Adaptation response of Pseudomonas fragi on refrigerated solid matrix to a moderate electric field

BACKGROUND

Moderate electric field (MEF) technology is a promising food preservation strategy since it relies on physical properties-rather than chemical additives-to preserve solid cellular foods during storage. However, the effectiveness of long-term MEF exposure on the psychrotrophic microorganisms responsible for the food spoilage at cool temperatures remains unclear.

RESULTS

The spoilage-associated psychrotroph Pseudomonas fragi MC16 was obtained from pork samples stored at 7 °C. Continuous MEF treatment attenuated growth and resulted in subsequent adaptation of M16 cultured on nutrient agar plates at 7 °C, compared to the control cultures, as determined by biomass analysis and plating procedures. Moreover, intracellular dehydrogenase activity and ATP levels also indicated an initial effect of MEF treatment followed by cellular recovery, and extracellular β-galactosidase activity assays indicated no obvious changes in cell membrane permeability. Furthermore, microscopic observations using scanning and transmission electron microscopy revealed that MEF induced sublethal cellular injury during early treatment stages, but no notable changes in morphology or cytology on subsequent days.

CONCLUSION

Our study provides direct evidence that psychrotrophic P. fragi MC16 cultured on nutrient agar plates at 7 °C are capable of adapting to MEF treatment.

Toward understanding the mechanism of chromophore-assisted laser inactivation--evidence for the primary photochemical steps

Chromophore-assisted laser inactivation (CALI) is a light-mediated technique used to selectively inactivate proteins of interest to elucidate their biological function. CALI has potential applications to a wide array of biological questions, and its efficiency allows for high-throughput application. A solid understanding of its underlying photochemical mechanism is still missing. In this study, we address the CALI mechanism using a simplified model system consisting of the enzyme beta-galactosidase as target protein and the common dye fluorescein. We demonstrate that protein photoinactivation is independent from dye photobleaching and provide evidence that the first singlet state of the chromophore is the relevant transient state for the initiation of CALI. Furthermore, the inactivation process was shown to be dependent on oxygen and likely to be based on photooxidation of the target protein via singlet oxygen. The simple model system used in this study may be further applied to identify and optimize other CALI chromophores.

A Simple Strategy for the Purification of Large Thermophilic Proteins Overexpressed in Mesophilic Microorganisms: Application to Multimeric Enzymes from Thermus sp. Strain T2 Expressed in Escherichia coli

The heating of protein preparations of mesophilic organism (e.g., E. coli) produces the obliteration of all soluble multimeric proteins from this organism. In this way, if a multimeric enzyme from a thermophilic microorganism is expressed in these mesophilic hosts, the only large protein remaining soluble in the preparation after heating is the thermophilic enzyme. These large proteins may be then selectively adsorbed on lowly activated anionic exchangers, enabling their full purification in just these two simple steps. This strategy has been applied to the purification of an alpha-galactosidase and a beta-galactosidase from Thermus sp. strain T2, both expressed in E. coli, achieving the almost full purification of both enzymes in only these two simple steps. This very simple strategy seems to be of general applicability to the purification of any thermophilic multimeric enzyme expressed in a mesophilic host.

Partial characterization of SUVi, a new mammalian gene induced by UV-C and expressed during the S phase of the cell cycle

By using a lacZ-based gene-trap approach, we identified a mammalian gene induced by UV-C in a Chinese hamster ovary cell clone (Menichini P et al. [1997]: Nucleic Acids Res 25:4803-4807). The activity of the encoded protein fused to a bacterial beta-galactosidase was followed through the hydrolysis of different beta-galactosidase substrates. In this study we describe how the expression of this gene is modulated during the cell cycle and in response to UV-irradiation. We show that the beta-galactosidase activity was virtually undetectable in quiescent cells (G[0]), started to increase when cells progressed in G(1), and reached a maximum in mid-S phase, indicating a possible role of the endogenous protein during DNA synthesis. Following UV-irradiation, besides a delay of the progression through the S phase, a twofold increase of the reporter protein activity in all phases of the cell cycle was observed. The partial sequence analysis showed that this gene, here named SUVi (for S phase UV-inducible), contains a domain that is highly conserved among different helicases. Together, these data suggest that the SUVi gene could be involved in DNA synthesis, a process that takes place both in the S phase and in the processing of UV-induced damage.

p53 deficiency alters the yield and spectrum of radiation-induced lacZ mutants in the brain of transgenic mice

