Characterization and modification of a monomeric mutant of the lactose repressor protein

A monomeric mutant lactose repressor protein (T-41), containing serine at position 282 in place of tyrosine [Schmitz, A., Schmeissner, U., Miller, J. H., & Lu, P. (1976) J. Biol. Chem. 251, 3359-3366], has been purified by a series of chromatographic and precipitation methods. The molecular weight of the mutant as determined by gel filtration was approximately 40,000. The inducer equilibrium binding constant for the mutant was comparable to that of the tetrameric wild-type repressor at pH 7.5, whereas operator DNA binding was not detectable. In contrast to wild-type repressor, equilibrium and kinetic rate constants for inducer binding to the monomer were largely independent of pH; thus, the quaternary structure of the wild-type repressor is required for the pH-associated effects on inducer binding. Although ultraviolet absorbance difference spectra indicated that inducer binding to T-41 protein elicited different changes in the environment of aromatic residues from those generated in wild-type repressor, the shift in the fluorescence emission maximum in response to inducer binding was similar for T-41 and wild-type repressors. Similarity in 1-anilinonaphthalene-8-sulfonic acid binding to monomer and tetramer suggests that this fluorophore does not bind at subunit interfaces. Modification of Cys-281 with methyl methanethiosulfonate was observed at low molar ratios of reagent per T-41 monomer (4-fold). This result is in contrast to data observed for tetrameric wild-type repressor which requires high molar ratios for this cysteine to react. We conclude that Cys-281, adjacent to the site of the T-41 mutation, is located on the surface of the monomer in this region crucial for subunit interaction.

A detailed examination of the iodination of beta-galactosidase: stoichiometric inactivation by nonspecific iodination

The incorporation of 125I, using lactoperoxidase, and the subsequent inactivation of beta-galactosidase in the period when incorporation and inactivation were stoichiometric were investigated in detail. The high pressure liquid chromatographic (HPLC) radioactive profiles of the tryptic peptides of samples taken in the stoichiometric period showed that, although two labelled peptides predominated, there were other labelled peptides. The predominating peptides were shown to be the mono- and di-iodinated forms of the peptide containing Tyr-253. This confirmed the result of an earlier study, but quantitation showed that this iodination accounted for only 15-18% of the total. To show that the other labelled peptides in the HPLC profiles were not merely oxidized or partially digested forms of the peptide containing Tyr-253, two experiments were carried out. In one of the experiments, two of the other labelled peptides were isolated and identified as iodinated forms of the peptide containing Tyr-285 (5-7% of the incorporation). In the other experiment, four additional labelled fractions from the HPLC eluate were treated further with trypsin. No further digestion was observed and thus these peptides did not result from incomplete digestion of the sequence containing Tyr-253. Overall, these results show that, although the incorporation of 125I was stoichiometric with inactivation, no single Tyr was responsible for the inactivation as was tentatively suggested previously. The competitive inhibitor isopropyl-beta-D-thiogalactopyranoside (IPTG) was effective in reducing the rates of inactivation of the enzyme and incorporation of 125I, but the same peptides were labelled in the presence of IPTG as in its absence.

In vivo and in vitro synthesis of Escherichia coli maltose-binding protein under regulatory control of the lacUV5 promoter-operator

It has not been possible to obtain in vitro expression of the positively regulated malE gene encoding the periplasmic maltose-binding protein (MBP) of Escherichia coli. To facilitate in vitro malE expression, we constructed plasmids that place the malE gene under transcriptional control of the lacUV5 promoter-operator. These plasmids could be grouped into three classes, based upon their ability to complement in vivo a chromosomal malE deletion in the presence or absence of isopropyl thiogalactoside. In the one class I plasmid analyzed, the lacUV5-malE junction was just 3' to the malE ATG initiation codon, and this plasmid did not complement the malE deletion. Class II and class III plasmids retained various amounts of the malE promoter. MBP synthesis was solely under control of the lacUV5 promoter in the class II plasmids, and MBP synthesis was under control of both the lacUV5 and malE promoters in the class III plasmids. A malE mutation that renders the MBP signal peptide export defective was genetically recombined onto one of the class II plasmids. The in vivo synthesis and export of plasmid-encoded MBP were studied in the presence and absence of isopropyl thiogalactoside and maltose and in a strain harboring a prlA mutation that suppresses the malE signal sequence mutation and is thought to alter the export machinery of cells. In addition, both class II and class III plasmids programmed the synthesis of precursor MBP in an in vitro-coupled transcription-translation system. When precursor MBP was synthesized in vitro in the presence of E. coli membrane vesicles, a significant portion of wild-type precursor MBP, but not export-defective precursor MBP, was converted to a form that migrated on sodium dodecyl sulfate-polyacrylamide gels identically to mature MBP synthesized in vivo.

