Facile synthesis of metal-chelating magnetic nanoparticles by exploiting organophosphorus coupling.

A new method is described for facile synthesis of metal-chelating magnetic nanoparticles by simply mixing iron oxide nanoparticles with a bifunctional organophosphorus compound, N-(phosphonomethyl)iminodiacetic acid (PM-IDA), in aqueous solution. On charging with nickel ions, the PM-IDA functionalized iron oxide nanoparticles exhibited high His-tag protein binding capacity (0.21 and 0.58 mg/mg for His-tagged green fluorescent protein and chloramphenicol acetyltransferase, respectively) and were successfully used to purify these proteins from bacterial cell extracts to high purity in a single step. Although other synthetic schemes for metal-chelating magnetic nanoparticles have been reported, the method described here is markedly simpler and involves only low-cost reagents.

Mutations in the chloramphenicol acetyltransferase (S61G, Y105C) increase accumulated amounts and resistance in Pseudomonas aeruginosa.

A chloramphenicol acetyltransferase (catB7) gene containing two point mutations, 181A/G and 314A/G, has been recently reported to be a determinant for high-level chloramphenicol resistance phenotype in a Pseudomonas aeruginosa strain PAhcr1. The mutant CATB7 was further characterized in vitro and in vivo to elucidate the molecular basis of high-level resistance. CAT assay demonstrated that the mutant and wild-type recombinant CATB7 had similar specific activities. Dot blotting revealed that the accumulated amounts of CATB7 in P. aeruginosa strains PAO1 and PAhcr1 were proportionate to the respective anti-chloramphenicol level. Site-directed mutagenesis showed that G61S and Y105C contributed synergistically to the PAhcr1 resistance phenotype. It could be proposed that the mutant CATB7 was more structurally stable than catalytically efficient as a chloramphenicol resistance determinant in PAhcr1.

Rapid expression and purification of 100 nmol quantities of active protein using cell-free protein synthesis.

Two strategies for ATP regeneration during cell-free protein synthesis were applied to the large-scale production and single-column purification of active chloramphenicol acetyl transferase (CAT). Fed-batch reactions were performed on a 5-10 mL scale, approximately 2 orders of magnitude greater than the typical reaction volume. The pyruvate oxidase system produced 104 nmol of active CAT in a 5 mL reaction over the course of 5 h. The PANOx system produced 261 +/- 42 nmol, about 7 mg, of active CAT in a 10 mL reaction over the course of 4 h. The reaction product was purified to apparent homogeneity with approximately 70% yield by a simple affinity chromatography adsorption and elution. To our knowledge, this is the largest amount of actively expressed protein to be reported in a simple, fed-batch cell-free protein synthesis reaction.

The Nano-tag, a streptavidin-binding peptide for the purification and detection of recombinant proteins.

We present a new streptavidin-binding peptide for both the purification and the detection of recombinant proteins. The peptide possesses nanomolar-affinity for streptavidin and therefore was termed Nano-tag. The Nano-tag(15) is 15 amino acids long and binds to streptavidin with a dissociation constant of 4 nM and the Nano-tag(9) is a 9-mer peptide with a dissociation constant of 17 nM. We demonstrate the one-step purification of Nano-tagged proteins, namely bovine heart fatty acid-binding protein (FABP), bacterial chloramphenicol acetyltransferase (CAT), and green fluorescent protein (GFP), from an in vitro translation system as well as from an Escherichia coli lysate. No significant influence of the Nano-tag(15) and of the conditions during affinity chromatography on maturation or activity of the proteins was observed whereas the Nano-tag(9) revealed a slight decline in the amount and activity of the synthesized proteins. The main advantage of the Nano-tag is the mild and specific elution with washing buffer plus biotin or related compounds, which enables the elution of the bound fusion protein from the streptavidin column in the native state. Additionally, the Nano-tag allowed the detection of recombinant proteins on Western blots by a streptavidin-alkaline phosphatase conjugate.

One-step metal-affinity purification of histidine-tagged proteins by temperature-triggered precipitation.

The feature of elastin-like proteins (ELPs) to reversibly precipitate above their transition temperature was exploited as a general method for the purification of histidine (His)-tagged proteins. The principle of the single-step metal-affinity method is based on coordinated ligand-bridging between the modified ELPs and the target proteins. ELPs with repeating sequences of [(VPGVG)(2)(VPGKG)(VPGVG)(2)](21) were synthesized and the free amino groups on the lysine residues were modified by reacting with imidazole-2-carboxyaldehyde to incorporate the metal-binding ligands into the ELP bio- polymers. Biopolymers charged with Ni(2+) were able to interact with a His tag on the target proteins based on metal coordination chemistry. Purifications of two His-tagged enzymes, beta-D-galactosidase and chloramphenicol acetyltransferase, were used to demonstrate the utility of this general method and over 85% recovery was observed in both cases. The bound enzymes were easily released by addition of either EDTA or imidazole. The recovered ELPs were reused four times with no observable decrease in the purification performance.

