An improved method for the enzymatic transformation of nucleosides into 5?-monophosphates

An improved method to transform nucleosides into 5'-monophosphates using nucleoside phosphotransferase from Erwinia herbicola is reported. The method is based on the shift in the equilibrium state of the reaction to the formation of desired product due to its precipitation by Zn2+. Under optimal conditions, the extent of nucleoside transformations into nucleoside-5'-monophosphates were 41-91% (mol).

Identification of a chromosome-borne class C beta-lactamase from Erwinia rhapontici

OBJECTIVES

To characterize the beta-lactamase gene content of Erwinia rhapontici.

METHODS

The beta-lactamase gene was cloned, sequenced and expressed in Escherichia coli.

RESULTS

The cloned gene conferred a resistance pattern of an Ambler class C beta-lactamase in E. coli. The AmpC-type enzyme had a pI value of 8.6 and shared 62% amino acid sequence identity with that of Escherichia fergusonii. The ampC gene was associated with a regulatory ampR gene and beta-lactamase production was inducible.

CONCLUSIONS

This work provides further evidence of the molecular heterogeneity of beta-lactamases in Erwinia spp. and that plant-pathogenic enterobacterial species may constitute a reservoir of antibiotic resistance genes.

Asparaginase pharmacodynamics differ by formulation among children with newly diagnosed acute lymphoblastic leukemia

Polyethylene glycol-conjugated (PEG) asparaginase is approved for use in patients who develop allergy to other forms of asparaginase, although its ability to deplete asparagine systemically in patients with hypersensitivity has not been well elucidated. In 53 children with newly diagnosed acute lymphoblastic leukemia, we serially assessed asparagine concentrations in cerebrospinal fluid (CSF) and plasma as well as serum anti-asparaginase antibodies. All patients received native Escherichia coli (Elspar) asparaginase during induction therapy; patients received PEG asparaginase during reinductions when available, and those who developed allergy received Erwinia asparaginase. All eight patients who developed clinical evidence of allergy to asparaginase had anti-asparaginase antibodies. Among patients who had no antibodies, those who received E. coli had lower mean (+/-s.d.) CSF asparagine (0.29+/-0.63, n=9) than those who received PEG (0.77+/-0.82, n=4) (P=0.007). Results were similar for plasma asparagine. There was no situation where asparagine concentrations were more effectively depleted by PEG than by other preparations. None of the five patients who developed thrombosis had an allergy or antibodies to asparaginase at the time of the thrombosis. We conclude that asparagine concentrations were less effectively depleted by PEG than by E. coli asparaginase at the doses commonly used. The risk of thrombosis may be affected by the intensity of asparaginase exposure.

Evolution of a pathway to novel long-chain carotenoids

Using methods of laboratory evolution to force the C(30) carotenoid synthase CrtM to function as a C(40) synthase, followed by further mutagenesis at functionally important amino acid residues, we have discovered that synthase specificity is controlled at the second (rearrangement) step of the two-step reaction. We used this information to engineer CrtM variants that can synthesize previously unknown C(45) and C(50) carotenoid backbones (mono- and diisopentenylphytoenes) from the appropriate isoprenyldiphosphate precursors. With this ability to produce new backbones in Escherichia coli comes the potential to generate whole series of novel carotenoids by using carotenoid-modifying enzymes, including desaturases, cyclases, hydroxylases, and dioxygenases, from naturally occurring pathways.

