l-ARABINOSE NEGATIVE MUTANTS OF THE l-RIBULOKINASE STRUCTURAL GENE AFFECTING THE LEVELS OF l-ARABINOSE ISOMERASE IN ESCHERICHIA COLI

Global identification of genes affecting iron-sulfur cluster biogenesis and iron homeostasis

Iron-sulfur (Fe-S) clusters are ubiquitous cofactors that are crucial for many physiological processes in all organisms. In Escherichia coli, assembly of Fe-S clusters depends on the activity of the iron-sulfur cluster (ISC) assembly and sulfur mobilization (SUF) apparatus. However, the underlying molecular mechanisms and the mechanisms that control Fe-S cluster biogenesis and iron homeostasis are still poorly defined. In this study, we performed a global screen to identify the factors affecting Fe-S cluster biogenesis and iron homeostasis using the Keio collection, which is a library of 3,815 single-gene E. coli knockout mutants. The approach was based on radiolabeling of the cells with [2-(14)C]dihydrouracil, which entirely depends on the activity of an Fe-S enzyme, dihydropyrimidine dehydrogenase. We identified 49 genes affecting Fe-S cluster biogenesis and/or iron homeostasis, including 23 genes important only under microaerobic/anaerobic conditions. This study defines key proteins associated with Fe-S cluster biogenesis and iron homeostasis, which will aid further understanding of the cellular mechanisms that coordinate the processes. In addition, we applied the [2-(14)C]dihydrouracil-labeling method to analyze the role of amino acid residues of an Fe-S cluster assembly scaffold (IscU) as a model of the Fe-S cluster assembly apparatus. The analysis showed that Cys37, Cys63, His105, and Cys106 are essential for the function of IscU in vivo, demonstrating the potential of the method to investigate in vivo function of proteins involved in Fe-S cluster assembly.

Characterization of the mmsAB-araD1 (gguABC) genes of Agrobacterium tumefaciens

The chvE-gguABC operon plays a critical role in both virulence and sugar utilization through the activities of the periplasmic ChvE protein, which binds to a variety of sugars. The roles of the GguA, GguB, and GguC are not known. While GguA and GguB are homologous to bacterial ABC transporters, earlier genetic analysis indicated that they were not necessary for utilization of sugars as the sole carbon source. To further examine this issue, in-frame deletions were constructed separately for each of the three genes. Our growth analysis clearly indicated that GguA and GguB play a role in sugar utilization and strongly suggests that GguAB constitute an ABC transporter with a wide range of substrates, including L-arabinose, D-fucose, D-galactose, D-glucose, and D-xylose. Site-directed mutagenesis showed that a Walker A motif was vital to the function of GguA. We therefore propose renaming gguAB as mmsAB, for multiple monosaccharide transport. A gguC deletion affected growth only on L-arabinose medium, suggesting that gguC encodes an enzyme specific to L-arabinose metabolism, and this gene was renamed araD1. Results from bioinformatics and experimental analyses indicate that Agrobacterium tumefaciens uses a pathway involving nonphosphorylated intermediates to catabolize L-arabinose via an L-arabinose dehydrogenase, AraA(At), encoded at the Atu1113 locus.

A proposed role for the Azotobacter vinelandii NfuA protein as an intermediate iron-sulfur cluster carrier

Iron-sulfur clusters ([Fe-S] clusters) are assembled on molecular scaffolds and subsequently used for maturation of proteins that require [Fe-S] clusters for their functions. Previous studies have shown that Azotobacter vinelandii produces at least two [Fe-S] cluster assembly scaffolds: NifU, required for the maturation of nitrogenase, and IscU, required for the general maturation of other [Fe-S] proteins. A. vinelandii also encodes a protein designated NfuA, which shares amino acid sequence similarity with the C-terminal region of NifU. The activity of aconitase, a [4Fe-4S] cluster-containing enzyme, is markedly diminished in a strain containing an inactivated nfuA gene. This inactivation also results in a null-growth phenotype when the strain is cultivated under elevated oxygen concentrations. NifU has a limited ability to serve the function of NfuA, as its expression at high levels corrects the defect of the nfuA-disrupted strain. Spectroscopic and analytical studies indicate that one [4Fe-4S] cluster can be assembled in vitro within a dimeric form of NfuA. The resultant [4Fe-4S] cluster-loaded form of NfuA is competent for rapid in vitro activation of apo-aconitase. Based on these results a model is proposed where NfuA could represent a class of intermediate [Fe-S] cluster carriers involved in [Fe-S] protein maturation.