An improved protein bioreactor: efficient product isolation during in vitro protein biosynthesis via affinity tag.

In vitro protein biosynthesis became a powerful technology for biochemical research. Beside the determination of structure and function in vitro selection of proteins is also of great interest. In most cases the use of a synthesized protein for further applications depends on its purity. For this purpose the in vitro production and purification of proteins with short affinity tails was established. A cell-free protein synthesis system was employed to produce bovine heart fatty acid-binding protein and bacterial chloramphenicol acetyltransferase with and without fusion of the Strep-tag affinity peptide. The quantitative removal of fusion protein during cell-free synthesis from a batch reaction and a semicontinuous flow cell-free reactor were achieved. No significant influence of the Strep-tag and the conditions during the affinity chromatography on maturation or activity of the proteins were observed. The product removal from the continuous flow cell-free reactor is still an only partially solved problem, because the use of ultrafiltration membranes has some limitations. The results document that it should be possible to avoid these limitations by introducing an affinity system.

High-yield, in vitro protein expression using a continuous-exchange, coupled transcription/ translation system.

The Rapid Translation System (RTS 500) (Roche Molecular Biochemicals) is a high-yield protein expression system that utilizes an enhanced E. coli lysate for an in vitro transcription/translation reaction. In contrast to conventional transcription/translation, this system allows protein expression to continue for more than 24 h. We demonstrated the utility of the RTS 500 by expressing different soluble and active proteins that generally pose problems in cell-based expression systems. We first expressed GFP-lunasin, a fusion protein that, because of its toxicity, has been impossible to produce in whole cells. The second protein we expressed, human interleukin-2 (IL-2), is generally difficult to produce, either as the native molecule or as a GSTfusion protein, in a soluble form in bacteria. Finally, we demonstrated the capacity of the RTS 500 to co-express proteins, by the simultaneous production of GFP and CAT in a single reaction. This new technology appears to be particularly usefulfor the convenient production of preparative amounts (100-900 microg) of proteins that are toxic or insoluble in cell-based systems.

Crystallization and preliminary X-ray diffraction analysis of the chloramphenicol acetyltransferase from Tn2424.

Crystals of chloramphenicol acetyltransferase B2, an enzyme encoded by the transposon Tn2424 from Escherichia coli, have been obtained utilizing polyethylene glycol as a precipitant. The enzyme inactivates the antibiotic chloramphenicol and is a member of the xenobiotic acetyltransferase family. Two crystal forms were obtained and complete data sets have been collected at a synchrotron source: form I, which diffracted to 3.2 A, and form II, grown in the presence of NiCl(2), for which crystals of the apoenzyme and of the enzyme-chloramphenicol complex have been obtained. For the form II crystals, complete data sets have been collected at 2.7 and 3.2 A resolution, respectively. The space group of the above two crystal forms is P2(1)3, with unit-cell parameter a = 130 A.

Crystallization of type I chloramphenicol acetyltransferase: an approach based on the concept of ionic strength reducers.

Chloramphenicol acetyltransferase (CAT) is responsible for bacterial resistance to chloramphenicol. It catalyzes inactivation of the antibiotic by acetyl-group transfer from acetyl CoA to one or both hydroxyl groups of chloramphenicol. Type I CAT possesses some unique properties which are not observed in other CAT variants. Type I CAT overexpressed in Escherichia coli was purified and crystals with a resolution limit of 2.22 A have been obtained using a novel procedure which is based on the concept of 'ionic strength reducers'. The crystals have the symmetry of space group P1 and unit-cell parameters a = 96.46, b = 113.86, c = 114.21 A, alpha = 119.9, beta = 94.1, gamma = 98.6 degrees. These dimensions are consistent with four to six trimers per unit cell, corresponding to a solvent fraction ranging from 65 to 47%.

Purification and crystallization of Pseudomonas aeruginosa chloramphenicol acetyltransferase.

A chloramphenicol acetyltransferase from Pseudomonas aeruginosa genomic DNA has been overexpressed, refolded, purified, and crystallized. Crystals suitable for a three-dimensional x-ray structure determination were obtained from solutions of polyethyleneglycol methyl ether 2000 containing NiCl2 at pH 8.5. These crystals belong to the cubic space group P4(1/3)32 (a = 154.8 A) and diffract x-rays to approximately 3.2 A resolution.