Purification and characterization of a lactonase from Erwinia cypripedii 314B that hydrolyzes ( S )-5-oxo-2-tetrahydrofurancarboxylic acid

A bacterium, strain 314B, able to assimilate ( S)-5-oxo-2-tetrahydrofurancarboxylic acid was isolated from soil and identified as Erwinia cypripedii. A lactonase hydrolyzing ( S)-5-oxo-2-tetrahydrofurancarboxylic acid to l-alpha-hydroxyglutaric acid was purified 63-fold with 2% recovery from crude extracts of this bacterium to homogeneity as judged by SDS-PAGE. The molecular masses estimated by SDS-PAGE and gel filtration were 41 kDa and 79 kDa, respectively. The maximum activity was observed at pH 6.5-7.5 and 35-45 degrees C. The enzyme showed lower activity toward dl-2-oxotetrahydrofuran-4,5-dicarboxylic acid, but did not act on ( R)-5-oxo-2-tetrahydrofurancarboxylic acid and other natural and synthetic lactones tested.

Tomato pectin methylesterase: Modeling, fluorescence, and inhibitor interaction studies?comparison with the bacterial (Erwinia chrysanthemi) enzyme

The molecular model of Lycopersicon esculentum (tomato) pectin methylesterase (PME) was built by using the X-ray crystal structure of PME from the phytopathogenic bacterium Erwinia chrysanthemi as a template. The overall structure and the position of catalytically important residues (Asp132, Asp 153, and Arg 221, located at the bottom of the active site cleft) are conserved. Instead, loop regions forming the walls of the catalytic site are much shorter and form a less deep cleft, as already revealed by the carrot PME crystal structure. The protein inhibitor of pectin methylesterase (PMEI) isolated from kiwi fruit binds tomato PME with high affinity. Conversely, no complex formation between the inhibitor and PME from E. chrysanthemi is observed, and the activity of this enzyme is unaffected by the presence of the inhibitor. Fluorescence quenching experiments on tomato PME and on PME-PMEI complex suggest that tryptophanyl residues present in the active site region are involved in the interaction and that the inhibitor interacts with plant PME at the level of the active site. We also suggest that the more open active site cleft of tomato PME allows the interaction with the inhibitor. Conversely, the narrow and deep cleft of the active site of E. chrysanthemi PME hinders this interaction. The pH-dependent changes in fluorescence emission intensity observed in tomato PME could arise as the result of protonation of an Asp residue with unusually high pKa, thus supporting the hypothesis that Asp132 acts as acid/base in the catalytic cycle.

Frameshift mutation events in β-glucosidases

Compensated frameshift mutation is a modification of the reading frame of a gene that takes place by way of various molecular events. It appears to be a widespread event that is only observed when homologous amino acid and nucleodotide sequences are compared. To identify these mutation events, the sequence analysis rationale was based on the search for short regions that would have much lower degrees of conservation in protein, but not in DNA, in well-conserved beta-glucosidase families. We have restricted our study to a seed set of sequences of O-glycoside hydrolase families 1 and 3. We found compensated frameshift mutation in the family of 1 beta-glucosidases for the Erwinia herbicola, Cellulomonas fimi, and (non-cyanogenic) Trifolium repens gene sequences, and in the family of 3 beta-glucosidases for the Clostridium thermocellum and Clostridium stercorarium gene sequences. By computational treatment, the observed mutation events in the gene frameshifting sub-sequence have been neutralised. Each nucleotide insertion must be eliminated and each nucleotide deletion must be substituted by the symbol N (any nucleotide). When the frameshifting fragments of the amino acid sequences were substituted by the computationally neutralised subsequences, the beta-glucosidase alignments were improved. We also discuss the structural implications of the compensated frameshift mutations events.

Evaluation of immunologic crossreaction of antiasparaginase antibodies in acute lymphoblastic leukemia (ALL) and lymphoma patients