Controlled expression of nif and isc iron-sulfur protein maturation components reveals target specificity and limited functional replacement between the two systems

The nitrogen-fixing organism Azotobacter vinelandii contains at least two systems that catalyze formation of [Fe-S] clusters. One of these systems is encoded by nif genes, whose products supply [Fe-S] clusters required for maturation of nitrogenase. The other system is encoded by isc genes, whose products are required for maturation of [Fe-S] proteins that participate in general metabolic processes. The two systems are similar in that they include an enzyme for the mobilization of sulfur (NifS or IscS) and an assembly scaffold (NifU or IscU) upon which [Fe-S] clusters are formed. Normal cellular levels of the Nif system, which supplies [Fe-S] clusters for the maturation of nitrogenase, cannot also supply [Fe-S] clusters for the maturation of other cellular [Fe-S] proteins. Conversely, when produced at the normal physiological levels, the Isc system cannot supply [Fe-S] clusters for the maturation of nitrogenase. In the present work we found that such target specificity for IscU can be overcome by elevated production of NifU. We also found that NifU, when expressed at normal levels, is able to partially replace the function of IscU if cells are cultured under low-oxygen-availability conditions. In contrast to the situation with IscU, we could not establish conditions in which the function of IscS could be replaced by NifS. We also found that elevated expression of the Isc components, as a result of deletion of the regulatory iscR gene, improved the capacity for nitrogen-fixing growth of strains deficient in either NifU or NifS.

Controlled expression and functional analysis of iron-sulfur cluster biosynthetic components within Azotobacter vinelandii

A system for the controlled expression of genes in Azotobacter vinelandii by using genomic fusions to the sucrose catabolic regulon was developed. This system was used for the functional analysis of the A. vinelandii isc genes, whose products are involved in the maturation of [Fe-S] proteins. For this analysis, the scrX gene, contained within the sucrose catabolic regulon, was replaced by the contiguous A. vinelandii iscS, iscU, iscA, hscB, hscA, fdx, and iscX genes, resulting in duplicate genomic copies of these genes: one whose expression is directed by the normal isc regulatory elements (Pisc) and the other whose expression is directed by the scrX promoter (PscrX). Functional analysis of [Fe-S] protein maturation components was achieved by placing a mutation within a particular Pisc-controlled gene with subsequent repression of the corresponding PscrX-controlled component by growth on glucose as the carbon source. This experimental strategy was used to show that IscS, IscU, HscBA, and Fdx are essential in A. vinelandii and that their depletion results in a deficiency in the maturation of aconitase, an enzyme that requires a [4Fe-4S] cluster for its catalytic activity. Depletion of IscA results in a null growth phenotype only when cells are cultured under conditions of elevated oxygen, marking the first null phenotype associated with the loss of a bacterial IscA-type protein. Furthermore, the null growth phenotype of cells depleted of HscBA could be partially reversed by culturing cells under conditions of low oxygen. Conserved amino acid residues within IscS, IscU, and IscA that are essential for their respective functions and/or whose replacement results in a partial or complete dominant-negative growth phenotype were also identified using this system.