Exposure to heavy particle radiation in the galacto-cosmic environment poses a significant risk in space exploration and the evaluation of radiation-induced genetic damage in tissues, especially in the central nervous system, is an important consideration in long-term manned space missions. We used a plasmid-based transgenic mouse model system, with the pUR288 lacZ transgene integrated in the genome of every cell of C57Bl/6(lacZ) mice, to evaluate the genetic damage induced by iron particle radiation. In order to examine the importance of genetic background on the radiation sensitivity of individuals, we cross-bred p53 wild-type lacZ transgenic mice with p53 nullizygous mice, producing lacZ transgenic mice that were either hemizygous or nullizygous for the p53 tumor suppressor gene. Animals were exposed to an acute dose of 1 Gy of iron particles and the lacZ mutation frequency (MF) in the brain was measured at time intervals from 1 to 16 weeks post-irradiation. Our results suggest that iron particles induced an increase in lacZ MF (2.4-fold increase in p53+/+ mice, 1.3-fold increase in p53+/- mice and 2.1-fold increase in p53-/- mice) and that this induction is both temporally regulated and p53 genotype dependent. Characterization of mutants based on their restriction patterns showed that the majority of the mutants arising spontaneously are derived from point mutations or small deletions in all three genotypes. Radiation induced alterations in the spectrum of deletion mutants and reorganization of the genome, as evidenced by the selection of mutants containing mouse genomic DNA. These observations are unique in that mutations in brain tissue after particle radiation exposure have never before been reported owing to technical limitations in most other mutation assays.

SOS induction by gamma-radiation in Escherichia coli strains defective in repair and/or recombination mechanisms

Ionizing radiation causes several types of DNA lesions, mainly single- or double-strand breaks and base damage. By means of the chromotest, an assay that allows the level of the SOS response to be monitored via beta-galactosidase enzymatic activity, the roles of several repair (uvrA, recN and oxyR) and recombination (recB, recJ and recO) genes in the response of Escherichia coli to gamma-radiation were studied. The results indicate that all the repair- and recombination-deficient strains were more sensitive to the lethal effects of ionizing radiation. However, the SOS activation pattern was somewhat different. The minimal inducing dose in uvrA and recN mutants was lower than in the wild-type, whereas their SOS response was higher at all doses. Conversely, in the strains lacking an active recB, recJ or recO gene, the doubling dose was almost the same as in the wild-type but the level of induction remained stable over a wide dose range. These findings suggest that neither single- nor double-strand breaks are in themselves direct SOS inducers and that while uvrA, recN and oxyR take part in different repair or protective pathways, apparently recB, recJ and recO participate in damage processing leading to SOS induction, as well as in recombination repair.

Heat-shock enhanced reactivation of a UV-damaged reporter gene in human cells involves the transcription coupled DNA repair pathway

A recombinant nonreplicating human adenovirus type 5, Ad5HCMVsp1lacZ, expressing the lacZ gene under control of the human cytomegalovirus immediate early promoter, was used to assess the effect of heat-shock (HS) on DNA repair of a UV-damaged reporter gene. Host cell reactivation (HCR) of beta-galactosidase (beta-gal) activity for UV-irradiated Ad5HCMVsp1lacZ was used as an indicator of DNA repair in the transcribed strand of an active gene. Repair was examined in heat-shock (HS) pretreated and mock-treated normal fibroblasts, normal lung epithelial cells, xeroderma pigmentosum group A, C, D and G fibroblasts (XP-A, XP-C, XP-D and XP-G), Cockayne's syndrome group A fibroblasts (CS-A), SV40-transformed normal fibroblasts (GM637f) and 5 tumour cell lines (SKOV-3, HeLa, HT29, SCC-25 and U20S). HS enhanced reactivation (HSER) of the reporter gene was detected in normal cells, HT29 tumour cells and XP-C fibroblasts. HSER was reduced or absent in all other XP, CS and tumour cell lines tested. HSER in normal and XP-C cell lines, but not CS-A, XP-A, XP-D or XP-G cells, suggests that HS treatment can enhance the repair of UV-damaged DNA through an enhancement of transcription coupled repair (TCR) or a mechanism which involves the TCR pathway. Since this response was absent in the SV40-transformed fibroblast cell line and 4 of 5 tumour cell lines examined, HSER of beta-gal activity for UV-irradiated Ad5HCMVsp1lacZ also requires some cellular function(s) affected by transformation.

