Cloning and overexpression of the oah1 gene encoding O-acetyl-L-homoserine sulfhydrylase of Thermus thermophilus HB8 and characterization of the gene product

The oah1 gene of an extremely thermophilic bacterium, Thermus thermophilus HB8, was cloned, sequenced, and overexpressed in Escherichia coli cells. The gene product having a high O-acetyl-L-homoserine sulfhydrylase (EC 4.2.99.10) activity was purified to homogeneity, with a recovery of approximately 40% and a purification ratio of 81-fold, both calculated from the cell-homogenate. The protein showed molecular masses of approximately 163000 (for the native form) and 47000 (for the subunit). The isoelectric point was pH 6.0. The optimum temperature and pH for the activity were approximately 70 degrees C and pH 7.8, respectively. The enzyme was also shown to be very stable at high temperature (90% activity remaining at 90 degrees C for 60 min at pH 7.8) and in a wide range of pH (pH 4-12 at room temperature). The absorption spectrum showed a peak at 425 nm, and hydroxylamine hydrochloride (0.1 mM) inhibited approximately 90% of the activity, suggesting formation of a Schiff base with pyridoxal 5'-phosphate. The enzyme showed an apparent K(m) value of 6.8 mM for O-acetyl-L-homoserine, a V(max) value of 165 micromol/min per mg of protein at a fixed sulfide concentration of 5 mM, and also an apparent K(m) value of approximately 1.3 mM for sulfide (with 25 mM acetylhomoserine). L-Methionine (1 mM) inhibited the enzyme activity by 67%. Based on these findings, it was discussed that this enzyme might be inactive under ordinary conditions but might become active as an alternative homocysteine synthase in T. thermophilus HB8, only under such conditions as deficiency in transsulfuration, bringing about a sufficient amount of sulfide available in the cell.     

Neonatal estrogenization leads to increased expression of cellular retinol binding protein 2 in the mouse reproductive tract

Exposure to estrogenic substances during a time window, the so-called critical period, in perinatal life causes an irregular development of the genital tract that leads to ovary-independent proliferation and cornification in the vaginal epithelium in mice. We have previously demonstrated that retinol inhibits the irreversible effects of estrogen on the vagina. Here, mice kept in a vitamin-A-deficient condition during perinatal life were shown to be more sensitive to the harmful effects of estrogen. In addition, expression of mRNA for retinol binding protein type 2 (CRBP2), a small intestine-specific cytosolic protein that captures intracellular retinal and retinol, was detected in the vaginal epithelium. Induction of increased expression of CRBP2 mRNA by estrogen was also evident in the uterus and epididymis. Both estradiol-17 and diethylstilbestrol markedly increased the tissue content of CRBP2 mRNA in the vagina and uterus during the neonatal critical period but not after 15 days of age. These results taken together imply that estrogen disrupts the local vitamin A balance by an induction of CRBP2 gene expression in the epithelium in the developing mouse genital tract, and that retinoid imbalance may contribute to the genesis of irreversible effects of estrogen on the vagina.This work was supported by Grants-in-Aid for Priority Area B (to T.M. and M.M.) and for Young Scientists (to M.M.) from the Ministry of Culture, Sports, Science and Technology, Japan     

Rapid and sensitive amino-acid sequencing of cloning Thermus thermophilus HB8 ferredoxin by proteomics

Recombinant holo Thermus thermophilus [7Fe-8S] ferredoxin was synthesized by cloning from Thermus thermophilus HB8 gene. A specific sequence (Pro-His-Val-Ile) at the N-terminus of the recombinant ferredoxin was determined by a rapid and highly sensitive mass spectral method using a novel Ru(II) Edman reagent, [(tpy)Ru(tpy-C6H4-NCS)](PF6)2 (tpy=terpyridine). The formation of the recombinant holoTtFd was established by the characteristic absorptions and CD extrema as [7Fe-8S] ferredoxin. The catalytic electron-transfer reactivity of the [7Fe-8S] ferredoxin between ferredoxin-NADP+ reductase and cytochrome c was recognized.     