Harvest protocol to reduce variability of soluble enzyme yield from cultured cells.

Many aspects of physiology and gene regulation can be studied by examining the levels of enzymes harvested from cultured cells. We found that the yield from cultured cells of two different cytosolic enzymes, creatine kinase and the common reporter gene product chloramphenicol acetyltransferase (CAT), could be highly variable despite superficially identical harvest procedures. Analysis of multiple harvest and assay parameters disclosed that fluctuations in enzyme yield were correlated with the time cells that were allowed to remain in an EDTA-containing buffered saline solution prior to scraping from the dishes with a rubber policeman. The highest and most consistent yields were obtained when the cells were allowed to remain in the solution for 6-10 min before scraping: this protocol has cut variability approximately by a factor of three.

Synthesis of chloramphenicol acetyltransferase in a coupled transcription-translation in vitro system lacking the chaperones DnaK and DnaJ.

A trimeric enzyme chloramphenicol acetyltransferase (CATI) has been synthesized in the Zubay system genetically depleted from DnaK and DnaJ. Most of CAT formed in the system fail to assemble into an active trimer. Instead CAT is accumulated in either a GroEL-bound complex or as an inactive monomer. Addition of purified DnaK and DnaJ to the system prior to the start of protein synthesis leads to the increase of the specific activity of formed CAT. A portion of exogenous DnaK and DnaJ added to the system associate with nascent polypeptide chains in the ribosomes. DnaK also comigrates with 50S-ribosomal subunits.

Tryptophan fluorescence of chloramphenicol acetyltransferase: resolution of individual excited-state lifetimes by site-directed mutagenesis and multifrequency phase fluorometry.

Multifrequency phase fluorometry, in conjunction with site-directed mutagenesis, has allowed the determination of the fluorescence lifetimes of each of the three tryptophan residues of the type III variant of chloramphenicol acetyltransferase (CATIII). The mutant proteins retaining a single tryptophan yield lifetimes of 1.36, 2.00, and 1.17 ns for Trp-16, -86, and -152, respectively. Binding of chloramphenicol shortens the fluorescence lifetimes of all three tryptophans to some extent, in particular those of Trp-86 and Trp-152 (decreases of 51% and 39%, respectively). The mechanism of fluorescence quenching is believed to be radiationless energy transfer. Estimates of Trp-chloramphenicol distances by energy-transfer calculations are in good agreement with those determined from the crystal structure of CATIII. Despite binding at the same site in wild-type CATIII, CoA and ethyl-S-CoA produce different responses in global lifetime measurements--increases of 8% and 31%, respectively. Examination of each of the one-Trp CATIII variants, generated by site-directed mutagenesis, yields a variety of responses. Trp-152, located within the CoA binding site, responds to both CoA and its thioalkyl derivative with a 27-30% increase in fluorescence lifetime. Trp-16, distant from the CoA site, does not differentiate between the two ligands (7% increase in lifetime). However, Trp-86 shows a striking difference in binding responses, only a 4% decrease with CoA but a 14% reduction with ethyl-S-CoA. Each of the two-Trp CAT variants shows little change in global fluorescence lifetime on association with CoA.(ABSTRACT TRUNCATED AT 250 WORDS)

Suppression of the transformed phenotype in hybrids of human T-cell leukemia virus type I tax-transformed rat fibroblasts and normal human fibroblasts.

We have isolated and characterized hybrid cell lines derived from Rat-2 cells transformed by the human T-cell leukemia virus type-I (HTLV-I) Tax protein fused with WI-38 normal human fibroblasts. These hybrid cells (designated as RTW cells) showed contact inhibition after growing to confluency, loss of anchorage-independent growth in 0.33% soft agar, and inability to form tumors in athymic mice. Assays of transcription from the HTLV-I long terminal repeat (LTR) demonstrated that Tax-mediated trans-activation of its own promoter was fully maintained in RTW cells, implying that functional Tax is expressed in these cells. Our results indicate that WI-38 cells contain a suppressor gene(s) which can block Tax-mediated cell transformation without interfering its trans-activation of the HTLV-I LTR.

Expression of a synthetic DNA region containing a consensus promoter and two lac operators in Brevibacterium sp.