Repair of oxidized iron-sulfur clusters in Escherichia coli

The [4Fe-4S]2+ clusters of dehydratases are rapidly damaged by univalent oxidants, including hydrogen peroxide, superoxide, and peroxynitrite. The loss of an electron destabilizes the cluster, causing it to release its catalytic iron atom and converting the cluster initially to an inactive [3Fe-4S]1+ form. Continued exposure to oxidants in vitro leads to further iron release. Experiments have shown that these clusters are repaired in vivo. We sought to determine whether repair is mediated by either the Isc or Suf cluster-assembly systems that have been identified in Escherichia coli. We found that all the proteins encoded by the isc operon were critical for de novo assembly, but most of these were unnecessary for cluster repair. IscS, a cysteine desulfurase, appeared to be an exception: although iscS mutants repaired damaged clusters, they did so substantially more slowly than did wild-type cells. Because sulfur mobilization should be required only if clusters degrade beyond the [3Fe-4S]1+ state, we used whole cell EPR to visualize the fate of oxidized enzymes in vivo. Fumarase A was overproduced. Brief exposure of cells to hydrogen peroxide resulted in the appearance of the characteristic [3Fe-4S]1+ signal of the oxidized enzyme. When hydrogen peroxide was then scavenged, the enzyme activity reappeared within minutes, in concert with the disappearance of the EPR signal. Thus it is unclear why IscS is required for efficient repair. The iscS mutants grew poorly, allowing the possibility that metabolic defects indirectly slow the repair process. Our data did indicate that damaged clusters decompose beyond the [3Fe-4S]1+ state in vivo when stress is prolonged. Under the conditions of our experiments, mutants that lacked other repair candidates--Suf proteins, glutathione, and NADPH: ferredoxin reductase--all repaired clusters at normal rates. We conclude that the mechanism of cluster repair is distinct from that of de novo assembly and that this is true because mild oxidative stress does not degrade clusters in vivo to the point of presenting an apoenzyme to the de novo cluster-assembly systems.

The cysteine desulfurase, IscS, has a major role in in vivo Fe-S cluster formation in Escherichia coli

The cysteine desulfurase, IscS, provides sulfur for Fe-S cluster synthesis in vitro, but a role for IscS in in vivo Fe-S cluster formation has yet to be established. To study the in vivo function of IscS in Escherichia coli, a strain lacking IscS was constructed and characterized. Using this iscS deletion strain, we have observed decreased specific activities for proteins containing [4Fe-4S] clusters from soluble (aconitase B, 6-phosphogluconate dehydratase, glutamate synthase, fumarase A, and FNR) and membrane-bound proteins (NADH dehydrogenase I and succinate dehydrogenase). A specific role for IscS in in vivo Fe-S cluster assembly was demonstrated by showing that an Fe-S cluster independent mutant of FNR is unaffected by the lack of IscS. These data support the conclusion that, via its cysteine desulfurase activity, IscS provides the sulfur that subsequently becomes incorporated during in vivo Fe-S cluster synthesis. We also have characterized a growth phenotype associated with the loss of IscS. Under aerobic conditions the deletion of IscS caused an auxotrophy for thiamine and nicotinic acid, whereas under anaerobic conditions, only nicotinic acid was required. The lack of IscS also had a general effect on the growth of E. coli because the iscS deletion strain grew at half the rate of wild type in many types of media even when the auxotrophies were satisfied.

The amino terminus of bacteriophage lambda integrase is involved in protein-protein interactions during recombination

Bacteriophage lambda integrase (Int) catalyzes at least four site-specific recombination pathways between pairs of attachment (att) sites. Protein-protein contacts between monomers of Int are presumed to be important for these site-specific recombination events for several reasons: Int binds to the att sites cooperatively, catalytic Int mutants can complement each other for strand cleavage, and crystal structures for two other recombinases in the Int family (Cre from phage P1 and Int from Haemophilus influenzae phage HP1) show extensive protein-protein contacts between monomers. We have begun to investigate interactions between Int monomers by three approaches. First, using a genetic assay, we show that regions of protein-protein interactions occur throughout Int, including in the amino-terminal domain. This domain was previously thought to be important only for high-affinity protein-DNA interactions. Second, we have found that an amino-terminal His tag reduces cooperative binding to DNA. This disruption in cooperativity decreases the stable interaction of Int with core sites, where catalysis occurs. Third, using protein-protein cross-linking to investigate the multimerization of Int during recombination, we show that Int predominantly forms dimers, trimers, and tetramers. Moreover, we show that the cysteine at position 25 is present at or near the interface between monomers that is involved in the formation of dimers and tetramers. Our evidence indicates that the amino-terminal domain of Int is involved in protein-protein interactions that are likely to be important for recombination.