To evaluate how well antibodies to one asparaginase preparation predict or correlate with antibodies to another preparation in acute lymphoblastic leukemia (ALL) and lymphoma patients who did and did not have hypersensitivity reactions during chemotherapy. In all, 24 children with newly diagnosed ALL or lymphoma, who received Escherichia coli asparaginase 10 000 IU/m(2) IM thrice weekly for nine doses as part of multiagent induction and reinduction chemotherapy, and seven monthly doses during the first 7 months of continuation treatment, were studied. Plasma samples were collected at postinduction and at postreinduction. Six of 24 patients had no overt clinical reactions (nonreacting) and received only the E. coli preparation. Of these, 18 patients who had allergic reactions were switched to Erwinia asparaginase. A total of 18 patients had an anaphylactoid reaction to Erwinia asparaginase and were switched to receive polyethylene glycol (PEG) asparaginase. Antibody levels were measured by enzyme-linked immunoadsorbent assay against all the three asparaginase preparations. At postinduction, antibodies against E. coli were higher in reacting patients (0.063+/-0.066) than in nonreacting patients (0.019+/-0.013) (P=0.03). At postreinduction, anti-Erwinia antibodies were significantly higher in reacting patients (0.431+/-0.727) than in nonreacting patients (0.018+/-0.009) (P=0.007). Anti-E. coli antibodies correlated with anti-PEG antibodies at postinduction (r=0.714, P<0.001) and at postreinduction (r=0.914, P<0.001), but did not correlate with anti-Erwinia antibodies at postinduction (r=0.119, P=0.580) and at postreinduction (r=0.078, P=0.716). The results indicate a crossreactivity between patient antibodies raised against natural E. coli and PEG asparaginase but not Erwinia asparaginase.

Distinct sucrose isomerases catalyze trehalulose synthesis in whiteflies, Bemisia argentifolii, and Erwinia rhapontici

Isomaltulose [alpha-D-glucopyranosyl-(1,6)-D-fructofuranose] and trehalulose [alpha-D-glucopyranosyl-(1,1)-D-fructofuranose] are commercially valuable sucrose-substitutes that are produced in several microorganisms by the palI gene product, a sucrose isomerase. Trehalulose also occurs in the silverleaf whitefly, Bemisia argentifoli, as the major carbohydrate in the insect's honeydew. To determine if the enzyme that synthesizes trehalulose in whiteflies was similar to the well-characterized sucrose isomerase from microbial sources, the properties of the enzymes from whiteflies and the bacterium, Erwinia rhapontici, were compared. Partial purification of both enzymes showed that the enzyme from whiteflies was a 116 kD membrane-associated polypeptide, in contrast to the enzyme from E. rhapontici, which was soluble and 66 kD. The enzyme from E. rhapontici converted sucrose to isomaltulose and trehalulose in a 5:1 ratio, whereas the enzyme from whiteflies produced only trehalulose. Unlike the E. rhapontici enzyme, the whitefly enzyme did not convert isomaltulose to trehalulose, but both enzymes catalyzed the transfer of fructose to trehalulose using sucrose as the glucosyl donor. The results indicate that trehalulose synthase from whiteflies is structurally and functionally distinct from the sucrose isomerases described in bacteria. The whitefly enzyme is the first reported case of an enzyme that converts sucrose to exclusively trehalulose.

A C35 carotenoid biosynthetic pathway

Upon coexpression with Erwinia geranylgeranyldiphosphate (GGDP) synthase in Escherichia coli, C(30) carotenoid synthase CrtM from Staphylococcus aureus produces novel carotenoids with the asymmetrical C(35) backbone. The products of condensation of farnesyldiphosphate and GDP, C(35) structures comprise 40 to 60% of total carotenoid accumulated. Carotene desaturases and carotene cyclases from C(40) or C(30) pathways accepted and converted the C(35) substrate, thus creating a C(35) carotenoid biosynthetic pathway in E. coli. Directed evolution to modulate desaturase step number, together with combinatorial expression of the desaturase variants with lycopene cyclases, allowed us to produce at least 10 compounds not previously described. This result highlights the plastic and expansible nature of carotenoid pathways and illustrates how combinatorial biosynthesis coupled with directed evolution can rapidly access diverse chemical structures.

