Specificity of the BAX Polymerase Chain Reaction System for Detection of the Foodborne Pathogen Listeria monocytogenes

The polymerase chain reaction (PCR) can be used for rapid and specific detection of foodborne pathogens. One commercial kit, the Qualicon BAX system uses PCR to detect Listeria monocytogenes in enrichment cultures derived from food and environmental samples. The specificity and sensitivity of the BAX system for detecting L. monocytogenes were characterized by using both pure and mixed cell cultures, and optimal conditions for production of cell lysates were determined. The BAX system was highly specific for L. monocytogenes, and no interference was seen in the presence of either other Listeria species or microbes from other genera. The assay detected L. monocytogenes at 105- 106 colony-forming units/mL. This sensitivity is adequate for detecting viable cells after enrichment but prevents false-positive signals from nonviable cells.

Case Report: Allergic Reactivity to Mahaleb (Prunus mahaleb) Spice in a Subject With Almond and Other Tree Nut Allergies

Background

Mahaleb is an aromatic spice prepared from the fruit stone of the St. Lucie Cherry that is used as a flavoring agent in traditional Turkish and Middle Eastern baking. Immunodiagnostic kits for almond, which are based on polyclonal almond-specific IgG antibodies, have been shown to demonstrate considerable cross-reactivity with mahaleb as was incidentally discovered during a cluster of allergen-related food recalls in 2015.

Objective

Though acute allergy to almond is somewhat common, allergies to mahaleb have not been previously documented. However, based on antigenic similarity observed with almond-specific IgG, it is predicted that mahaleb nut proteins would exhibit some level of cross-reactivity with almond-specific IgE and may therefore potentiate acute allergic symptoms in individuals with food allergy to almond.

Case Presentation: Herein, we report on a 40-year old Caucasian female with longitudinal history of multiple tree nut allergies including allergy to almond, presenting with moderate pruritus and oropharyngeal swelling shortly following ingestion of mahaleb seed kernels.

Methods and Results

Skin-prick testing using extracts compounded from pistachio, almond, and mahaleb revealed positive wheals measuring 8, 3, and 7 mm respectfully. Indirect enzyme-linked immunosorbent assay (ELISA) using plate-bound antigens prepared from pistachio, almond, and mahaleb revealed IgG positive responses to all three targets. ELISA and Western blot analysis performed using goat anti-almond polyclonal IgG demonstrated significant cross-reactivity between almond and mahaleb, but not to pistachio.

Conclusion

This is the first documented case of acute allergy to mahaleb, co-occurring in the context of plural tree nut allergies, providing novel evidence that mahaleb may pose a risk to nut-allergic individuals and indicating a need for awareness of spice contamination with nut and mahaleb residues.

Bioinformatics and Food Allergens

Bioinformatics can play an important role in developing improved technology for the detection and characterization of food allergens. However, the full realization of this potential will depend on the development of allergen-specific databases as well as improved methods for data mining within these databases. Examples of existing allergen databases and analysis tools are described, as are the most important issues that need to be addressed in the next stage of database development.

Opportunities and Challenges Using Non-targeted Methods for Food Fraud Detection

In recent years, non-targeted methods have been a popular "buzz" phrase in food fraud detection. Using analytical instrumentation techniques, non-targeted methods have been developed and applied in many food and agricultural situations. However, confusion and misstatements remain regarding how the methods are used. This perspective will discuss the definitions related to non-targeted testing, the procedure of developing and validating methods, the techniques and data analysis, and opportunities and challenges regarding the use of this class of analytical methods. The perspective seeks to provide readers with the latest information regarding recent advances in the use of non-targeted methods.