Construction and properties of aconitase mutants of Escherichia coli

Escherichia coli contains two genes (acnA and acnB) encoding aconitase activities. An acnB mutant was engineered by replacing the chromosomal acnB gene by an internally deleted derivative containing a tetR cassette. An acnB double mutant was then made by transducing a previously constructed acnA::kanR mutation into the acnB::tetR strain. Western blotting confirmed that the AcnA and AcnB proteins were no longer produced by the corresponding mutants and PCR analysis showed that the chromosomal acnB gene had been replaced by the disrupted gene. Aerobic and anaerobic growth in glucose minimal medium were impaired but not abolished by the acnB mutation, indicating that the lesion is partially complemented by the acnA+ gene, and growth was enhanced by glutamate. The acnAB double mutant would not grow on unsupplemented glucose minimal medium and although it responded to glutamate like a typical auxotroph under anaerobic conditions, under aerobic conditions no response to glutamate was observed before it was over-grown by 'revertants' lacking citrate synthase (acnAB gltA). The acnAB double mutant retained a low but significant aconitase activity (< or = 5% of wild-type), designated AcnC. Enzymological and regulatory studies with acn-lacZ fusions indicated that AcnB is the major aconitase, which is synthesized earlier in the growth cycle than AcnA, and subject to catabolite and anaerobic repression.

Isolation and characterization of Escherichia coli mutants able to utilize the novel pentose L-ribose

Wild-type strains of Escherichia coli were unable to utilize L-ribose for growth. However, L-ribose-positive mutants could be isolated from strains of E. coli K-12 which contained a ribitol operon. L-ribose-positive strains of E. coli, isolated after 15 to 20 days, had a growth rate of 0.22 generation per h on L-ribose. Growth on L-ribose was found to induce the enzymes of the L-arabinose and ribitol pathways, but only ribitol-negative mutants derived from strains originally L-ribose positive lost the ability to grow on L-ribose, showing that a functional ribitol pathway was required. One of the mutations permitting growth on L-ribose enabled the mutants to produce constitutively an NADPH-linked reductase which converted L-ribose to ribitol. L-ribose is not metabolized by an isomerization to L-ribulose, as would be predicted on the basis of other pentose pathways in enteric bacteria. Instead, L-ribose was metabolized by the reduction of L-ribose to ribitol, followed by the conversion to D-ribulose by enzymes of the ribitol pathway.

Arabinose-induced binding of AraC protein to araI2 activates the araBAD operon promoter

The state of Escherichia coli araI DNA occupancy by AraC protein has been found to change from a two-turn to a four-turn occupancy upon the addition of the inducer arabinose. The araI site is separable into two contiguous regions, araI1 and araI2. araI1 binds both ligand-bound and ligand-free AraC protein, whereas araI2 binds AraC protein in the presence of arabinose only. A mutation in araI and a known mutation in araC led to the loss of araI2 binding, while binding to araI1 was unaffected. Both mutants failed to activate the promoter of the araBAD operon. We propose that araI2 occupancy by AraC protein leads to RNA polymerase recognition of the araBAD promoter and that araI1 acts as a switch mechanism allowing both the repressor and the activator forms of AraC protein to regulate the araBAD promoter.

Repression and catabolite gene activation in the araBAD operon

Catabolite gene activation of the araBAD operon was examined by using catabolite gene activator protein (CAP) site deletion mutants. A high-affinity CAP-binding site between the divergently orientated araBAD and araC operons has been previously identified by DNase I footprinting techniques. Subsequent experiments disagreed as to whether this site is directly involved in stimulating araBAD expression. In this paper, we present data showing that deletions generated by in vitro mutagenesis of the CAP site led to a five- to sixfold reduction in single-copy araBAD promoter activity in vivo. We concluded that catabolite gene activation of araBAD involves this CAP site. The hypothesis that CAP stimulates the araBAD promoter primarily by relieving repression was then tested. The upstream operator araO2 was required for repression, but we observed that the magnitude of CAP stimulation was unaffected by the presence or absence of araO2. We concluded that CAP plays no role in relieving repression. Other experiments showed that when CAP binds it induces a bend in the ara DNA; similar bending has been reported upon CAP binding to lac DNA. This conformational change in the DNA may be essential to the mechanism of CAP activation.