Successful treatment with Erwinia L-asparaginase for recurrent natural killer/T cell lymphoma

We describe a patient with natural killer (NK)/T cell lymphoma who relapsed after autologous peripheral blood stem cell transplantation (auto-PBSCT) and was successfully treated with Escherichia coli (E. coli) and Erwinia L-asparaginase. A 38-year-old male patient with ulcerated tumor at the left thigh was diagnosed as having nasal type NK/T cell lymphoma on the basis of histopathological and flowcytometric findings of tumor, revealing diffuse infiltration of atypical lymphoid cells into blood vessels and expression of CD7 and CD56 antigens, but not CD3. He had tumor infiltration in the bone marrow and at the right lower lung field. After five cycles of CHOP (cyclophosphamide, doxorubicin, vincristine and prednisolone) therapy, the patient achieved complete remission and received high-dose chemotherapy with auto-PBSCT, although the tumor recurred in the right leg 10 months later. Despite salvage chemotherapy, followed by local irradiation and surgical amputation, a tumor recurred at the left upper gingiva 10 days after. Using E. coli L-asparaginase (6000 U/m2/day), the tumor regressed, fever was alleviated and the serum lactate dehydrogenase decreased to normal range after several days. The asparagine synthetase expression in tumor cells was immunohistochemically negative on paraffin-embedded tissues. Because of the anaphylactoid reaction developing after E. coli L-asparaginase, alternative Erwinia L-asparaginase (6000 U/m2/day) was administered, resulting in regression of tumor and fever lysis. L-asparaginase is a promising agent for the treatment of NK/T cell lymphoma.

Bacterial phytoene synthase: molecular cloning, expression, and characterization of Erwinia herbicola phytoene synthase

Phytoene synthase (PSase) catalyzes the condensation of two molecules of geranylgeranyl diphosphate (GGPP) to give prephytoene diphosphate (PPPP) and the subsequent rearrangement of the cyclopropylcarbinyl intermediate to phytoene. These reactions constitute the first pathway specific step in carotenoid biosynthesis. The crtB gene encoding phytoene synthase was isolated from a plasmid containing the carotenoid gene cluster in Erwinia herbicola and cloned into an Escherichia coli expression system. Upon induction, recombinant phytoene synthase constituted 5-10% of total soluble protein. To facilitate purification of the recombinant enzyme, the structural gene for PSase was modified by site-directed mutagenesis to incorporate a C-terminal Glu-Glu-Phe (EEF) tripepetide to allow purification by immunoaffinity chromatography on an immobilized monoclonal anti-alpha-tubulin antibody YL1/2 column. Purified recombinant PSase-EEF gave a band at 34.5 kDa upon SDS-PAGE. Recombinant PSase-EEF was then purified to >90% homogeneity in two steps by ion-exchange and immunoaffinity chromatography. The enzyme required Mn(2+) for activity, had a pH optimum of 8.2, and was strongly stimulated by detergent. The concentration of GGPP needed for half-maximal activity was approximately 35 microM, and a significant inhibition of activity was seen at GGPP concentrations above 100 microM. The sole product of the reaction was 15,15'-Z-phytoene.

Surrogate biochemistry: use of Escherichia coli to identify plant cDNAs that impact metabolic engineering of carotenoid accumulation

Carotenoids synthesized in plants but not animals are essential for human nutrition. Therefore, ongoing efforts to metabolically engineer plants for improved carotenoid content benefit from the identification of genes that affect carotenoid accumulation, possibly highlighting potential challenges when pyramiding traits represented by multiple biosynthetic pathways. We employed a heterologous bacterial system to screen for maize cDNAs encoding products that alter carotenoid accumulation either positively or negatively. Genes encoding carotenoid biosynthetic enzymes from the bacterium Erwinia uredovora were introduced into Escherichia coli cells that were subsequently transfected with a maize endosperm cDNA expression library; and these doubly transformed cells were then screened for altered carotenoid accumulation. DNA sequencing and characterization of one cDNA class conferring increased carotenoid content led to the identification of maize cDNAs encoding isopentenyl diphosphate isomerase. A cDNA that caused a reduced carotenoid content in E. coli was also identified. Based on DNA sequence analysis, DNA hybridization, and further functional testing, this latter cDNA was found to encode the small subunit of ADP-glucose pyrophosphorylase, a rate-controlling enzyme in starch biosynthesis that has been of interest for enhancing plant starch content.