Assessment of Avenins from Different Oat Varieties Using R5-Based Sandwich ELISA

Gluten derived from wheat and related triticeae cereals possesses distinct amino acid sequences that provoke the immunopathogenic features of celiac disease (CD) in genetically susceptible individuals. However, the role of oat-derived gluten, or avenins, in CD pathogenesis remains a disputed matter, as evidenced by a lack in harmonized legislation regarding gluten classification in relation to gluten-free labeling. In this study, we have analyzed a panel of pure oat cultivars using a sandwich ELISA based on the R5 monoclonal antibody (mAb), which binds to canonical epitopes occurring within celiagenic peptides present in triticeae-derived gluten but reportedly not present in avenins. We have identified three varieties of oats that reproducibly bind R5 antibodies and levels indicating the presence of gluten at more than the 20 ppm gluten regulatory threshold. Nested assessment using Western blot analysis and alternative gluten detection systems corroborated these results. Collectively, these data suggest that select oat varieties may prove problematic to patients with CD and to food companies and regulatory agencies and will extend our basic understanding of current gluten detection systems.

Analysis of U.S. Food and Drug Administration food allergen recalls after implementation of the food allergen labeling and consumer protection act

To avoid potentially life-threatening reactions, food allergic consumers rely on information on food labels to help them avoid exposure to a food or ingredient that could trigger a reaction. To help consumers in the United States obtain the information that they need, the Food Allergen Labeling and Consumer Protection Act of 2004 defined a major food allergen as being one of eight foods or food groups and any ingredient that contains protein from one of these foods or food groups. A food that contains an undeclared major food allergen is misbranded under the U.S. Food, Drug, and Cosmetic Act and is subject to recall. Food allergen labeling problems are the most common cause of recalls for U.S. Food and Drug Administration (FDA)-regulated food products. To help understand why food allergen recalls continue to occur at a high rate, information on each food allergen recall that occurred in fiscal years 2007 through 2012 was obtained from the FDA recall database. This information was analyzed to identify the food, allergen, root cause, and mode of discovery for each food allergen recall. Bakery products were the most frequently recalled food type, and milk was the most frequently undeclared major food allergen. Use of the wrong package or label was the most frequent problem leading to food allergen recalls. These data are the first reported that indicate the importance of label and package controls as public health measures.

The regulatory challenge of food allergens

Food allergy is an important public health issue worldwide. Allergic consumers must avoid eating foods that could provoke potentially life-threatening reactions, and successful avoidance depends on having complete and accurate information on food labels. Regulatory agencies support allergic consumers by working with industry to ensure that all food allergens that are intended to be present in a food are declared on the label and that effective controls are used to prevent the presence of unintended allergens. These regulatory activities take place in a complex legal and policy environment both domestically and internationally. Protecting allergic consumers in this complex environment requires effective use of public health data and risk assessments.

A survey of food allergen control practices in the U.S. food industry

Despite awareness of the importance of food allergy as a public health issue, recalls and adverse reactions linked to undeclared allergens in foods continue to occur with high frequency. To reduce the overall incidence of such problems and to ensure that food-allergic consumers have the information they need to prevent adverse reactions, it is important to understand which allergen control practices are currently used by the food industry. Therefore, the U.S. Food and Drug Administration carried out directed inspections of registered food facilities in 2010 to obtain a broader understanding of industry allergen control practices in the United States. The results of these inspections show that allergen awareness and the use of allergen controls have increased greatly in the last decade, but that small facilities lag in implementing allergen controls.

Workshop proceedings: challenges and opportunities in evaluating protein allergenicity across biotechnology industries

A workshop entitled "Challenges and Opportunities in Evaluating Protein Allergenicity across Biotechnology Industries" was held at the 51st Annual Meeting of the Society of Toxicology (SOT) in San Francisco, California. The workshop was sponsored by the Biotechnology Specialty Section of SOT and was designed to present the science-based approaches used in biotechnology industries to evaluate and regulate protein allergenicity. A panel of experts from industry and government highlighted the allergenicity testing requirements and research in the agricultural, pharmaceutical/biopharma, and vaccine biotechnology industries and addressed challenges and opportunities for advancing the science of protein allergenicity. The main learning from the workshop was that immunoglobulin E-mediated allergenicity of biotechnology-derived products is difficult to assess without human data. The approaches currently being used to evaluate potential for allergenicity across biotechnology industries are very different and range from bioinformatics, in vitro serology, in vivo animal testing, in vitro and in vivo functional assays, and "biosimilar" assessments (ie, biotherapeutic equivalents to innovator products). The challenge remains with regard to the different or lack of regulatory requirements for allergenicity testing across industries, but the novel approaches being used with bioinformatics and biosimilars may lead to opportunities in the future to collaborate across biotechnology industries.