Modulation of gill Na+,K+-ATPase activity by ammonium ions: Putative coupling of nitrogen excretion and ion uptake in the freshwater shrimp Macrobrachium olfersii

The effect of NH4+ ions on (Na+,K+)-ATPase hydrolytic activity was examined in a gill microsomal fraction from M. olfersii. In the absence of NH4+ ions, K+ ions stimulated ATP hydrolysis, exhibiting cooperative kinetics (nH=0.8), to a maximal specific activity of V=556.1+/-22.2 nmol.min(-1).mg(-1) with K(0.5)=2.4+/-0.1 mmol.L(-1). No further stimulation by K+ ions was observed in the presence of 50 mmol.L(-1) NH4+ ions. ATP hydrolysis was also stimulated by NH4+ ions obeying Michaelian kinetics to a maximal specific activity of V=744.8+/-22.3 nmol.min(-1).mg(-1) and KM=8.4+/-0.2 mmol.L(-1). In the presence of 10 mmol.L(-1) K+ ions, ATP hydrolysis was synergistically stimulated by NH4+ ions to V=689.8+/-13.8 nmol.min(-1).mg(-1) and K(0.5)=6.6+/-0.1 mmol.L(-1), suggesting that NH4+ ions bind to different sites than K+ ions. PNPP hydrolysis was also stimulated cooperatively by K+ or NH4+ ions to maximal values of V= 235.5+/-11.8 nmol.min(-1).mg(-1) and V=234.8+/-7.0 nmol.min(-1).mg(-1), respectively. In contrast to ATP hydrolysis, K(+)-phosphatase activity was not synergistically stimulated by NH4+ and K+ ions. These data suggest that at high NH4+ ion concentrations, the (Na+, K+)-ATPase exposes a new site; the subsequent binding of NH4+ ions stimulates ATP hydrolysis to rates higher than those for K+ ions alone. This is the first demonstration that (Na+, K+)-ATPase activity in a freshwater shrimp gill is modulated by ammonium ions, independently of K+ ions, an effect that may constitute a fine-tuning mechanism of physiological relevance to osmoregulatory and excretory processes in palaemonid shrimps.     

Biochemical characterization of TT1383 from Thermus thermophilus identifies a novel dNTP triphosphohydrolase activity stimulated by dATP and dTTP

The HD domain motif is found in a superfamily of proteins in bacteria, archaea and eukaryotes. A few of these proteins are known to have metal-dependant phosphohydrolase activity, but the others are functionally unknown. Here we have characterized an HD domain-containing protein, TT1383, from Thermus thermophilus HB8. This protein has sequence similarity to Escherichia coli dGTP triphosphohydrolase, however, no dGTP hydrolytic activity was detected. The hydrolytic activity of the protein was determined in the presence of more than two kinds of deoxyribonucleoside triphosphates (dNTPs), which were hydrolyzed to their respective deoxyribonucleosides and triphosphates, and was found to be strictly specific for dNTPs in the following order of relative activity: dCTP > dGTP > dTTP > dATP. Interestingly, this dNTP triphosphohydrolase (dNTPase) activity requires the presence of dATP or dTTP in the dNTP mixture. dADP, dTDP, dAMP, and dTMP, which themselves were not hydrolyzed, were nonetheless able to stimulate the hydrolysis of dCTP. These results suggest the existence of binding sites specific for dATP and dTTP as positive modulators, distinct from the dNTPase catalytic site. This is, to our knowledge, the first report of a non-specific dNTPase that is activated by dNTP itself.     

Trigger factor from Thermus thermophilus is a Zn2+-dependent chaperone

The ribosome-associated chaperone trigger factor (TF) of Escherichia coli interacts with a variety of newly synthesized polypeptides to assist their correct folding. Here, we report that the TF of thermophilic eubacterium, Thermus thermophilus, arrested spontaneous folding of green fluorescent protein by forming a 1:1 binary complex. The complex was isolable by gel-filtration but was shown to be dynamic because green fluorescent protein was released by alpha-casein in large excess. Unexpectedly, EDTA completely abolished the folding-arrest activity of TF, and analysis revealed that the TF from our preparation contained approximately 0.5 mol Zn2+/mol TF. The folding-arrest activity of TF that was saturated with Zn2+ (approximately 1 mol/mol TF) was twice as efficient as that of untreated TF. Thus, chaperone activity of thermophilic TF is Zn2+-dependent.     