Benzyl derivative facilitation of transcription in Escherichia coli at the ara and lac operon promoters: metabolite gene regulation (MGR)

A number of benzyl derivatives have been tested for their ability to induce the expression of the araBAD operon in an Escherichia coli K-12 strain. Those derivatives shown to be stimulatory include: benzoic acid (BA), para-amino benzoic acid (PABA), para-hydroxy benzoic acid (PHBA), ortho-amino benzoic acid (OABA), 3-hydroxy-4-methoxy phenylethylamine (MTA), and 4-hydroxy-3-methoxyphenol acetic acid (HVA). The araC gene product was necessary to facilitate the induction. To further characterize if the inductive effect was mediated at the level of transcription, an araBAD-tetracycline resistant (Tcr) operon fusion plasmid (pAP-B) was employed. Benzyl derivatives which induce expression of the araBAD operon in situ also induced a Tcr phenotype with pAP-B. Both indole acetic acid (IAA) and imidazole (IM), which were previously shown to circumvent the necessity for cAMP in the induction of the araBAD operon, also induced a Tcr phenotype with pAP-B. Induction of lac or other cAMP responding operons with the inducing molecules at the chromosomal level was not detectable when assessed by carbon utilization. However, a lacZYA-Tcr operon fusion plasmid (pLPI) did respond to IAA and several of the inducing benzyl derivatives. Catabolite repression of chromosomal araBAD expression was reversed when the exogenous concentration of OABA was elevated. Similar effects on the Tcr phenotypes conferred by pAP-B and pLP1 were observed when OABA or several other inducing benzyl derivatives were present exogenously.

Five mutations in the promoter region of the araBAD operon of Escherichia coli B/r

Five mutations that result in reduced expression of the araBAD operon were cloned onto the plasmid pBR322. The position of each mutation was determined by DNA sequence analysis. Three of the mutations were located in the RNA polymerase binding site of the araBAD promoter. The first, ara-1016, was a one-base-pair deletion at position -35; the second, ara-1036, was a transversion at position -13; the third, ara-1027, was a nine-base-pair deletion from +5 to +13. S1 nuclease mapping showed that mutations ara-1016 and ara-1036 greatly reduced transcription and that mutation ara-1027 had little, if any, effect on transcription. Two other mutations resulted from the transposition of the insertion element, IS1, downstream from the transcriptional start site of the operon. Molecular mechanisms for all of the mutations are discussed.

Cyclic adenosine 3',5'-monophosphate-mediated hyperinduction of araBAD and lacZYA expression in a crp mutant of Escherichia coli K-12

A spontaneous lac+ revertant of an adenylate cyclase deletion strain of Escherichia coli K-12 was isolated and characterized. This revertant, designated strain KC20, exhibited a pleiotropic suppression of the adenylate cyclase defect, with the crp locus being the site of the suppressor mutation. Cyclic adenosine 3',5'-monophosphate at an exogenous concentration of 1 mM severely inhibited the growth of strain KC20 in minimal media. Lower concentrations of the cyclic nucleotide elicited less pronounced effects. Studies on araBAD and lacZYA expression showed that cyclic adenosine 3',5'-monophosphate elicited an initial dose-dependent hyperinduction of these systems. Hyperinduction of araBAD, in L-arabinose grown cultures of strain KC20, resulted in accumulation of inhibitory concentrations of methylglyoxal. Hyperinduction of lacZYA in lactose-grown cultures of strain KC20 did not result in any such methylglyoxal production.

Metabolite gene regulation: imidazole and imidazole derivatives which circumvent cyclic adenosine 3',5'-monophosphate in induction of the Escherichia coli L-arabinose operon

Imidazole, histidine, histamine, histidinol phosphate, urocanic acid, or imidazolepropionic acid were shown to induce the L-arabinose operon in the absence of cyclic adenosine 3',5'-monophosphate. Induction was quantitated by measuring the increased differential rate of synthesis of L-arabinose isomerase in Escherichia coli strains which carried a deletion of the adenyl cyclase gene. The crp gene product (cyclic adenosine 3',5'-monophosphate receptor protein) and the araC gene product (P2) were essential for induction of the L-arabinose operon by imidazole and its derivatives. These compounds were unable to circumvent the cyclic adenosine 3',5'-monophosphate in the induction of the lactose or the maltose operons. The L-arabinose regulon was catabolite repressed upon the addition of glucose to a strain carrying an adenyl cyclase deletion growing in the presence of L-arabinose with imidazole. These results demonstrated that several imidazole derivatives may be involved in metabolite gene regulation (23).