Evolution of the C30 carotenoid synthase CrtM for function in a C40 pathway

The C30 carotene synthase CrtM from Staphylococcus aureus and the C40 carotene synthase CrtB from Erwinia uredovora were swapped into their respective foreign C40 and C30 biosynthetic pathways (heterologously expressed in Escherichia coli) and evaluated for function. Each displayed negligible ability to synthesize the natural carotenoid product of the other. After one round of mutagenesis and screening, we isolated 116 variants of CrtM able to synthesize C40 carotenoids. In contrast, we failed to find a single variant of CrtB with detectable C30 activity. Subsequent analysis revealed that the best CrtM mutants performed comparably to CrtB in an in vivo C40 pathway. These mutants showed significant variation in performance in their original C30 pathway, indicating the emergence of enzymes with broadened substrate specificity as well as those with shifted specificity. We discovered that Phe 26 alone determines the specificity of CrtM. The plasticity of CrtM with respect to its substrate and product range highlights the potential for creating further new carotenoid backbone structures.

Identification of a chromosome-borne expanded-spectrum class a beta-lactamase from Erwinia persicina

From whole-cell DNA of an enterobacterial Erwinia persicina reference strain that displayed a penicillinase-related antibiotic-resistant phenotype, a beta-lactamase gene was cloned and expressed in Escherichia coli. It encoded a clavulanic-acid-inhibited Ambler class A beta-lactamase, ERP-1, with a pI value of 8.1 and a relative molecular mass of ca. 28 kDa. ERP-1 shared 45 to 50% amino acid identity with the most closely related enzymes, the chromosomally encoded enzymes from Citrobacter koseri, Kluyvera ascorbata, Kluyvera cryocrescens, Klebsiella oxytoca, Proteus vulgaris, Proteus penneri, Rahnella aquatilis, Serratia fonticola, Yersinia enterocolitica, and the plasmid-mediated enzymes CTX-M-8 and CTX-M-9. The substrate profile of the noninducible ERP-1 was similar to that of these beta-lactamases. ERP-1 is the first extended-spectrum beta-lactamase from an enterobacterial species that is plant associated and plant pathogenic.

Expression of a bacterial carotene hydroxylase gene (crtZ) enhances UV tolerance in tobacco

Carotenoids are essential components of the photosynthetic apparatus involved in plant photoprotection. To investigate the protective role of zeaxanthin under high light and UV stress we have increased the capacity for its biosynthesis in tobacco plants (Nicotiana tabacum L. cv. Samsun) by transformation with a heterologous carotenoid gene encoding beta-carotene hydroxylase (crtZ) from Erwinia uredovora under constitutive promoter control. This enzyme is responsible for the conversion of beta-carotene into zeaxanthin. Although the total pigment content of the transgenics was similar to control plants, the transformants synthesized zeaxanthin more rapidly and in larger quantities than controls upon transfer to high-intensity white light. Low-light-adapted tobacco plants were shown to be susceptible to UV exposure and therefore chosen for comparative analysis of wild-type and transgenics. Overall effects of UV irradiation were studied by measuring bioproductivity and pigment content. The UV exposed transformed plants maintained a higher biomass and a greater amount of photosynthetic pigments than controls. For revelation of direct effects, photosynthesis, pigment composition and chlorophyll fluorescence were examined immediately after UV treatment. Low-light-adapted plants of the crtZ transgenics showed less reduction in photosynthetic oxygen evolution and had higher chlorophyll fluorescence levels in comparison to control plants. After 1 h of high-light pre-illumination and subsequent UV exposure a greater amount of xanthophyll cycle pigments was retained in the transformants. In addition, the transgenic plants suffered less lipid peroxidation than the wild-type after treatment with the singlet-oxygen generator rose bengal. Our results indicate that an enhancement of zeaxanthin formation in the presence of a functional xanthophyll cycle contributes to UV stress protection and prevention of UV damage.