Comparison of international food allergen labeling regulations

Food allergy is a significant public health issue worldwide. Regulatory risk management strategies for allergic consumers have focused on providing information about the presence of food allergens through label declarations. A number of countries and regulatory bodies have recognized the importance of providing this information by enacting laws, regulations or standards for food allergen labeling of "priority allergens". However, different governments and organizations have taken different approaches to identifying these "priority allergens" and to designing labeling declaration regulatory frameworks. The increasing volume of the international food trade suggests that there would be value in supporting sensitive consumers by harmonizing (to the extent possible) these regulatory frameworks. As a first step toward this goal, an inventory of allergen labeling regulations was assembled and analyzed to identify commonalities, differences, and future needs.

Determination of protein levels in soy and peanut oils by colorimetric assay and ELISA

Analytical methods are needed for measuring the levels of protein from allergenic food transferred into cooking oil. A simple method for determination of total protein in cooking oils was developed. Oil was extracted with phosphate-buffered saline with 0.05% Tween (PBST) and the extracts were partitioned with hexane to remove residual oil. Total protein in the PBST extracts was assayed with bicinchoninic acid (BCA), micro-BCA, reducing-agent compatible BCA and CB-XT kits. These methods were used to measure recovery of protein from peanut butter spikes of soy and peanut oil in the range of 50-1000 ppm. Recoveries were generally above 70%. However, the BCA and micro-BCA assays were subject to interference and enhanced color formation which were probably due to co-extracted antioxidants present in oil. The reducing agent-compatible BCA and CB-X protein assays reduced interference and gave lower protein values in crude, cold-pressed, and refined peanut oils. Heating oil to 180 degrees C before extraction also reduced interference-induced color enhancement. A commercial ELISA test kit was also used to measure peanut protein in oil spiked with peanut butter. Recovery of peanut residues measured by ELISA was significantly decreased when the peanut butter-spiked oil was heated to 180 degrees C compared to unheated oil. Recovery of spiked peanut butter protein measured by the buffer extraction-colorimetric method was not decreased in heated oil. The method developed here could be used to determine protein levels in crude and refined oil, and to assess the potential for allergen cross-contact from reused cooking oil.

Cleaning and other control and validation strategies to prevent allergen cross-contact in food-processing operations

Food allergies affect an estimated 10 to 12 million people in the United States. Some of these individuals can develop life-threatening allergic reactions when exposed to allergenic proteins. At present, the only successful method to manage food allergies is to avoid foods containing allergens. Consumers with food allergies rely on food labels to disclose the presence of allergenic ingredients. However, undeclared allergens can be inadvertently introduced into a food via cross-contact during manufacturing. Although allergen removal through cleaning of shared equipment or processing lines has been identified as one of the critical points for effective allergen control, there is little published information on the effectiveness of cleaning procedures for removing allergenic materials from processing equipment. There also is no consensus on how to validate or verify the efficacy of cleaning procedures. The objectives of this review were (i) to study the incidence and cause of allergen cross-contact, (ii) to assess the science upon which the cleaning of food contact surfaces is based, (iii) to identify best practices for cleaning allergenic foods from food contact surfaces in wet and dry manufacturing environments, and (iv) to present best practices for validating and verifying the efficacy of allergen cleaning protocols.