A mutagen assay detecting forward mutations in an arabinose-sensitive strain of Salmonella typhimurium

Strain SV3 of Salmonella typhimurium is sensitive to arabinose, that is, unable to grow in a medium containing arabinose plus glycerol as carbon source. Arabinose resistance is the consequence of the mutational inactivation of one of at least three different genes. The selection of arabinose-resistant mutants provides a simple and sensitive assay for the detection of weak mutagens and for refined quantitative studies of strong ones. The assay is not influenced by experimental artifacts derived from physiological or lethal effects or from differences in plating density. Such artifacts are common with other bacterial mutagen assays, including those using strains analogous to SV3. As practical examples, the assay was used with N-methyl-N'-nitro-N-nitrosoguanidine and the fungicide captafol.

Constitutive mutations in the controlling site region of the araBAD operon of Escherichia coli B/r that decrease sensitivity to catabolite repression

Strains of Escherichia coli B/r containing a deletion of the regulatory gene araC are Ara-. Slow-growing revertants of these strains were isolated and designated aralc because they contain a second mutation in a controlling site, aral, that allows for a low level of constitutive expression of the araBAD operon (Englesbert et al., 1969). We mutagenized aralc delta C strains and selected mutants that grow faster in mineral L-arabinose medium. The new mutations, called araXc, map very close to the original aralc mutations and are in the controlling site region between araB and araC. The aralcXc delta C strains have a higher constitutive level of expression of the araBAD operon than the aralc delta C parents. The araXc mutations are cis acting and decrease the araBAD operon's sensitivity to catabolite repression. The araBAD operon is expressed equally well in ara delta C and ara C cya crp backgrounds. The repressor form of ara C protein is able to repress the constitutive synthesis due to the ara Xc allele.

Mutations affecting catabolite repression of the L-arabinose regulon in Escherichia coli B/r

Expression of the L-arabinose regulon in Escherichia coli B/r requires, among other things, cyclic adenosine-3', 5'-monophosphate (cAMP) and the cAMP receptor protein (CRP). Mutants deficient in adenyl cyclase (cya-), the enzyme which synthesizes cAMP, or CRP (crp-) are unable to utilize a variety of carbohydrates, including L-arabinose. Ara+ revertants of a cya-crp- strain were isolated on 0.2% minimal L-arabinose plates, conditions which require the entire ara regulon to be activated in the absence of cAMP and CRP. Evidence from genetic and physiological studies is consistent with placing these mutations in the araC regulatory gene. Deletion mapping with one mutant localized the site within either araO or araC, and complementation tests indicated the mutants acted trans to confer the ability to utilize L-arabinose in a cya-crp- genetic background. Since genetic analysis supports the conclusion, that the mutant sites are in the araC regulatory gene, the mutants were designated araCi, indicating a mutation in the regulatory gene affecting the cAMP-CRP requirement. Physiological analysis of one mutant, araCi1, illustrates the trans-acting nature of the mutation. In a cya-crp- genetic background, araCi1 promoted synthesis of both isomerase, a product of the araBAD operon, and permease, a product of the araE operon. Isomerase and permease levels in araCi1 cya+ crp+ were hyperinducible, and the sensitivity of each to cAMP was altered. Two models are presented that show the possible mutational lesion in the araCi strains.

Suppressor-induced structural changes in a missense L-ribulokinase of Escherichia coli

A suppressor mutation specific for a missense codon in the L-ribulokinase structural gene of the L-arabinose operon of Escherichia coli B/r enhanced L-arabinose utilization by the strain containing the missense codon. Electrophoretic comparisons of the wild-type, missense, and suppressed missense L-ribulokinases indicated that the suppressor changed the structure of the missense kinase, thereby increasing its catalytic activity. Hyperinducibility imposed on an operator-distal gene by the missense codon was not affected by the suppressor mutation.