Anti-Erwinia asparaginase antibodies during treatment of childhood acute lymphoblastic leukemia and their relationship to outcome: a case-control study

PURPOSE

A case-control study was performed to determine whether patients who had been treated with Erwinia asparaginase as part of their treatment for childhood acute lymphoblastic leukemia (ALL) and who showed relapsed of their disease more often developed anti-asparaginase antibodies than patients who remained in remission.

METHODS

A group of 13 patients who showed relapsed of their disease (median follow-up 35 months) were randomly matched with control patients of the same risk group (two control patients to each case), who had received therapy of the same intensity during the same period (median follow-up 70 months). Anti- Erwinia asparaginase antibodies were measured (ELISA method) during maintenance therapy after asparaginase treatment (30,000 IU/m(2) daily for 10 days in all patients plus twice weekly for 2 weeks in intermediate-risk and high-risk ALL patients).

RESULTS

The overall incidence of anti- Erwinia asparaginase antibodies was 8% (3 of 39 patients). There was no statistically significant difference in the incidence of antibody formation between patients who had suffered relapse (1 of 13) and those who had not (2 of 26). In two of the three patients who developed antibodies, the antibodies disappeared after some time, whereas one patient had measurable antibody levels for more than a year after asparaginase therapy.

CONCLUSIONS

In this study, the development of anti-Erwinia asparaginase antibodies was rare and was unrelated to the risk of relapse.

Antibody formation during intravenous and intramuscular therapy with Erwinia asparaginase

BACKGROUND

Determination of the frequency of antibody formation during first and second exposure to Erwinia asparaginase after i.v. and i.m. administration.

PROCEDURE

Thirty-nine children with newly diagnosed acute lymphoblastic leukemia (ALL) were included in this prospective study. Antibodies were determined (ELISA method) in plasma from these patients on specific days during and after therapy with 30,000 IU/m(2) i.v. or i.m. every day for ten days during the induction phase (first exposure). For 19 children, antibodies were measured in plasma during and after the re-induction phase (second exposure) following treatment with 30,000 IU/m(2) i.v. or i.m. twice a week for two weeks (Mondays and Thursdays). On the same days of therapy, enzyme activity (spectrophotometric method) and the concentration of asparagine (HPLC) was determined.

RESULTS

During the first exposure, none of the patients developed anti-Erwinia asparaginase antibodies. During the second exposure, one patient (1 of 8 patients) treated intravenously developed antibodies, which were associated with disappearance of enzyme activity and reappearance of asparagine. Three of eleven patients developed antibodies of pharmacokinetic importance after i.m. therapy. None of the children had any clinical symptoms of hypersensitivity.

CONCLUSIONS

The formation of antibodies and subsequently altered pharmacokinetics of Erwinia asparaginase seemed to be of importance only during a second period of asparaginase therapy.

[Isolation and genetic study of Erwinia mutants devoid of common components of the phosphoenolpyruvate-dependent phosphotransferase system]

Mutants of bacteria belonging the genus Erwinia (Erwinia chrysanthemi and Erwinia carotovora) with pleiotropic disturbances in the utilization of many substrates were obtained through chemical and transposon mutagenesis. Genetic studies revealed that these mutants had defective ptsI or ptsH genes responsible for the synthesis of common components of the phosphoenolpyruvate-dependent phosphotransferase system, enzyme I and the HPr protein, respectively. The ptsI+ allele in both Erwinia species was cloned in vivo. Mapping of obtained mutations indicated that the ptsI and ptsH genes of E. chrysanthemi do not constitute a linkage group. The ptsI gene is located at 100 min of the chromosomal map, whereas the ptsH gene is located at 175 min. Sequencing of a portion of the E. chrysanthemi ptsI gene showed that a product of the cloned DNA region had up to 68% homology with the N terminus of Escherichia coli enzyme I.