Initiation of chromosome replication in Escherichia coli after induction of dnaA gene expression from a lac promoter

Escherichia coli HB282 carries a dnaA46(Ts) allele on the chromosome, a wild-type dnaA allele under the control of the lacUV5 promoter on the multicopy plasmid pBC32, and an overproducing lac repressor allele on an F' factor. When the plasmid dnaA gene is repressed, the strain is thermosensitive. After a temporary deficiency in active dnaA protein at nonpermissive temperature, the addition of isopropyl-beta-D-thiogalactopyranoside to the culture was found to produce a burst of initiations within 5 to 10 min at 30% of the origins in 90% of the cells. Initiations then continued at a rate slightly faster than the mass-doubling time such that after 2 h the origin-to-mass ratio of the control culture was restored.

Lactose repressor protein modified with dansyl chloride: activity effects and fluorescence properties

Chemical modification using 5-(dimethylamino)naphthalene-1-sulfonyl chloride (dansyl chloride) has been used to explore the importance of lysine residues involved in the binding activities of the lactose repressor and to introduce a fluorescent probe into the protein. Dansyl chloride modification of lac repressor resulted in loss of operator DNA binding at low molar ratios of reagent/monomer. Loss of nonspecific DNA binding was observed only at higher molar ratios, while isopropyl beta-D-thiogalactoside binding was not affected at any of the reagent levels studied. Lysine residues were the only modified amino acids detected. Protection of lysines-33 and -37 from modification by the presence of nonspecific DNA correlated with maintenance of operator DNA binding activity, and reaction of lysine-37 paralleled operator binding activity loss. Energy transfer between dansyl incorporated in the core region of the repressor protein and tryptophan-201 was observed, with an approximate distance of 23 A calculated between these two moieties.

Effect of iso-propyl-thio-beta-D-galactoside concentration on the level of lac-operon induction in steady state Escherichia coli

In steady state E. coli cells growing at their maximal rate in broth, maximum induction of beta-galactosidase occurs at 0.10 mM isopropyl-thio-beta-D-galactoside (IPTG). Although induction of lac is near zero in steady state cells that are growing in 0.01 mM IPTG, induction at mildly subdued levels persists down to at least 0.001 mM in post-steady state cultures. Meanwhile, thiogalactoside transacetylase remains uninduced over the full range in which the cells are in steady state.

Entry exclusion determinant(s) of IncN plasmid pKM101

pKM101 renders its host a poor recipient in conjugal matings with genetically distinguishable derivatives of itself. The gene(s) primarily responsible for this, denoted eex, is located in between genes required for both conjugal transfer and sensitivity to donor-specific bacteriophage, although it itself is not necessary for transfer. A gene linked to, or coincident with, the region needed for vegetative plasmid replication also inhibited establishment of related plasmids under certain conditions. Construction of an operon fusion between eex and the Escherichia coli lac promoter has shown that this gene is transcribed in a clockwise fashion on the circular map of pKM101. To date, we have not been able to visualize a protein product(s) of the eex gene(s).

Trinitrobenzenesulfonate modification of the lysine residues in lactose repressor protein