Requirement of adenosine-3',5'-cyclic monophosphate for L-arabinose isomerase synthesis in Escherichia coli

Adenosine-3', 5'-cyclic monophosphate (cyclic AMP) is essential for the synthesis of l-arabinose isomerase in Escherichia coli. Cyclic AMP appears to be required for the transcription of deoxyribonucleic acid into messenger ribonucleic acid (RNA), since the enzyme synthesis is not observed in induced cells to which cyclic AMP is added after messenger RNA synthesis is arrested by rifampin or after inducer removal.

Hyperinducibility as a result of mutation in structural genes and self-catabolite repression in the ara operon

Mutations in gene araB producing an l-arabinose-negative phenotype cause either an increase (hyperinducible), decrease (polar), or have no effect at all on the inducible rate of expression of the l-arabinose operon. Fourteen araB gene mutants exhibiting such effects were shown to be the result of: nonsense, frameshift, or missense mutations. All missense mutants were hyperinducible, exhibiting approximately a twofold increase in rate of l-arabinose isomerase production. All frameshift and most nonsense mutants exhibited polar effect. One nonsense mutant was hyperinducible. The cis-dominant polar effect of nonsense and frameshift mutants (as compared to induced wild type) were more pronounced in arabinose-utilizing merodiploids and in araBaraC(c) double mutants where inducible and constitutive enzyme levels are respectively determined. On the other hand, in arabinose-utilizing merodiploids, missense mutations no longer exhibited hyperinducibility but displayed a wild-type level of operon expression. Increases in the wild type-inducible rate of expression of the operon were found when growth rate was dependent on the concentration of l-arabinose. Cyclic 3',5'-adenosine monophosphate also stimulated expression of the operon with the wild type in a mineral l-arabinose medium. These observations are explained on the basis that the steady-state expression of the l-arabinose operon OIBAD is dependent on the concentration of (i) l-arabinose, the effector of this system, which stimulates the expression of the operon, and (ii) catabolite repressors, produced from l-arabinose, which dampen the expression of the operon. We have termed the latter phenomenon "self-catabolite" repression.

D-Fucose as a gratuitous inducer of the L-arabinose operon in strains of Escherichia coli B-r mutant in gene araC

d-Fucose, a nonmetabolizable analogue of l-arabinose, prevents growth of Escherichia coli B/r on a mineral salts medium plus l-arabinose by inhibiting induction of the l-arabinose operon. Mutations giving rise to d-fucose resistance map in gene araC and result in constitutive expression of the l-arabinose operon. Most of these mutations also permit d-fucose to serve as a gratuitous inducer. It is concluded that d-fucose-resistant mutants produce an araC gene product with an altered inducer specificity. Addition of l-arabinose to cells induced with the gratuitous inducer, d-fucose, resulted in severe transient repression of operon expression followed by permanent catabolite repression. Transient repression but no permanent catabolite repression was obtained when cells unable to metabolize l-arabinose were employed. It is concluded that transport of l-arabinose alone is sufficient to achieve transient repression of its own operon, but that metabolism of l-arabinose must occur to achieve permanent catabolite repression of the l-arabinose operon. This general effect has been termed "self-catabolite repression."

Nalidixic acd-resistant mutants of Escherichia coli deficient in isocitrate dehydrogenase

icd Mutants of Escherichia coli K-12, selected for their resistance to nalidixic acid, are deficient in isocitrate dehydrogenase.

Control of expression of the L-arabinose operon in temperature-sensitive mutants of gene araC in Escherichia coli B-r

Expression of the l-arabinose operon in Escherichia coli B/r is dependent on the temperature of growth of the araC mutants reported in this paper. Analysis of these temperature-sensitive regulatory mutants indicates that both repressor and activator activities are thermolabile. The simplest model to explain the manner in which the operon is controlled is one suggesting that the regulatory gene, araC, codes for a protein which upon synthesis acts as a repressor molecule and prevents operon function. When inducer is added, the repressor undergoes a conformational shift and becomes an activator which switches on enzyme synthesis, provided the repressor concentration is reduced to a sufficiently low level in the cell. These data lend strong support to the model that both activities are the result of the same gene product.

