluded however.

IS3 can function as a mobile promoter in E. coli

We had shown previously (3) that the E. coli argE gene could be turned-on by an IS3 element inserted in orientation II near the 5' end of the gene. Here we show that this effect is due to the presence of an outward promoter located on IS3. The exact site of insertion of IS3 was determined by DNA sequencing. Using the S1 nuclease mapping technique with in vivo transcribed RNA we located the promoter responsible for argE transcription on IS3 itself outside the region involved in the inverted repeats of this element. IS3 may therefore be considered as a mobile promoter.

Analysis of btuB receptor function by use of nonsense suppression

Informational suppression of btuB nonsense mutants allows the study of the effect of known, single amino acid substitutions on receptor function. We found that ligand uptake is largely unaffected by such amino acid changes. The few instances in which certain substitutions destroyed sensitivity to the two lethal agents (phage BF23, colicin E3) without affecting vitamin B12 uptake suggest a common region on the btuB receptor involved in the binding of these proteinaceous agents.

Transcription of Regions within the divergent argECBH operon of Escherichia coli: evidence for lack of an attenuation mechanism

Using in vitro and in vivo assays, we could detect no early termination of DNA transcription in the proximal part of the argCBH arm of the argECBH divergent operon. The discrepancy noted previously between the respective amplitudes of variation of messenger and enzyme synthesis must therefore be attributed to other causes than a difference in efficiency between attenuation and repression.

Promoter mapping and selection of operator mutants by using insertion of bacteriophage Mu in the argECBH divergent operon of Escherichia coli K-12

The analysis of a large number of Arg mutants obtained by inserting phage Mu in the argECBH cluster of genes confirmed the "facing" arrangement proposed earlier for the promoters of argE (argEp) and argCBH (argCBHp) and clarified remaining ambiguities regarding the localization of argEp. Casadaban and Cohen's Mu d lac phages (M. Casadaban and S. N. Cohen, Proc. Natl. Acad. Sci. U.S.A. 76:4530-4533, 1979) were used to construct strains where either an intact or a truncated lacZ gene was fused to argC or argB. Several operator-constitutive mutations could be selected for in such strains; the mutations affected both arms of the cluster, thereby defining one common operator region for both directions of transcription.

Structural studies of lambda transducing bacteriophage carrying bacterial deoxyribonucleic acid from the metBJLF region of the Escherichia coli chromosome

The structures of several lambda dmet and related lambda darg transducing phage were studied by restriction fragment mapping and electron microscopic measurements of homoduplexes and heteroduplexes. A new transducing phage (lambda dmet141), in which metF is the only functional gene of the cluster, was isolated. In contrast, lambda dmet117, which expresses the entire metBJLF cluster, has only 3 kilobases more bacterial deoxyribonucleic acid (DNA) than lambda dmet141. An EcoRI restriction fragment of lambda dmet117, which carries the leftmost 6 kilobases of the bacterial DNA insert, was isolated and shown to contain a functional copy of metB. Small structural differences at the attachment sites of some of the phage were shown to result from different sites of lambda integration in the two parent insertion lysogens.

Structure of a naturally occurring plasmid with genes for enterotoxin production and drug resistance

A physical map of the 117-kilobase conjugative plasmid pCG86 was constructed using electron microscope heteroduplex analysis. This plasmid carries the genes elt, for heat-labile enterotoxin, and estA, for heat-stable enterotoxin, as well as the genes for resistance to tetracycline, streptomycin, sulfonamides, and mercury. These genes were mapped using deletions and Tn5 insertions as physical markers. Analysis of a heteroduplex between pCG86 and a previously described enterotoxin plasmid (EntP307) showed a 48-kilobase region of complete homology which included the genes elt and estA. An 8.8-kilobase BamHI fragment of EntP307 carrying elt, cloned by others, was also shown to be completely homologous with pCG86. The position of elt on the fragment was verified, and it was shown to carry estA as well. A 44-kilobase region of pCG86 showed partial homology with the region of EntP307 previously shown to contain conjugal transfer genes. The gene for tetracycline resistance is carried on a stem-loop structure with the dimensions of Tn10, and the genes for the other drug resistance markers are carried on a 14.6-kilobase segment that forms an insertion loop in heteroduplexes with EntP307. These studies suggest that pCG86 arose either by recombination between an enterotoxin plasmid of incompatibility group FI, like EntP307, and a multiple resistance factor of incompatibility group FII, or by transposition into EntP307 of two transposons.