Allergen sequence databases

A number of specialized databases have been developed to facilitate studies of human allergens. These include molecular databases focused on protein sequences and structures, informational databases focused on clinical, biochemical and epidemiological data related to protein allergens, a database on allergen nomenclature, and other knowledge bases or informational websites that are peripherally-related to research on allergens. Examples of each type of databases are listed and described briefly in this review. Database construction and maintenance and their impact on database quality and usefulness are also discussed.

Bioinformatics and food allergens

Bioinformatics can play an important role in developing improved technology for the detection and characterization of food allergens. However, the full realization of this potential will depend on the development of allergen-specific databases as well as improved methods for data mining within these databases. Examples of existing allergen databases and analysis tools are described, as are the most important issues that need to be addressed in the next stage of database development.

Computational tools for the study of allergens

Allergy is a major cause of morbidity worldwide. The number of characterized allergens and related information is increasing rapidly creating demands for advanced information storage, retrieval and analysis. Bioinformatics provides useful tools for analysing allergens and these are complementary to traditional laboratory techniques for the study of allergens. Specific applications include structural analysis of allergens, identification of B- and T-cell epitopes, assessment of allergenicity and cross-reactivity, and genome analysis. In this paper, the most important bioinformatic tools and methods with relevance to the study of allergy have been reviewed.

Allergen databases

Allergies represent a significant medical and industrial problem. Molecular and clinical data on allergens are growing exponentially and in this article we have reviewed nine specialized allergen databases and identified data sources related to protein allergens contained in general purpose molecular databases. An analysis of allergens contained in public databases indicates a high level of redundancy of entries and a relatively low coverage of allergens by individual databases. From this analysis we identify current database needs for allergy research and, in particular, highlight the need for a centralized reference allergen database.

Molecular, serological, and virulence characteristics of Vibrio parahaemolyticus isolated from environmental, food, and clinical sources in North America and Asia

Potential virulence attributes, serotypes, and ribotypes were determined for 178 pathogenic Vibrio parahaemolyticus isolates from clinical, environmental, and food sources on the Pacific, Atlantic, and Gulf Coasts of the United States and from clinical sources in Asia. The food and environmental isolates were generally from oysters, and they were defined as being pathogenic by using DNA probes to detect the presence of the thermostable direct hemolysin (tdh) gene. The clinical isolates from the United States were generally associated with oyster consumption, and most were obtained from outbreaks in Washington, Texas, and New York. Multiplex PCR was used to confirm the species identification and the presence of tdh and to test for the tdh-related hemolysin trh. Most of the environmental, food, and clinical isolates from the United States were positive for tdh, trh, and urease production. Outbreak-associated isolates from Texas, New York, and Asia were predominantly serotype O3:K6 and possessed only tdh. A total of 27 serotypes and 28 ribogroups were identified among the isolates, but the patterns of strain distribution differed between the serotypes and ribogroups. All but one of the O3:K6 isolates from Texas were in a different ribogroup from the O3:K6 isolates from New York or Asia. The O3:K6 serotype was not detected in any of the environmental and food isolates from the United States, and none of the food or environmental isolates belonged to any of the three ribogroups that contained all of the O3:K6 and related clinical isolates. The combination of serotyping and ribotyping showed that the Pacific Coast V. parahaemolyticus population appeared to be distinct from that of either the Atlantic Coast or Gulf Coast. The fact that certain serotypes and ribotypes contained both clinical and environmental isolates while many others contained only environmental isolates implies that certain serotypes or ribotypes are more relevant for human disease.

Sequence analysis for assessing potential allergenicity

Sequence analysis plays an important role in assessing the potential allergenicity of proteins used in transgenic foods, particularly for proteins that have not previously been part of the food supply. Sequence comparisons are used to indicate potential unexpected cross reactivity to existing allergens and to assess the potential for developing new sensitivities. Although the concept of using sequence analysis is straightforward, implementing a bioinformatic analysis that is accurate and complete can be complex. Several factors need to be considered, including the design and content of the sequence database, the analysis strategy, and the criteria for evaluating the results.