Polarity in gene araB of the l-Arabinose operon in Escherichia coli B/r

A series of mutations are described which map in the araB gene of the l-arabinose operon and exert a polar effect on gene araA, the structural gene for the l-arabinose isomerase. Ten of the 20 araB point mutants examined exhibited absolute polarity and may represent insertions of genetic material into the araB gene. The remaining 10 point mutants exhibit strong polarity (less than 10% of the normal wild-type inducible level of isomerase) and may represent a class of externally suppressible polar mutations other than amber or ochre. Seven of the 12 araB deletion mutants examined, or 58%, exhibit polarity, suggesting that a shift in the reading frame has been generated in the polycistronic message for the l-arabinose operon. The remaining, presumably in-phase, deletion mutants exhibit hyperinducible levels of isomerase, an effect that is eliminated when an araB(+) gene is introduced in the trans position. The hyperinducibility effect is discussed in terms of a model for self-catabolite repression, originally proposed by Katz and Englesberg.

Arabinose-leucine deletion mutants of Escherichia coli B-r

The control of ara gene expression was studied in mutants of Escherichia coli B/r containing deletions which fused the l-arabinose gene complex with the leucine operon (the normal gene order being araDABIOC...leuDCBAO). Complementation experiments with stable merodiploids showed that expression of ara genes cis to araC-leu deletions was controlled by the trans-acting product of the araC gene. Expression of ara genes cis to araB-leu deletions was under leucine control. These studies confirm the existence of a region between genes araC and araB essential for normal activator controlled expression of the ara structural genes. One deletion was characterized as an araO-leu deletion. Its effect on ara gene expression was unique in that ara genes were susceptible to potential regulation by both l-arabinose and leucine. These experiments suggest that two different species of messenger ribonucleic acid (mRNA) may be produced for the ara-leu region as a result of this deletion. One, under l-arabinose-activator control, is initiated in the l-arabinose region; the other, under leucine control, is initiated in the leucine region. The latter indicates that araI can be transcribed. Whether araI is transcribed in the former instance (mRNA made under activator control) remains to be established.

Selection of a mutant of Escherichia coli which has high mutation rates

A mutation which causes high mutation rates in all other loci tested was induced with nitrosoguanidine and was selected through the ability of the progeny of such mutant cells to mutate to streptomycin resistance at a higher rate than the wild-type cells. This mutation (mut-2) and the Treffers' mutation (mutT1) mapped at approximately the same position to the right of leu. Specificity studies showed that the two mutations differ in rates of mutation produced.

Positive control of enzyme synthesis by gene C in the L-arabinose system

Englesberg, Ellis (University of Pittsburgh, Pittsburgh, Pa.), Joseph Irr, Joseph Power, and Nancy Lee. Positive control of enzyme synthesis by gene C in the l-arabinose system. J. Bacteriol 90:946-957. 1965.-The l-arabinose gene complex consists of genes D, A, B, and C, linked in that order between the markers thr and leu, and an unlinked gene E. Genes D, A, B, and E are the structural genes for three inducible enzymes and permease, respectively. Gene C, with two mutant alleles, C(-) and C(c), is the regulatory gene exhibiting positive and negative control. C(-) mutants are deficient and C(c) mutants are constitutive for all three enzymes and permease. Complementation analysis, employing sexual merozygotes (A(-)C(+) x A(+)C(-)), with six different C(-) mutants, demonstrates that C(-) is recessive to C(+) (positive control). A total of 61 C(c) mutants, isolated as clones resistant to d-fucose inhibition, are linked to the leu ara region of the chromosome, and the 22 C(c) mutants that were analyzed in detail mapped within the C gene among the C(-) mutant sites. C(c) mutants produce various but coordinate levels of the two enzymes measured, and permease. Complementation analysis (A(-)C(c) x A(+)C(-), A(-)C(c) x A(+)C(+)) shows that C(c) is dominant to C(-) (positive control) and recessive to C(+) (negative control). Deletion mutants that extend into the C gene are l-arabinose permease-negative, thus supporting the positive regulatory role of the C gene. The name "activator gene" is proposed for genes of the C type to accentuate their positive role in gene expression. A working model consistent with these results is presented.