The cat reporter gene was used to assess expression of two promoters, previously strongly expressed in Escherichia coli, in Brevibacterium sp. R312 strain. The tac promoter (de Boer et al., 1983, Proc. Natl. Acad. Sci. USA 80, 21-25) was poorly expressed in Brevibacterium sp. In contrast, the AatII-SalI fragment of plasmid pYEJ001 (Pharmacia LKB Biotechnology, Sweden) containing two lac operators, a consensus sequence promoter and the cat structural gene clearly revealed chloramphenicol acetyltransferase activity and the presence of a 25,600-kDa protein, corresponding to the monomeric CAT protein, in cell extracts.

Multiple control elements regulate transcription from the most distal promoter of human aldolase A gene.

Three major transcripts, differing in their 5'-untranslated regions, are produced from the human aldolase A gene by alternative usage of three promoters, designated distal, middle and proximal. We report that the genomic region (distal promoter) upstream from the first leader exon (exon L1) efficiently directs transcription of a reporter CAT gene after transient transfections in human hepatoma cells (Hep3B) in a fashion independent from the other two promoters of the same gene. The distal promoter region contains cis-acting elements that regulate transcription both positively and negatively.

Isolation of lambda repressor mutants with defects in cooperative operator binding.

A hybrid operator-promoter region was designed to aid in a screen for cooperativity mutants of the lambda repressor. In this system, lambda repressor mutants with defects in pairwise cooperative binding are unable to act as efficient transcriptional repressors. Four single amino acid substitutions in the C-terminal domain of the repressor were isolated. Studies of the DNA binding properties of the purified mutant proteins show that a repressor bearing the Gly147-->Asp mutation binds with normal affinity to single operator sites but is defective in pairwise cooperative site binding. Quantitative footprinting studies show that the free energy of interaction between repressor dimers bound at operator sites OR1 and OR2 is reduced from -2.4 kcal/mol for the wild-type repressor to 0 kcal/mol for the GD147 mutant.

Expression of the rat brain creatine kinase gene in C6 glioma cells.

We have recently shown that while brain creatine kinase (CKB) mRNA was detectable in RNA from cultured primary rat brain neurons, CKB mRNA was about 15-fold higher in primary astrocytes and 17-fold higher in oligodendrocytes (Molloy et al., J Neurochem 59:1925-1932, 1992). To begin to understand the molecular mechanisms responsible for brain glial cells containing the highest levels of CKB mRNA in the body, we have examined the expression of rat CKB mRNA in established C6 glioma cells. RNase-protection analysis showed the endogenous CKB mRNA levels in exponentially growing C6 were high and measured 50% of that in total RNA from rat brain lysate and 60% of that in cultured primary astrocytes and oligodendrocytes. The 5' and 3' ends of CKB mRNA in C6 were mapped to the same nucleotides as CKB mRNA from rat brain, indicating that the sites of in vivo transcription initiation and termination/polyadenylation of CKB mRNA in C6 are the same as in total rat brain RNA. The level of CKB enzyme activity in C6 whole cell lysates was among the highest of the glial cell lines which we measured. All creatine kinase enzyme activity present in C6 was found in the dimeric CKB isoform (BB), which is characteristic of CKB expression in the brain. A 2.9 kb gene fragment containing the basal CKB promoter and far-upstream 5' sequences was cloned upstream of the chloramphenicol acetyltransferase (CAT) gene and transfected into C6 cells. CAT activity was readily detectable in C6 and mapping of the 5' end of the CAT mRNA showed that transcription was directed from the correct initiation site. Since we found C6 cells were difficult to transfect, conditions were established which both maximized transfection efficiency and maintained normal C6 cell morphology. These results should permit the future identification of the nuclear trans-acting factors and the cognate cis-acting regulatory elements responsible for high CKB mRNA expression in brain glial cells.

Glycosome assembly in trypanosomes: variations in the acceptable degeneracy of a COOH-terminal microbody targeting signal.

Trypanosomes compartmentalize most of their glycolytic enzymes in a peroxisome-like microbody, the glycosome. The specificity of glycosomal targeting was examined by expression of chloramphenicol acetyltransferase fusion proteins in trypanosomes and monkey cells. Compartmentalization was assessed by cell fractionation, differential detergent permeabilization, and immunofluorescence. The targeting signal of trypanosome phosphoglycerate kinase resides in the COOH-terminal hexapeptide, NRWSSL; a basic amino acid is not required. The minimal targeting signal is, as for mammalian cells, a COOH-terminal tripeptide related to -SKL. However, the acceptable degeneracy of the signal for glycosomal targeting in trypanosomes is considerably greater than that for peroxisomal targeting in mammals, with particularly relaxed requirements in the penultimate position.