Automated ribotyping differentiates vibrio parahaemolyticus O3:K6 strains associated with a Texas outbreak from other clinical strains

Automated ribotyping with a Qualicon Riboprinter was used to determine whether clinical isolates of Vibrio parahaemolyticus O3:K6 recovered during two U.S. outbreaks of oyster-associated gastroenteritis in 1998 were related to each other and to a previously identified highly virulent Asian clone of this serotype. The patterns produced using the restriction enzymes Eco RI and Pst I suggest that the outbreak in the Northeastern United States was caused by a single strain closely related to the Asian clone. In contrast, it appears that multiple strains were involved in the Texas outbreak and that the predominant type was genetically distinct from the Northeastern and Asian clone.

Differentiation between types and strains of Clostridium botulinum by riboprinting

The ability of automated ribotyping to differentiate between major types and individual strains of Clostridium botulinum was tested using the Qualicon Riboprinter Microbial Characterization System. Pure spores of C. botulinum type A, proteolytic type B, nonproteolytic type B, and type E strains were inoculated onto modified anaerobic egg yolk agar and incubated 24 h at 35 degrees C. Plates were rinsed with buffer (2 mM Tris + 20 mM EDTA) to remove vegetative cells that were heated for 10 min at 80 degrees C, treated with a lysing agent, and ribotyped in the Qualicon Riboprinter utilizing the enzyme EcoRI. Riboprint patterns were obtained for 30 strains of the four major types of C. botulinum most commonly involved in human foodborne botulism. Proteolytic strains yielded the best and most consistent results. Fifteen ribogroups were identified among the 31 strains tested. Interestingly, in two cases, a single ribogroup contained patterns from isolates belonging to evolutionarily distinct Clostridium lineages. This degree of differentiation between strains of C. botulinum may be useful in hazard analysis and identification, hazard analysis and critical control point monitoring and validation, environmental monitoring, and in inoculation studies.

Ribotype analysis of strain distribution in Listeria monocytogenes

Changes in the temporal and spatial patterns of strain distribution for the foodborne pathogen Listeria monocytogenes were studied by ribotyping using the Qualicon Riboprinter system. Ribotype patterns were obtained by using the restriction enzymes EcoRI and PvuII for 72 isolates of L. monocytogenes recovered from smoked salmon samples over a period of 3 years. Each pattern was classified both by comparison to a pattern library and by comparison among the 72 isolate patterns. Eleven EcoRI-based ribogroups and 16 PvuII groups were identified. Eight of the 11 EcoRI ribogroups were found in isolates obtained over a period of >12 months, and 75% of the EcoRI ribogroups that were found in more than one food sample were distributed nationally. Within the set of isolates, there were 26 instances where more than one isolate was obtained from a single food sample. In 35% of these instances, the co-isolates produced different ribotype patterns, indicating that multiple strains of L. monocytogenes commonly coexist in the same environment. Overall, these data indicate that the population of L. monocytogenes consists of a number of widely dispersed strains with little geographic or temporal stratification.

Specificity of the BAX polymerase chain reaction system for detection of the foodborne pathogen Listeria monocytogenes

The polymerase chain reaction (PCR) can be used for rapid and specific detection of foodborne pathogens. One commercial kit, the Qualicon BAX system uses PCR to detect Listeria monocytogenes in enrichment cultures derived from food and environmental samples. The specificity and sensitivity of the BAX system for detecting L. monocytogenes were characterized by using both pure and mixed cell cultures, and optimal conditions for production of cell lysates were determined. The BAX system was highly specific for L. monocytogenes, and no interference was seen in the presence of either other Listeria species or microbes from other genera. The assay detected L. monocytogenes at 10(5)-10(6) colony-forming units/mL. This sensitivity is adequate for detecting viable cells after enrichment but prevents false-positive signals from nonviable cells.