Comparison of Escherichia coli-asparaginase with Erwinia-asparaginase in the treatment of childhood lymphoid malignancies: results of a randomized European Organisation for Research and Treatment of Cancer-Children's Leukemia Group phase 3 trial

Asparaginase is an enzyme used in the treatment of acute lymphoblastic leukemia and lymphoblastic lymphoma in children. It has minimal bone marrow toxicity. Its major side effects are anaphylaxis, pancreatitis, diabetes, coagulation abnormalities, and thrombosis, especially intracranial. It is derived from 2 different sources: Escherichia coli and Erwinia chrysanthemi. Nonrandomized clinical studies have suggested a similar efficacy of these 2 types of asparaginases and a lower toxicity for Erwinia-asparaginase. The European Organisation for Research and Treatment of Cancer-Children's Leukemia Group (EORTC-CLG) 58881 trial randomized 700 children with acute lymphoblastic leukemia or lymphoblastic lymphoma to either E coli- or Erwinia-asparaginase at the same dosage of 10 000 IU/m(2) twice weekly to compare toxicity and efficacy. Coagulation abnormalities were more frequent in the E coli-asparaginase than in the Erwinia-asparaginase arm of the study (30.2% versus 11.9%, P <.0001). The incidence of other toxicity was not significantly different. In the Erwinia-asparaginase arm, more patients failed to achieve complete remission (4.9% versus 2.0%; P =.038) and the relapse rate was higher, leading to shorter event-free survival (hazard ratio,1.59; 95% CI, 1.23-2.06; P =.0004). The estimate of event-free survival rate (SE) at 6 years was 59.8% (2.6%) versus 73.4% (2.4%). Overall survival rate at 6 years was also lower in the Erwinia-asparaginase arm at 75.1% (2.3%) versus 83.9% (2.0%), P =.002. With the dose scheduling used in this protocol, E coli-asparaginase induced more coagulation abnormalities but was superior to Erwinia-asparaginase for the treatment of childhood lymphoid malignancies.

Functional assembly of the foreign carotenoid lycopene into the photosynthetic apparatus of Rhodobacter sphaeroides, achieved by replacement of the native 3-step phytoene desaturase with its 4-step counterpart from Erwinia herbicola

Photosynthetic organisms synthesize a diverse range of carotenoids. These pigments are important for the assembly, function and stability of photosynthetic pigment-protein complexes, and they are used to quench harmful radicals. The photosynthetic bacterium Rhodobacter sphaeroides was used as a model system to explore the origin of carotenoid diversity. Replacing the native 3-step phytoene desaturase (CrtI) with the 4-step enzyme from Erwinia herbicola results in significant flux down the spirilloxanthin pathway for the first time in Rb. sphaeroides. In Rb. sphaeroides, the completion of four desaturations to lycopene by the Erwinia CrtI appears to require the absence of CrtC and, in a crtC background, even the native 3-step enzyme can synthesize a significant amount (13%) of lycopene, in addition to the expected neurosporene. We suggest that the CrtC hydroxylase can intervene in the sequence of reactions catalyzed by phytoene desaturase. We investigated the properties of the lycopene-synthesizing strain of Rb. sphaeroides. In the LH2 light-harvesting complex, lycopene transfers absorbed light energy to the bacteriochlorophylls with an efficiency of 54%, which compares favourably with other LH2 complexes that contain carotenoids with 11 conjugated double bonds. Thus, lycopene can join the assembly pathway for photosynthetic complexes in Rb. sphaeroides, and can perform its role as an energy donor to bacteriochlorophylls.