DNA electron microscopy

In recent years DNA electron microscopy has become a tool of increasing interest in the fields of molecular genetics and molecular and cell biology. Together with the development of in vitro recombination and DNA cloning, new electron microscope techniques have been developed with the aim of studying the structural and functional organization of genetic material. The most important methods are based on nucleic acid hybridizations: DNA-DNA hybridization (heteroduplex, D-loop), RNA-DNA hybridization (R-loop), or combinations of both (R-hybrid). They allow both qualitative and quantitative analysis of gene organization, position and extension of homology regions, and characterization of transcription. The reproducibility and resolution of these methods make it possible to map a specific DNA region within 50 to 100 nucleotides. Therefore they have become a prerequisite for determining regions of interest for subsequent nucleotide sequencing. Special methods have been developed also for the analysis of protein-DNA interaction: e.g., direct visualization of specific protein-DNA complexes (enzymes, regulatory proteins), and analysis of structures with higher complexity (chromatin, transcription complexes).

Use of gene cloning to determine polarity of an operon: genes carAB of Escherichia coli

A gene-cloning approach was used to determine the transcription polarity of the carbamoylphosphate operon (carAB) of Escherichia coli. In agreement with the accompanying paper (J. Bacteriol. 143:914-920, 1980), our results lead to the conclusion that carA is the proximal gene of the carAB operon.

Cloning and restriction mapping of the trmA gene coding for transfer ribonucleic acid (5-methyluridine)-methyltransferase in Escherichia coli K-12

A hybrid plasmid from the Clarke and Carbon collection has been isolated. This plasmid carries the trmA gene of E. coli, which is necessary for the formation of 5-methyluridine (m5U,ribothymidine) present in all transfer ribonucleic acid (tRNA) chains of the organism so far sequenced. A restriction map of the argCBH-trmA regions is presented. By using cloning in vitro, the trmA gene was located on a 2.9-kilobase pair deoxyribonucleic acid (DNA) fragment. These results and comparison with lambda dargECBH transducing phages established the gene order: argECBH trmA bfe in the 88-min region of the E. coli chromosomal map. Plasmids carrying this 2.9-kilobase pair DNA fragment overproduce the enzyme tRNA(m5U)methyltransferase (EC 2.1.1.35) 20 to 40 times. When this 2.9-kilobase pair chromosomal DNA fragment was expressed in a minicell system, a polypeptide of a molecular weight of 42,000 was synthesized. This polypeptide was tentatively identified as the tRNA(m5U)methyltransferase. These results support the earlier suggestion that the trmA gene is the structural gene for the tRNA(m5U)methyltransferase.

Clustered arg genes on a BamHI segment of the Escherichia coli chromosome

BamHI cleavage of DNAs from transducing phages gamma darg13 (ppc, argECBH, bfe), gamma darg14 (ppc, argECBH) and gamma darg23 (argECBH) yields three purely gamma DNA segments (and, in one case, a fourth), as well as several Excherichia coli-DNA-containing segments. The length (in kilobases, kb) of the segments, determined by electron microscopy and ararose gel electrophoresis is 4.2, 7.5, 8.4, 6.2, 6.9, and 6.4 kb for gamma darg13; 13.0, 7.5, 4.7, 6.2, 6.9, and 6.4 kb for gamma darg 14: and 5.3, 11.0, 4.7, 6.2, 6.9, and 6.4 kb for gamma darg23. Ordering of the segments (in relation to the gamma genetic map and with the direction from left to right corresponding to the clockwise orientation of the E. coli genetic map and to each of the numerical sequences given) reveals, on 26 kb of bacterial DNA, two cleavage sites defining the 7.5-kb segment obtainable from the DNA of either gamam darg13 or gamma darg14. These and analogous findings with argEC and argCB deletion-bearing strains, together with results from heteroduplex experiments, locate argE, argC, argB, and presumably argH on the 7.5-kb segment.

Tandem and inverted repeats of arginine genes in Escherichia coli: structural and evolutionary considerations

Duplications of arg genes produced in the Rec+ and in the recA genetic backgrounds are shown by heteroduplex analysis to be strictly tandem at the level of resolution of this technique. The formation of these particular rearrangements therefore does not require the inclusion of transposons or other sequences of an appreciable size in their final structure. Duplications of short segments (about 2,000 nucleotides) appear unexpectedly stable when compared with duplications of longer segments (about 10,000 nucleotides). One of the structures analyzed displays two inversely repeated argE genes rearranged into an artificial divergent operon. The bearing of this observation on the origin of bipolar operons, of "mirror-image" map symmetries and on the production of inverted repeats in general, is discussed.