Modification of the lysine residues in the lactose repressor protein has been carried out with trinitrobenzenesulfonate. Reaction of lysine residues at positions 33, 37, 108, 290, and 327 was observed. Inducer binding was increased by modification with this reagent, while both nonspecific DNA binding and operator DNA binding were diminished, although to differing degrees. The loss in operator DNA binding capacity was complete with modification of approximately 2 equiv of lysine per monomer. The extent of reaction was affected by the presence of both sugar and DNA ligands; binding activities of the modified protein and reaction pattern of the lysines were perturbed by these ligands. The presence of operator or nonspecific DNA during the reaction protected against specific and nonspecific DNA binding activity loss. This protection presumably occurs by steric restriction of reagent access to lysine residues which are essential for both nonspecific and operator binding interactions. Lysines-33 and -108 were protected from modification in the presence of DNA. These experiments suggest that the charge on the lysine residues is important for protein interaction with DNA and that steric constraints for operator DNA interaction with the protein are more restrictive than for nonspecific DNA binding. In contrast, inducer (isopropyl beta-D-thiogalactoside) presence partially protected lysine-290 from modification while significantly enhancing reaction at lysine-327. Conformational alterations consequent to inducer binding are apparently reflected in these altered lysine reactivities.

The 4.5 S RNA gene of Escherichia coli is essential for cell growth

The Escherichia coli gene coding for the metabolically stable 4.5 S RNA (ffs) has been shown to be required for cell viability. Essentiality was demonstrated by examining the recombination behavior of substitution mutations of ffs generated in vitro. Substitution mutants of ffs are able to replace the chromosomal allele only in the presence of a second, intact copy of ffs. Independent evidence of essentiality and the finding that 4.5 S RNA is important for protein synthetic activity came from characterization of cells dependent on the lac operon inducer isopropyl-beta-D-thiogalactoside for ffs gene expression. Here, a strain dependent on isopropyl-beta-D-thiogalactoside for 4.5 S RNA synthesis was developed by inactivation of the chromosomal ffs allele and lysogenization by a lambda phage containing 4.5 S DNA fused to a hybrid trp-lac promoter. Withdrawal of the thiogalactoside leads to a deficiency in 4.5 S RNA, a dramatic loss in protein synthesis activity, and eventual cell death. Tagging of the chromosomal ffs region with a kanamycin-resistance gene allowed mapping of the 4.5 S RNA gene. Results from this analysis place ffs near lon at approximately ten minutes on the E. coli linkage map.

Cell-division control in Escherichia coli: specific induction of the SOS function SfiA protein is sufficient to block septation

Blocks in DNA replication cause a rapid arrest of cell division in Escherichia coli. We have previously established that the function SfiA (SulA), induced under these conditions as part of the SOS response, is involved in this inhibition of division. To separate the effects of SfiA from those of other SOS functions, we have constructed a plac-sfiA operon fusion, permitting specific induction of SfiA protein by addition of the lac operon inducer isopropyl beta-D-thiogalactopyranoside (IPTG). In lon mutants, in which the unstable SfiA protein has a longer half-life, IPTG caused a rapid arrest of cell division. Under these conditions, there is no concomitant induction of the SOS response. IPTG also caused a rapid arrest of cell division in lon+ strains. These results demonstrate that induction of the SfiA protein is sufficient to cause inhibition of division. Mutations in the sfiB gene can suppress IPTG-induced SfiA-mediated inhibition of division. At higher SfiA concentrations, however, even sfiB mutants cease division; an additional mutation genetically inseparable from sfiB restores normal division. These observations reinforce the hypothesis that the SfiB protein, probably required for cell septation, is the target of action of the SfiA division inhibitor.

Facile and gentle method for quantitative lysis of Escherichia coli and Salmonella typhimurium

Garrett et al. (Mol. Gen. Genet. 182:326-331, 1981) constructed strains of Escherichia coli harboring derivatives of plasmid pBR322 that carry the lysis genes (S, R, and Rz) of phage lambda. The plasmid construction placed the genes under control of the lactose operon operator-promotor (and thus of lac repressor). Induction of E. coli strains carrying these plasmids resulted in rapid lysis of the culture unless the S gene was defective, in which case the cells grew normally. A freeze-thaw treatment of induced cells carrying an S- plasmid gave quantitative lysis of either E. coli or Salmonella typhimurium cells under exceptionally gentle conditions. The method was equally effective on exponential phase cells and stationary phase cells and was readily extended to a large number of independent cultures.

