Thiamine uptake in Escherichia coli. 3. Regulation of thiamine uptake in Escherichia coli

Thiamine transport in Escherichia coli: the mechanism of inhibition by the sulfhydryl-specific modifier N-ethylmaleimide

Active transport of thiamin (vitamin B(1)) into Escherichia coli occurs through a member of the superfamily of transporters known as ATP-binding cassette (ABC) transporters. Although it was demonstrated that the sulfhydryl-specific modifier N-ethylmaleimide (NEM) inhibited thiamin transport, the exact mechanism of this inhibition is unknown. Therefore, we have carried out a kinetic analysis of thiamin transport to determine the mechanism of inhibition by NEM. Thiamin transport in vivo exhibits Michaelis-Menten kinetics with K(M)=15 nM and V(max)=46 U mg(-1). Treatment of intact E. coli KG33 with saturating NEM exhibited apparent noncompetitive inhibition, decreasing V(max) by approximately 50% without effecting K(M) or the apparent first-order rate constant (k(obsd)). Apparent noncompetitive inhibition is consistent with an irreversible covalent modification of a cysteine(s) that is critical for the transport process. A primary amino acid analysis of the subunits of the thiamin permease combined with our kinetic analysis suggests that inhibition of thiamin transport by NEM is different from other ABC transporters and occurs at the level of protein-protein interactions between the membrane-bound carrier protein and the ATPase subunit.

thiBPQ encodes an ABC transporter required for transport of thiamine and thiamine pyrophosphate in Salmonella typhimurium

In Salmonella typhimurium, thiamine pyrophosphate (TPP) is a required cofactor for several enzymes in central metabolism. Herein we identify a new thi operon, thiBPQ (designated sfuABC in Escherichia coli), required for the transport of thiamine and TPP into the cell. Insertions in the operon result in strains that are phenotypically and biochemically defective in thiamine and TPP transport. Data presented herein show that this operon is transcriptionally repressed in the presence of exogenous thiamine, with TPP the likely regulatory molecule. This work represents the first identification of thiamine transport genes in bacteria and demonstrates the function of a proposed ABC transporter in E. coli.

Synthesis and accumulation of thiamin triphosphate in Escherichia coli cells expressing chicken cytosolic adenylate kinase

To examine whether cytosolic adenylate kinase (AK1, EC 2.7.4.3) catalyzes synthesis of thiamin triphosphate (TTP) in vivo, chicken AK1 was expressed in Escherichia coli, and cellular AK1 activity and TTP content were determined. E. coli harboring the vector plasmid was used as a control. Chicken AK1 was expressed in the producer strain at a high level (83 U/mg protein) even without inducers, and this expression was doubled (153 U/mg protein) by beta-D-isopropylthiogalactopyranoside (IPTG). TTP was accumulated in the producer cells at a high level (5.7 nmol/g dry weight) without IPTG and this was also doubled (10.2 nmol/g dry weight) by IPTG. TTP content in the control strain was very low (0.2-0.9 nmol/g dry weight) and was unaffected by IPTG. Neither bacterial growth curve nor cellular content of AMP, ADP, ATP, thiamin diphosphate or total thiamin (sum of thiamin and its phosphate esters) was different between the producer and the control strains. These results indicate that chicken AK1 expressed in E. coli catalyzed the synthesis and accumulation of TTP within the bacterial cells.

Transport of thiamine and 4-methyl-5-hydroxyethylthiazole by Salmonella typhimurium

The transport of thiamine and 4-methyl-5-hydroxyethylthiazole (MHET), its thiazole moiety, was studied using whole cells of Salmonella typhimurium. It was found that the bacteria possessed an active transport system for thiamine that had Km 0.21 microM and Vmax 33 nmol.min-1.(mg dry wt. cells)-1. Transport of thiamine was glucose dependent, whereas MHET uptake was dependent on both glucose and 2-methyl-4-amino-5-hydroxymethylpyrimidine (MAHMP), the pyrimidine moiety of thiamine. Uptake of both thiamine and MHET was severely curtailed by cyanide, azide, N-ethylmaleimide and carbonyl cyanide m-chlorophenylhydrazone. Oxythiamine inhibited thiamine, but not MHET, uptake and thiamine slightly inhibited MHET uptake. 2-Methyl-4-amino-5-methoxymethylpyrimidine and 4-amino-5-hydroxymethylpyrimidine were unable to replace MAHMP as stimulators of MHET uptake, but 2-methyl-4-amino-5-aminomethylpyrimidine was marginally effective in this regard. Similar results were obtained with attempts to replace MAHMP as a growth requirement for a purD mutant of Salmonella typhimurium. MHET uptake showed saturation kinetics only in the presence of MAHMP, and is not otherwise actively transported.

Transport of 2-methyl-4-amino-5-hydroxymethylpyrimidine by Salmonella typhimurium

The transport of 2-methyl-4-amino-5-hydroxymethylpyrimidine (MAHMP) by Salmonella typhimurium was studied using synthetic [methyl-3H3]MAHMP. It was found that an active transport system existed for MAHMP, having Km of 0.07 microM and Vmax 45 nmol.min-1.(g dry wt. cells)-1, that required glucose as a source of energy and was pH and temperature dependent. Uptake was inhibited by cyanide, azide, N-ethylmaleimide, 2,4-dinitrophenol and carbonyl cyanide m-chlorophenylhydrazone. Uptake was also weakly inhibited by oxythiamine, but not by thiamine, 2-methyl-4-amino-5-aminomethylpyrimidine, or 4-amino-5-hydroxymethylpyrimidine, indicating that the transport system is specific for MAHMP.