Human lysosomal beta-galactosidase-cathepsin A complex: definition of the beta-galactosidase-binding interface on cathepsin A

Human lysosomal beta-galactosidase is organized as a 680-kDa complex with cathepsin A (also named carboxypeptidase L and protective protein), which is necessary to protect beta-galactosidase from intralysosomal proteolysis. To understand the molecular mechanism of beta-galactosidase protection by cathepsin A, we defined the structural organization of their complex including the beta-galactosidase-binding interface on cathepsin A. Radiation inactivation analysis suggested the existence of a 168-kDa structural subunit of the complex containing both beta-galactosidase and cathepsin A. Chemical cross-linking of the complex confirmed the existence of this subunit and showed that it is composed of one cathepsin A dimer and one beta-galactosidase monomer. The modeling of the cathepsin A dimer tertiary structure based on atomic coordinates of a wheat carboxypeptidase suggested a putative beta-galactosidase-binding cavity formed by the association of two cathepsin A monomers. According to this model two exposed loops of cathepsin A bordering the cavity were chosen as part of a putative beta-galactosidase-binding interface. Synthetic peptides corresponding to these loops were found both to dissociate the complex and to inhibit its in vitro reconstitution from purified cathepsin A and beta-galactosidase. The defined location of the GAL monomer in the complex with 35% of its surface covered by the CathA dimer may explain the stabilizing effect of CathA on GAL in lysosome.

Role of vitamin A in modulating the radiation-induced changes in intestinal disaccharidases of rats exposed to multifractionated gamma-radiations

PURPOSE

The aim of this investigation was to determine whether pre- or post-administration of vitamin A will be effective in reducing the radiation-induced alterations in intestinal disaccharidases in rats.

MATERIAL AND METHODS

Rats were subjected to fractionated whole-body irradiation (20 x 0.5 Gy). Intestinal lactase activity as well as maltase and sucrase activities were assessed. Vitamin A was administered at daily intraperitoneal dose of 15,000 IU/kg body weight for 7 days prior to radiotherapy and thereafter twice weekly throughout therapy up to 7 days post irradiation.

RESULTS

In irradiated rats a marked decrease in intestinal lactase activity to about one-fourth of those in non-irradiated rats was observed. In addition, a significant reduction in maltase and sucrase activities to one half of the control group was observed. The application of vitamin A significantly improved the radiation-induced inhibition of intestinal enzymes. Pretreatment application of vitamin A is more efficient to protect against radiation injury than a posttreatment application.

CONCLUSIONS

The usage of vitamin A for modulation of radiation-induced changes in intestinal enzymes provides sufficient protection against treatment side effects induced by large volume radiotherapy.

The Azotobacter vinelandii recA gene: sequence analysis and regulation of expression

The nucleotide (nt) sequence of the Azotobacter vinelandii recA gene (Av-recA) was determined and compared with the recA sequences from Pseudomonas aeruginosa (Pa-recA), a soil bacterium, and Escherichia coli (Ec-recA), an enteric bacterium. The Av-recA gene and the deduced aa sequence were found to be more similar to their Pa-recA counterparts than to the Ec-recA gene and protein. Expression of Av-recA was found to be autoregulatory. Unlike Ec-recA and Pa-recA, however, expression of Av-recA was weakly enhanced upon DNA damage. In E. coli, expression of an Av-recA::lacZ fusion was poor, but its autoregulation was similar to that of Ec-recA. Av-recA expression, however, could not induce the repair system response in E. coli.

Inactivation mechanism of tetrameric beta-galactosidase by gamma-rays involves both fragmentation and temperature-dependent denaturation of protomers