Evaluation of DNA preparation techniques for detection of the SLT-1 gene of Escherichia coli O157: H7 in bovine faeces using the polymerase chain reaction

The polymerase chain reaction (PCR) has the potential to detect low levels of the human pathogen Escherichia coli O157: H7 in bovine faeces. To improve the utility of PCR for this application, several methods for preparing template DNA from bovine faeces, both directly and after non-selective enrichment, were tested. These were boiling, enzyme treatment, enzyme treatment plus phenol-chloroform extraction, and enzyme treatment plus phenol-chloroform extraction plus Geneclean purification. Of these, the boiling method was the most consistent and had a sensitivity of approximately 3 cfu g-1 faeces, with an assay time of less than 32 h. The boiling method was also combined with immunomagnetic separation (IMS) to detect E. coli O157: H7 in less than 8 h, but with a sensitivity of approximately 10(3) cfu g-1 faeces. These methods can be used to prepare template for PCR screening of bovine faeces using any appropriate PCR primers.

Effects of sodium saccharin and linoleic acid on mRNA levels of Her2/neu and p53 in a human breast epithelial cell line

The effects of two food-related chemicals (sodium saccharin and linoleic acid) on the levels of Her2/neu and p53 mRNA in a non-cancerous human breast epithelial cell line (HBL-100) were tested in comparison with the effects of the known tumor promoter phorbol 12-myristate 13-acetate (TPA). Treatments were made both with and without prior treatment with two well-known tumor initiators, N-nitroso-N-methylurea (NMU) or 7,12-dimethylbenz[a]anthracene (DMBA). The effects in general were small, the greatest being increases of 46-67% in Her2/neu mRNA levels in response to treatments with TPA or sodium saccharin following NMU treatments. These results demonstrate that sodium saccharin following NMU treatments might be involved in transcriptional regulation of Her2/neu in HBL-100 cells and suggest that its effects may not be limited to urinary bladder.

Impedance based sensing of the specific binding reaction between Staphylococcus enterotoxin B and its antibody on an ultra-thin platinum film

Immunobiosensing techniques to measure specific antigen-antibody binding reactions are important in the development of biosensor applications in biotechnology, in vitro diagnosis, medicine and food technology. An immunobiosensor was constructed to measure the specific binding reaction between Staphylococcus enterotoxin B (SEB) and anti-SEB antibodies. The biosensor comprised an anti-SEB bioactive layer covalently immobilized on an ultra-thin platinum (Pt) film sputtered onto a 100 nm thick silicon dioxide layer on a silicon chip. The Pt film was discontinuous with a normal thickness of 25 A. The impedance of the Pt film decreased during the binding of the anti-SEB to SEB in phosphate buffered saline (PBS) at room temperature. The impedance decreases were irreversible in PBS before saturation of the specific binding sites. When saturated, the impedance at 100 Hz was 14% of the value obtained for the fresh anti-SEB layer in PBS. The magnitude of the impedance (Z) decrease followed a simple relationship with SEB concentration in the range between 0.389 and 10.70 ng/ml SEB. The specificity of the biosensor was demonstrated by showing that no irreversible impedance decreases occurred when the sensor was exposed to 100 ng/ml kappa-casein, or alpha-lactalbumin, in PBS.

Fluorescent Antibodies Applied to Direct Epifluorescent Filter Technique for Microscopic Enumeration of Escherichia coli 0157:H7 in Milk and Juice

In a modification of the direct epifluorescent filter technique (DEFT), direct fluorescent antibody staining was used for the rapid (<1 h), specific enumeration of foodbome Escherichia coli 0157:H7 by epifluorescence microscopy. Cell counts obtained by this method (Ab-DEFT) correlated well with DEFT counts obtained by acridine orange staining and with viable plate counts ranging from 10⁸ to 10¹ cells per ml for pure cultures in buffer. Ab-DEFT also was effective for counting E. coli 0157:H7 cells inoculated into milk and juice; the sensitivity limit was about 10³ for milk. The highly specific nature of the technique was demonstrated by enumeration of E. coli 0157:H7 cells in the presence of large numbers of indigenous spoilage microorganisms in milk. This is the first known demonstration of the combination of DEFT and antibody probe technology for the specific enumeration of a microbe directly in food without a growth or enrichment step.