Properties of the thiamine transport system in Escherichia coli

Thiamine transport was studied with a mutant (KG1976) of Escherichia coli K-12 deficient in thiamine kinase (EC 2.7.1.89), which catalyzes the formation of thiamine monophosphate from thiamine. Mutant cells accumulated thiamine 390-fold as the free form against a concentration gradient in the absence of added carbon sources at the steady state. Thiamine taken up from the medium, or thiamine preloaded in the absence of glucose, was expelled into the medium when glucose, d-lactate, or succinate was added, whereas exit in the absence of glucose was very slow. The rate of thiamine entry was therefore determined in the absence of glucose, and that of thiamine exit was followed by the addition of glucose to thiamine-preloaded cells. The activities of thiamine entry and exit were optimal at 42 and 37 degrees C, respectively. Hyperbolic saturation kinetics were obtained for the entry rate with a K(m) value of 5.9 nM. The exit rate showed a sigmoidal dependence on cellular thiamine concentrations, and a half-maximal velocity was observed at 31 muM. The rates of both entry and exit were lowered by electron transport inhibitors and uncouplers, suggesting that the energy coupled to both processes was provided through substrate oxidation. Thiamine exit from K(+)-depleted cells was enhanced by K(+) alone and by Na(+) to a much lesser extent, and K(+) and glucose were found to be synergistic for thiamine exit. These cations had no effect on the entry of thiamine into KG1676 cells in the absence of glucose. These properties of the entry and exit of thiamine in KG1676 are discussed from the standpoint of the possible involvement of different membrane components or different sites of identical thiamine carrier protein.

[Studies on the thiamine transport system in Bacillus cereus (author's transl)]

The thiamine transport system in Bacillus cereus exhibits rhythmical changes of resorption- and excretion-phases lasting 1-2 h. These main phases are subdivided in shorter ones with an average duration of 45 s. The velocity of the thiamine uptake is influenced by pH, temperature, age of cells, energy and substrate supply and thiamine concentration of the medium. The Michaelis-Menten-Kinetic can be used to describe the uptake: Km = 1.98 x 10(-8) M; Vmax = 1.19 x 10(-6) mol/g dry weight x min. The rate is enhanced by K+, Ca2+ and Mg2+, and inhibited by Pyrithiamin, EDTA, H+-ions, proton donors and proton acceptors; OH(-)-ions cause a change in the direction of transport. A theoretical explanation can be given by assuming a coupling of the thiamine permeation with proton movements in the membrane.

Inhibition of thiamine pyrophosphate utilization by thiamine or its monophosphate in Escherichia coli

The growth of a thiamine pyrophosphate auxotroph of Escherichi coli was inhibited by either thiamine or thiamine monophosphate, and the growth of a thiamine monophosphate auxotroph was inhibited by thiamine. The thiamine pyrophosphate-dependent oxidation of pyruvate was inhibited by thiamine with whole cells of the thiamine pyrophosphate auxotroph but not with cell extracts prepared from the same organism. In addition, the thiamine pyrophosphate uptake of the thiamine pyrophosphate auxotroph was inhibited by either thiamine or thiamine monophosphate. Although the thiamine pyrophosphate uptake of a revertant, selected for prototrophy from the thiamine monophosphate auxotroph, was inhibited by thiamine to an extent comparable to that observed with the thiamine monophosphate auxotroph, its growth was no longer inhibited by thiamine. A possible mechanism for the inhibition by thiamine and thiamine monophosphate in the utilization of thiamine pyrophosphate is discussed.

Hydroxyethylthiazole uptake in Escherichia coli: general properties and relationship between uptake and thiamine biosynthesis

Uptake of (35)S-hydroxyethylthiazole (4-methyl-5-hydroxyethylthiazole) by Escherichia coli intact cells was studied. Hydroxyethylthiazole was taken up in the presence and absence of glucose at the same rate. The uptake was almost proportional to a hydroxyethylthiazole concentration gradient up to 0.1 mM with no tendency of saturation, and reached a steady state within 2 min. When the cells were treated with 1 mM N-ethylmaleimide, about 50% inhibition of hydroxyethylthiazole uptake was observed. Hydroxyethylthiazole uptake was stimulated by the addition of hydroxymethylpyrimidine (2-methyl-4-amino-5-hydroxymethylpyrimidine), and this effect was further enhanced in the presence of glucose. For full activation of hydroxyethylthiazole uptake, 1 muM hydroxymethylpyrimidine was necessary in the presence of glucose. The rate of hydroxyethylthiazole uptake was almost linear up to 60 min in the presence of hydroxymethylpyrimidine and glucose. Hydroxymethylpyrimidine monophosphate and its pyrophosphate could not stimulate the uptake. Thiamine and 2-amino-hydroxyethylthiazole were inhibitory on hydroxyethylthiazole uptake in the presence of hydroxymethylpyrimidine and glucose. N-ethylmaleimide and 2, 4-dinitrophenol were also inhibitory. No stimulatory effect of hydroxymethylpyrimidine on hydroxyethylthiazole uptake was observed in mutant cells lacking either thiaminephosphate pyrophosphorylase or hydroxymethylpyrimidine monophosphate kinase. The possibility of direct participation of thiamine-synthesizing enzymes in hydroxyethylthiazole uptake was discussed.

Thiamine-binding protein of Escherichia coli

The ability to transport thiamine in Escherichia coli was reduced by osmotic shock treatment with a concomitant release of a thiamine-binding protein; its formation was repressed by thiamine added to the growth medium.