Cloning and endonuclease restriction analysis of argF and of the control region of the argECBH bipolar operon in Escherichia coli

A 1.8 kb DNA fragment, liberated by endonuclease HindIII, contains the control region of the argECBH bipolar operon near one end and the weak secondary promoter of argH at the other extremity; it has been cloned in plasmid pBR322. The same plasmid vector has been used to clone the argF gene liberated from the chromosome by endonuclease BamHI. Restriction patterns for the two hybrid plasmids have been determined, using enzymes AluI, BglI, EcoRI, HaeIII, HincII, HindIII, HpaI and II, PstI and SalI. Two AluI sites situated on either side of and close to a HincII target delineate two short fragments covering the whole of the argECBH control region. The argF control elements are located in a region accessible to further dissection by BamHI, EcoRI, PstI and HindIII. Carriers of the argF plasmid produce extremely high amounts of ornithine carbamoyltransferase, a feature useful for purification of this enzyme.

Studies on the control region of the bipolar argECBH operon of Escherichia coli. I. Effect of regulatory mutations and IS2 insertions

Several mutations affecting the control or the potential of gene expression in the argECBH bipolar operon have been characterized by enzyme assays, genetic mapping, dominance tests and pulse labelled RNA determinations. None of the mutations involves DNA rearrangements detectable by heteroduplex analysis (Charlier et al., 1978). Partially constitutive transcription of both argE and argCBH has been observed in mutant L10 while constitutive argE transcription and normal argCBH control characterize mutants L9, LL13 and LL2. The control region thus appears to contain two overlapping operators, as suggested previously (Elseviers et al., 1972). Two mutants (L2, LL1) and strain 6-8 from Bretscher and Baumberg (1976) display an increase in acetylornithinase specific activity (argE product) without concommittant increased argE transcription. In addition, they exhibit a decreased argCBH transcription. It is suggested that in these organisms, argE translation and argCBH transcription may be affected by the same genetic event; this explanation is compatible with present working hypothesis for the structure of the control region. An interpretation in terms of messenger attenuation also appears possible. From the properties of two strains harbouring an IS2 insertion in the control region (Charlier et al., 1978) the following conclusion may be drawn: 1. When inserted in orientation I close to the proximal end of a silent gene IS2 appears to promote a low but detectable transcription readthrough into that gene. 2. Insertion of an IS2 element in orientation II close to a neighbouring gene is not a sufficient condition to express that gene at a high rate. The properties of the two insertions appear compatible with the structure proposed for the control region.

Adjacent insertion sequences IS2 and IS5 in bacteriophage Mu mutants and an IS5 in a lambda darg bacteriophage

Using electron microscopic heteroduplex analysis, we have demonstrated that an insertion found in a Mu prophage and in some infectious. Mu deletion-substitution mutants derived from it consists of bacterial insertion sequence IS2 linked directly to IS5. Other infectious Mu mutants derived from the same lysogen have only IS5 or a portion of IS2. In addition, we have found that an independent insertion in a transducing phage, lambda 13 dargB2, is IS5. The ends of IS5 are short, inverted duplications of each other. These observations support the notion that the DNA insertion previously designated IS5 on the basis of a single example in lambda KH100 is a bona fide bacterial insertion sequence.

Isolation and characterization of specialized lambda transducing bacteriophage carrying the metBJF methionine gene cluster

Secondary attachment site lysogens of Deltaatt(lambda)Deltappc-argECBH strains of Escherichia coli with lambdacI857 integrated into the bfe gene (88 min) were isolated. Of 20 such lysogens examined, 2 produce lysates with transducing phage containing the metBJF gene cluster (87 min). Reintroduction of the ppc-argECBH chromosome segment (which lies between the bfe and met genes) into these strains virtually abolishes the production of met transducing phage. All of the phage examined have lost essential genes from the left arm of the lambda chromosome. Approximately 85% of the phage appear to have the same genetic composition, containing the metBJF gene cluster, but not the closely linked gene cytR, and having lost phage genes G and J. Analytical CsCl density gradient centrifugation of five representatives of this major class of phage shows four of them to have identical densities (lighter than lambda), while the fifth cannot be resolved from lambda. The four apparently identical phage were isolated from three separate lysates, which suggests the existence of preferred sites for illegitimate recombination on the bacterial and phage chromosomes. Three specialized transducing phage that carry cytR in addition to metB, metJ, and metF have also been studied. Each of these viruses has a different amount of phage deoxyribonucleic acid. Two of them have less deoxyribonucleic acid than lambda, whereas the third has about the same amount. The metB, metF, and cytR genes of the transducing phage have been shown to function in vivo. The phage-borne metB and metF genes are subject to metJ-mediated repression.