Effects of staining of RNA with ethidium bromide before electrophoresis on performance of northern blots

Staining of RNA with ethidium bromide (EtdBr) prior to running agarose gels has been reported to afford certain advantages over staining gels after electrophoresis. We have examined prior staining of RNA with a wide range of EtdBr concentrations, particularly with respect to its effects on Northern blot hybridizations using antisense RNA probes. Prior staining with EtdBr at concentrations of 100-1000 micrograms/ml caused significant alterations in RNA mobilities and significantly decreased hybridization with antisense RNA probes compared with unstained RNA. Prior staining with EtdBr at 10-50 micrograms/ml resulted in the best combination of staining sensitivity, absence of alterations in RNA mobility and efficiency of hybridization. Conventional staining of gels after electrophoresis also resulted in decreased hybridization efficiency with RNA probes compared with unstained RNA.

Instability of Tn5 in a plasmid cloning vector for the cyanobacterium Anacystis nidulans

Transposon Tn5 was used to produce insertions within the region of a cyanobacterial shuttle vector previously identified as necessary for transformation of Anacystis nidulans. These transposon-containing plasmids were used to transform a plasmid-cured derivative of Anacystis strain R2 and tested for structural stability of the transforming plasmid. The transposon DNA was deleted from all the plasmids containing Tn5 within the cyanobacterial replication region. Inserts in the vector DNA were physically stable and expressed the kanr gene. The internal Tn5 HindIII fragment was also cloned into each of the three HindIII sites in the shuttle plasmid. Inserts in two of these sites were stable, whereas inserts into the third site were not.

Polyelectrolyte precipitation of β-galactosidase fusions containing poly-aspartic acid tails

Protein recovery from industrial microbial processes can be very expensive, often exceeding the cost of protein production. We have genetically engineered 3 beta-galactosidase (beta-gal) fusion proteins containing poly-aspartic acid tails to test the effect of the tails on recovery by the relatively inexpensive method of polyelectrolyte precipitation. The fusion proteins, designated T1, T2, and T3, were constructed with C-terminal tails of 5, 11, and 16 aspartic acid residues, respectively. The fusion proteins were expressed in Escherichia coli, and purified by affinity chromatography. T1 and T2 had specific activities similar to that of wildtype beta-gal, whereas the specific activity of T3 was about half that of T1 and T2. The increased net charge of the fusion proteins compared to wildtype beta-gal was indicated both by ion-exchange chromatography and their migration pattern in non-denaturing polyacrylamide gel electrophoresis. All three tails enhanced polyethyleneimine (PEI) precipitation of the fusion proteins compared to wildtype beta-gal. At a low PEI/protein ratio (0.01, g g-1), recovery by precipitation of T2 and T3 was more than 2 X that of the beta-gal control, whereas that of T1 was only slightly greater than that of the control. At a higher PEI/protein ratio (0.03, g g-1) the amount of precipitation of all three fusion proteins was nearly the same, about 1.5 X that of the control.

Instability of Tn5 inserts in cyanobacterial cloning vectors

Transposon Tn5 was used to produce random insertions in two hybrid cloning vectors for the unicellular cyanobacterium Anacystis nidulans. The transposon-containing plasmids were used to localize essential replication functions and to characterize the stability of large inserts in these vectors. The effect of the insertions on plasmid function was tested by transformation into a derivative of A. nidulans that had been cured of the endogenous plasmid used to construct the vectors. A region of approximately 4 kilobases was essential for successful plasmid transformation and replication. This region has also been shown to be involved in plasmid replication by deletion analysis. High rates of excision of Tn5 inserts within this region and restoration of normal replication function were observed when transformants were selected by using a resistance marker outside the replication region in the absence of selection for the transposon-coded kanamycin resistance. Transposon inserts outside this region were not deleted.