The radiation inactivation method is widely used to estimate the molecular size of membrane-bound enzymes, receptors, and transport systems in situ. The method is based on the principle that exposure of frozen solutions or lyophilized protein preparations to increasing doses of ionizing radiations results in a first-order decay of biological activity proportional to radiation inactivation size of the protein. This parameter is believed to reflect the "functional unit" of the protein defined as the minimal assembly of structure (protomers) required for expression of a given biological activity. We tested the functional unit as a concept to interpret radiation inactivation data of proteins with Escherichia coli beta-galactosidase, where the protomers are active only when associated in a tetramer. Gamma-Irradiation of beta-galactosidase at both -78 and 38 degrees C followed by quantitation of the residual unfragmented promoter band by SDS-polyacrylamide gel electrophoresis yielded the protomer size, indicating that only one protomer is fragmented by each radiation hit. By following the enzyme activity as a function of dose it was found that only the protomer that has been directly hit and fragmented at -78 degrees C was effectively inactivated. In contrast, at 38 degrees C, it was the whole tetramer that was inactivated. beta-Galactosidase cannot have two different functional units depending on temperature. The inactivation of the whole beta-galactosidase tetramer at 38 degrees C is in fact related to protomer fragmentation but also to the production of stable denatured protomers (detected by gel-filtration HPLC and differential UV spectroscopy) due to energy transfer from fragmented protomers toward unhit protomers.(ABSTRACT TRUNCATED AT 250 WORDS)

Deletions induced by gamma rays in the genome of Escherichia coli

An Escherichia coli lysogen was constructed with a lambda phage bearing a lacZ gene surrounded by about 100 x 10(3) base-pairs of dispensable DNA. The lacZ mutants induced by gamma rays in this lysogen were more than 10% large deletions, ranging in size from 0.6 x 10(-3) to 70 x 10(3) base-pairs. These deletions were centered, not on lacZ, but on a ColE1 origin of DNA replication located 1.2 x 10(3) bases downstream from lacZ. This suggested that this origin of replication was involved in the process by which the deletions were formed. In agreement with this hypothesis, a lysogen of the same phage without the ColE1 origin showed a very much lower percentage of radiation-induced deletions, as did a second lysogen of a lambda phage without any known plasmid origin of replication. Indirect evidence is presented for radiation-induced deletions centered on the lambda origin of DNA replication in a lysogen. It is suggested that high percentages of large deletions may occur among radiation-induced mutations in mammalian cells because deletions centered on some of the thousands of origins of replication in these genomes do not kill the cells.

Sensitive model with which to detect athermal effects of non-ionizing electromagnetic radiation

To clarify the potential of non-ionizing electromagnetic radiation to cause biological effects by athermal mechanisms, and to initiate elucidation of those mechanisms, a model system amenable to scrutiny at the molecular level has been designed and characterized. Assessment of beta-galactosidase activity in E. coli JM101 containing the plasmid pUC8 provides a sensitive assay with many important advantages. The ability to examine at the molecular level each of the processes involved in producing beta-galactosidase should permit elucidation of the molecular mechanism(s) that give rises to an observed effect.

Glucose-6-phosphate dehydrogenase from Leuconostoc mesenteroides is a reliable internal standard for radiation-inactivation studies of membranes in the frozen state

The target size of four soluble enzymes (beta-galactosidase, pyruvate kinase, alcohol dehydrogenase, and glucose-6-phosphate dehydrogenase) in the presence or absence of subcellular membrane fractions has been determined by the radiation-inactivation method using samples in the frozen state. For each of the four enzymes, full activity was recovered after freezing and thawing in the absence of radiation. We found minimal (less than 20%) binding of the enzymes to either submitochondrial vesicles or sarcoplasmic reticulum vesicles. Under the conditions tested, beta-galactosidase, pyruvate kinase, and alcohol dehydrogenase exhibited target sizes which varied according to the experimental conditions, i.e., the buffer selected and also the presence or absence of membrane preparations. For these tetrameric enzymes, the target sizes were generally comparable to either a monomer or a dimer. By contrast, the target size of glucose-6-phosphate dehydrogenase from Leuconostoc mesenteroides was found to be essentially invariant when frozen in a variety of buffers and in the presence or absence of either cryoprotectant (sucrose or glycerol) or different membrane preparations. The target size from 19 separate determinations gave an average value of 104 +/- 16 kDa, which is comparable to the molecular weight of the enzyme (104 kDa). We conclude that glucose-6-phosphate dehydrogenase from L. mesenteroides is a reliable internal standard for radiation-inactivation studies of membrane preparations in the frozen state.

Suppression of induced beta-galactosidase synthesis by cysteamine and its reversion by gamma-irradiation in the presence of ascorbate

The induced synthesis of beta-galactosidase in E. coli was found to be inhibited by cysteamine. This inhibitory effect of the SH compound was antagonized by the addition of ascorbate followed by gamma-irradiation with relatively low doses. The cAMP level which, it has been suggested, plays a role in the radioprotective action of cysteamine, is stabilized by ascorbate against changes induced by irradiation.