A lacZ-ftsZ gene fusion is an analog of the cell division inhibitor sulA

An in-frame lacZ-ftsZ gene fusion under lac control was fortuitously constructed by subcloning an EcoRI fragment that contains approximately 90% of the ftsZ gene. The identity of the gene fusion was confirmed by isolating an amber mutation in the hybrid gene and then using it to reconstruct the ftsZ gene, which now contained an amber mutation. The hybrid protein (ZZ), which does not possess ftsZ activity, contains seven amino acids of lacZ at its amino terminal end, followed by 35,000 daltons of the carboxyl end of the ftsZ protein. Induction of the hybrid protein resulted in a rapid cessation of cell division which could be reversed by removing the lac inducer. This inhibition of division could be prevented by an increased gene dosage of ftsZ or the presence of the sulB allele of ftsZ, which is known to code for an altered but functional ftsZ protein. An increased gene dosage of ftsZ or the presence of the sulB allele of ftsZ is known to overcome sulA-mediated inhibition of division during the SOS response. Thus, our results suggest that ZZ is an analog of sulA and may aid in determining how sulA inhibits cell division.

Expression of a gene in a 400-base-pair fragment of colicin plasmid ColE2-P9 is sufficient to cause host cell lysis

The colicin E2 immunity (ceiB) and lysis (celB) genes of colicin plasmid ColE2-P9 were cloned as a 900-base-pair insert under the control of the lac promoter in high-copy-number plasmid pUR222. Hosts carrying this plasmid were immune to colicin E2, produced increased amounts of immunity protein (molecular weight, 9,000) and two smaller proteins (molecular weights, 5,000 and 3,000), and lysed when incubated in medium containing isopropyl-beta-D-thiogalactopyranoside (IPTG). A 400-base-pair lacp-distal fragment derived from the insert in this plasmid was recloned in the same orientation into pUR222. Although hosts carrying this plasmid also lysed when grown in the presence of IPTG, they were sensitive to colicin E2 and produced increased amounts of the 5,000- and 3,000-molecular-weight proteins (but not the full-length immunity protein) when treated with IPTG. The results were consistent with the idea that expression of celB (production of the 5,000- and 3,000-molecular-weight proteins) is sufficient to cause host cell lysis in the absence of colicin production and derepression of the host cell SOS system.

A calorimetric investigation of the interaction of the lac repressor with inducer

A calorimetric study has been made of the interaction between the lac repressor and isopropyl-1-thio-beta-D-galactopyranoside (IPTG). The buffer-corrected enthalpy of reaction at 25 degrees C was found to be -15.6, -24.7, -4.6 kJ/mol of bound IPTG at pH 7.0, pH 8.1, and pH 9.0, respectively. This large range of enthalpy values is in contrast to a maximum difference in the free energy of the reaction of only 1.5 kJ/mol of bound IPTG between these pH values. The reaction was found by calorimetric measurements in different buffers to be accompanied by an uptake of 0.29 mol of protons/mol of bound IPTG at pH 8.1. The pH dependency of the reaction enthalpy suggests differences in the extent of protonation of the binding site and the involvement of H bonding with IPTG. The lack of strong hydrophobic contributions in the IPTG binding process is revealed by the absence of any determinable heat capacity change for the reaction at pH 7.0. The presence of phosphate buffer significantly alters the enthalpy of IPTG binding at higher pH values, but has little effect upon the binding constant. This implies that highly negative phosphate species change the nature of the IPTG binding site without any displacement of phosphate upon IPTG binding.

Inactivation of beta-Galactosidase by iodination of tyrosine-253

Beta-Galactosidase is rapidly inactivated by iodination catalyzed by lactoperoxidase but is not inactivated in the presence of the substrate analogue, isopropyl beta-D-thiogalactoside (IPTG). Enzyme activity is lost upon the incorporation of 1 mol of iodine per mol of monomer, without dissociation of the tetrameric structure. Tryptic digests of beta-galactosidase iodinated with 125I in the presence and absence of IPTG were separated by high-performance liquid chromatography and were compared. One fraction was found to be more highly labeled in the digest from the inactivated protein. After isolation of the peptide, amino acid analysis indicated it to be Asp-Tyr-Leu-Arg, residues 252-255. Thus, Tyr-253 is the most reactive tyrosine in beta-galactosidase. This suggests that the conformation of this region of the protein may be altered by binding of IPTG to make Tyr-253 less accessible to iodination. Alternatively, Tyr-253 could be an active-site residue.