High-level production of the non-cariogenic sucrose isomer palatinose in transgenic tobacco plants strongly impairs development

Palatinose (isomaltulose, 6-O-alpha-D-glucopyranosyl-D-fructose) is a structural isomer of sucrose which is produced from sucrose by some bacterial strains as a reserve material during periods of low carbon availability. The ability to synthesise palatinose is not only advantageous for the bacteria but is also of industrial interest since palatinose is used as a sucrose substitute in food production. To explore the possibility of palatinose production in plants a recently isolated sucrose isomerase gene (palI; EC 5.4.99.11) from Erwinia rhapontici [F. Börnke et al. (2001) J Bacteriol 183: 2425-2430] was cloned into a plant expression vector between the constitutive 35S CaMV promoter and the octopine synthase polyadenylation signal. To allow secretion of the protein into the apoplast the signal peptide of the potato proteinase inhibitor II was N-terminally fused to the pall coding region. Expression of the protein was verified by northern and western blot analyses. Efficient secretion of the protein was demonstrated by palI detection in intercellular fluids. Transgenic plants expressing palI accumulated high levels of palatinose. As a consequence, transgenic plants showed severe phenotypic alterations. Young leaves were curled and developed bleached areas during maturation. Flowers were misshapen and sterile. Based on nonaqueous fractionation experiments palatinose was found in several subcellular compartments, indicating limited membrane transport of the sugar. In contrast to results obtained with short-term feeding experiments, no evidence for palatinose-mediated regulation of photosynthetic or defence genes could be obtained in the transgenic palI-expressing tobacco plants. Based on our results we conclude that plants can efficiently be used as bioreactors for the production of palatinose. Furthermore, tissue-specific expression of palI should allow carbon allocation to specific tissues and/or cell-types to be modulated.

Production and properties of asparaginase from a new Erwinia sp

Asparaginase production by a mesophilic strain of Erwinia sp. was examined; the maximum of activity was found at 40 degrees C and pH 8.5. Among the various carbon sources, mannitol was shown to be the best for production of activity. Inorganic nitrogen sources were better than the organic ones. The enzyme activity was not inhibited by 10 mmol/L metal ions (Na+, K+, Mg2+, Ca2+, Ba2+, Co2+, Ni2+, Zn2+); the activity was strongly inhibited by addition of EDTA. L-Arginine, DL-alanine, L-asparagine and L-glutamine stimulated the L-asparaginase production by 3.9, 1.7, 4.3 and 4.0 fold, respectively. The combination of L-arginine, L-asparagine and L-glutamine synergistically stimulated the asparaginase up to 5.8 fold.

Do bacterial L-asparaginases utilize a catalytic triad Thr-Tyr-Glu?

The structures of Erwinia chrysanthemi L-asparaginase (ErA) complexed with the L- and D-stereoisomers of the suicide inhibitor, 6-diazo-5-oxy-norleucine, have been solved using X-ray crystallography and refined with data extending to 1.7 A. The distances between the Calpha atoms of the inhibitor molecules and the hydroxyl oxygen atoms of Thr-15 and Tyr-29 (1.20 and 1.60 A, respectively) clearly indicate the presence of covalent bonds between these moieties, confirming the nucleophilic role of Thr-15 during the first stage of enzymatic reactions and also indicating direct involvement of Tyr-29. The factors responsible for activating Tyr-29 remain unclear, although some structural changes around Ser-254', Asp-96, and Glu-63, common to both complexes, suggest that those residues play a function. The role of Glu-289' as the activator of Tyr-29, previously postulated for the closely related Pseudomonas 7A L-glutaminase-asparaginase, is not confirmed in this study, due to the lack of interactions between these residues in these complexes and in holoenzymes. The results reported here are consistent with previous reports that mutants of Escherichia coli L-asparaginase lacking Glu-289 remain catalytically active and prove the catalytic roles of both Thr-15 and Tyr-29, while still leaving open the question of the exact mechanism resulting in the unusual chemical properties of these residues.