Functional reconstitution of a crenarchaeal splicing endonuclease in vitro

Sulfolobus tokodaii strain 7 is one of Crenarchaea whose entire genome has been sequenced. The genome sequence revealed that it possesses two open reading frames (ORFs) that are homologous to EndA, a protein responsible for splicing endonuclease activity in Archaea. Interestingly, one of the two ORFs lacks a putative catalytic amino acid residue for the nuclease activity. To investigate their functions, the two ORF products were individually expressed in Escherichia coli, partially purified, and tested for their nuclease activities in vitro. Using in vitro transcribed tRNA precursor as a substrate, we found that the two ORF products are concurrently required to cleave exon-intron junctions. Our finding implies that the splicing endonuclease for the organism is a multi-subunit complex composed of the two endA gene products.     

Multidrug transporter MexB of Pseudomonas aeruginosa: overexpression, purification, and initial structural characterization

Structural and functional characterization of the multidrug transporter, MexB, of Pseudomonas aeruginosa is significantly restricted due to a low yield of approximately 0.1 mg/L of culture from natural sources. To facilitate structural studies of this medically important transporter protein, we developed a large-scale system for expression of the genetically engineered recombinant, MexB, in the Escherichia coli cell. Using the system, the eventual yield of MexB attained was about 10mg/L of culture. The optimized purification protocol in the presence of dodecyl beta-D-maltoside allowed isolation of highly homogeneous MexB. The oligomeric state of the protein in detergent solution has been characterized to verify that the native state of the purified protein has been preserved. The molecular mass of the protein-detergent complex was found to be 380-450kDa. The MexB-dodecyl beta-d-maltoside mass ratio was determined to be 1.8 +/- 0.05. Taking into account the monomeric MexB molecular mass deduced from its amino acid sequence (112.8 kDa), we concluded that the purified MexB exists as the homotrimer in the surfactant solution. Circular dichroism analysis of MexB showed dominance of the alpha-helix structures. High yield, homogeneity, and stability of MexB position it as a good candidate for structural and functional characterization.     

Midblastula transition (MBT) of the cell cycles in the yolk and pigment granule-free translucent blastomeres obtained from centrifuged Xenopus embryos

We obtained translucent blastomeres free of yolk and pigment granules from Xenopus embryos which had been centrifuged at the beginning of the 8-cell stage with cellular integrity. They divided synchronously regardless of their cell size until they had decreased to 37.5 microm in radius; those smaller than this critical size, however, divided asynchronously with cell cycle times inversely proportional to the square of the cell radius after midblastula transition (MBT). The length of the S phase was determined as the time during which nuclear DNA fluorescence increased in Hoechst-stained blastomeres. When the cell cycle time exceeded 45 min, S and M phases were lengthened; when the cell cycle times exceeded 70 min, the G2 phase appeared; and after cell cycle times became longer than 150 min, the G1 phase appeared. Lengths of G1, S and M phases increased linearly with increasing cell cycle time. Enhanced green fluorescent protein (EGFP)-tagged proliferating cell nuclear antigen (PCNA) expressed in the blastomeres appeared in the S phase nucleus, but suddenly dispersed into the cytoplasm at the M phase. The system developed in this study is useful for examining the cell cycle behavior of the cell cycle-regulating molecules in living Xenopus blastomeres by fluorescence microscopy in real time.     

A novel metal-activated L-serine O-acetyltransferase from Thermus thermophilus HB8

L-Cysteine is an important amino acid in terms of its industrial applications. The biosynthesis of L-cysteine in enteric bacteria is regulated through the feedback inhibition by L-cysteine of L-serine O-acetyltransferase (SAT), a key enzyme in L-cysteine biosynthesis. We recently found that L-cysteine is overproduced in Escherichia coli strains expressing a gene encoding feedback inhibition-insensitive SAT. Further improvements in L-cysteine production are expected by the use of SAT with high stability. We report here the sat1 gene encoding SAT of an extreme thermophile, Thermus thermophilus HB8. The sat1 gene was cloned and overexpressed in E. coli cells based on the genome sequence in T. thermophilus HB8. The predicted amino acid sequence consists of 295 amino acids and is homologous to other O-acetyltransferase members. In particular, the carboxyl-terminal region shares approximately 30% identities with SATs found in bacteria and plants, despite showing only about 15% identity in the overall sequence. Enzymatic analysis and an atomic absorption study of the purified recombinant proteins revealed that the enzyme is highly activated by Co(2+) or Ni(2+), and contains Zn(2+) and Fe(2+). These results indicate that the T. thermophilus SAT is a novel type of enzyme different from other members of this protein family.