Alternative-substrate inhibition and the kinetic mechanism of the beta-galactoside/proton symport of Escherichia coli

The effects of competing alternative substrates on the rate of uptake by galactoside/proton symport were investigated. These experiments produced a decrease in apparent maximum velocity with increased alternative-substrate concentration that cannot be accounted for by a simple ordered mechanism. This, together with non-linearities in the variation of the apparent kinetic constants with alternative-substrate concentration, can be accounted for by a random mechanism for galactoside and proton binding.

Feedback regulation of ribosomal protein gene expression in Escherichia coli

The structural genes for Escherichia coli ribosomal protein (r-protein) genes L1, S4, and S11 were inserted into a plasmid vector containing the lac operator and promoter such that the synthesis of L1, S4, and S11 was controlled by lac regulatory elements. Synthesis of L1, S4, and S11 was stimulated by addition of an inducer of the lac operon (isopropyl thiogalactoside) to exponentially growing cells. Elevated synthesis of L1 caused a specific decrease in L11 synthesis, whereas overproduction of S4 resulted in lowered synthesis of S13 and L17. Stimulation of L1 or S4 synthesis also inhibited cell growth. Overproduction of S11 did not affect synthesis of other r-proteins or alter growth. These results confirm previous in vitro studies [Yates, J. L., Arfsten, A. E. & Nomura, M. (1980) Proc. Natl. Acad. Sci. USA 77, 1837-1841] and support the hypothesis that certain r-proteins have the capacity to selectively inhibit synthesis of r-proteins whose genes are in the same operon as their own.

Inhibition of Clostridium botulinum types A and B hemagglutinins by sugars

Chromatographically isolated hemagglutinins of Clostridium botulinum types A and B are serologically related but not identical. Of the sugars (5, 6, 12, 18 carbons, some derivatives, L and D forms) tested, only D-galactose and some of tis derivatives were inhibitors of these hemagglutinins. O-Nitrophenyl-beta-D-galactopyranoside and isopropyl-beta-D-thiogalactoside were the most potent inhibitors. The two hemagglutinins were bound tightly by p-aminophenyl-beta-D-thiogalactopyranoside coupled to CH-Sepharose 4B. The ligands to which these hemagglutinins bind were determined as the sugars which inhibited the hemagglutinating activity.

Some biochemical effects of 4-deoxy-4-fluoro-D-glucose on Escherichia coli

The uptake of 4-deoxy-4-fluoro-D-glucose (4FG), without subsequent catabolism, by resting cells of Escherichia coli (ATCC 11775) is 0.06 mg/mg dry weight. In frozen-thawed cells of this organism, 4FG is a substrate for the phosphoenolpyruvate phosphotransferase system with a rate of phosphorylation twice that found for the isomeric 3-deoxy-3-fluoro-D-glucose. 4FG is not a carbon source for growth of this organism and it inhibits the extent of growth of cells in the presence of glucose. The inhibition of growth of E. coli K12 on lactose by 4FG is also observed and this is considered to be consistent with the fact that 4FG is an uncompetitive inhibitor of beta-galactosidase (EC 3.2.1.23) activity and that 4FG or 4-deoxy-4-fluoro-D-glucose-6 phosphate repress beta-galactosidase synthesis. These results support the view that catabolite repression may be produced by compounds which are not necessarily metabolised further than hexose-6-phosphates.

The synthesis of beta-galactosidase by constitutive and other regulatory mutants of Escherichia coli in chemostat culture

The synthesis of beta-galactosidase by an E. coli constitutive mutant was examined in a chemostat using glucose-, glycerol-, succinate- or N-limited growth media. Except for glucose-grown bacteria, the steady-state intracellular level of beta-galactosidase was maximal at dilution rates between 0-2 and 0-3 h-1. At higher dilution rates enzyme synthesis was reduced by catabolite repression, which could be relieved by the addition of cyclic AMP. With a catabolite-resistant mutant (UV5c), no decrease in enzyme level at high dilution rates were observed. All mutants examined were constitutive and gave decreased enzyme levels at low dilution rates, with the exception of lac-/F'lac UV5c mutants where the enzyme levels rose at low dilution rates. Hyper-producing mutants were isolated but were unstable. A constitutive mutant growing on glycerol-limited media was considered the most suitable for large-scale production of beta-galactosidase in a chemostat.