Studies on the bipolar argECBH operon of E. coli: characterization of restriction endonuclease fragments obtained from gammadargECBH transducing phages and a ColE1 argECBH plasmid

The isolation of a new type of gamma transducing phage carrying the bipolar argECBH operon of E. coli K12 is described. The argECBH segment is inserted in the phage in a direction which is opposite from that of previously isolated argECBH-carrying phages. A colE1 argECBH plasmid has been constructed. DNA fragments resulting from digestion of these genetic elements with Eco RI and Hind III restriction enzymes have been characterized by agarose gel electrophoresis and electron microscopy, including hetero-duplex analysis. Two fragments are of special significance for studies on the control of arginine synthesis, one of length 9.8 kilobases carrying the whole argECBH region, the other of length 2 kilobases carrying most or all of the control region between argE and argC.

EcoRI cleavage sites in the argECBH region of the Escherichia coli chromosome

The EcoRI cleavage of deoxyribonucleic acids (DNAs) from lambdadarg phages, carrying argECBH, has been examined. The phages are derived from the heat-inducible, lysis-defective strain lambda y199, and their bacterial DNA, including argECBH, is derived from Escherichia coli K-12. Such cleavage of the phage DNAs, in each case, produces the D, E, and F segments of lambda. Additionally, these DNAs yield segments, ordered from left to right, of length (in kilobases [kb]) determined by electron microscopy and 0.7% agarose slab gel electrophoresis as follows: lambdadarg13 (ppc argECBH bfe), 13.9, 2.8, 1.5, and 5.6; lambdadarg14 (ppc argECBH), 3.0, 2.0, 17.3, and 6.2; and lambdadarg23 (argECBH), 18.4 and 6.2. For lambdadarg13 sup102 DNA, the segment analogous to the 13.9-kb segment measures 12.2 kb. The direction from left to right corresponds to the clockwise orientation of the E. coli genetic map. The EcoRI segments define five cleavage sites near the arg region of the E. coli chromosome. For each of the DNAs, the arg genes occur on the largest segment produced. The 17.3-kb segment, being entirely bacterial, represents the argECBH-bearing EcoRI segment of the E. coli chromosome. The location of the arg genes was demonstrated electron microscopically in heteroduplex experiments.

Escherichia coli ornithine carbamolytransferase isoenzymes: evolutionary significance and the isolation of lambdaargF and lambdaargI transducing bacteriophages

Among the Enterobacteriaceae, Escherichia coli K-12 is the only strain known to have two structural genes (argF and argI) for ornithine carbamoyltransferase. The two gene products interact to form a family of four functional isoenzymes, respectively designated FFF, FFI, FII, and III. The FFF and III isoenzymes exhibit nearly identical kinetic parameters in the conditions applied. FFF is more thermolabile than III; this allows the straightforward characterization of new transducing phages carrying either argF or argI. The bearing of the available information regarding ornithine carbamoyltransferase isoenzymes on the evolution of the ancestral E. coli chromosome is reconsidered.

Isolation and heteroduplex mapping of a lambda transducing bacteriophage carrying the structural genes for carbamoylphosphate synthase: regulation of enzyme synthesis in Escherichia coli K-12 lysogens

A N-lambda bacteriophage transducing the structural genes for Escherichia coli K-12 carbamoylphosphate synthase (glutamine) (CPSase; EC 2.7.2.9) has been isolated and analyzed both genetically and physically. The whole int-N region is substituted for a short chromosomal segment corresponding almost exactly to the car locus. The study of CPSase, ornithine carbamoyltransferase, and aspartate carbamoyltransferase regulation in carriers of lambdadcar confirms the previously reported participation of the argR gene product in the control of CPSase synthesis and points to the existence of a regulatory molecule involved in the control of both CPSase and aspartate carbamoyltransferase synthesis. The general usefulness of using N- lambda transducing bacteriophages for the recovery of large amounts of gene products is discussed.