Analysis of a metalloporphyrin antioxidant mimetic (MnTE-2-PyP) as a radiomitigator: prostate tumor and immune status

Due to radiation-induced immune depression and development of pathologies such as cancer, there is increasing urgency to identify radiomitigators that are effective when administered after radiation exposure. The main goal of this study was to determine the radiomitigation capacity of MnTE-2-PyP[Mn(III) tetrakis (N-ethylpyridinium-2-yl) porphyrin], a superoxide dismutase (SOD) mimetic, and evaluate leukocyte parameters in spleen and blood. C57BL/6 mice were total-body exposed to 2 Gy γ-rays (Co-60), i.e., well below a lethal dose, followed by subcutaneous implantation of 5 × 10(5) RM-9 prostate tumor cells and initiation of MnTE-2-PyP treatment (day 0); interval between each procedure was 1-2 h. The drug was administered daily (12 times). Tumor progression was monitored and immunological analyses were performed on a subset per group on day 12. Animals treated with MnTE-2-PyP alone had significantly slower tumor growth compared to mice that did not receive the drug (P < 0.05), while radiation alone had no effect. Treatment of tumor-bearing mice with MnTE-2-PyP alone significantly increased spleen mass relative to body mass; the numbers of splenic white blood cells (WBC) and lymphocytes (B and T), as well as circulating WBC, granulocytes, and platelets, were high compared to one of more of the other groups (P < 0.05). The results show that MnTE-2-PyP slowed RM-9 tumor progression and up-regulated immune parameters, but mitigation of the effects of 2 Gy total-body irradiation were minimal.

Improved performance control on the Grid

In the 2005 paper by Mayes et al., a threshold-based performance control system (PerCo) was described and an initial experimental evaluation was presented. The objective of the current paper is to investigate the role of the threshold value in PerCo and to place the threshold-based rescheduling heuristic on a more principled footing. Simulation enables us to identify the 'optimal' threshold value for a particular application scenario, and we show that this optimal value results in a 10 per cent improvement in performance for the application considered by Mayes et al. Furthermore, we find that the execution time of this optimal threshold-based schedule is very close (within 0.5%) to the execution time that results from a linear programming optimal schedule.

Rat sinusoidal liver endothelial cells (SECs) produce pro-fibrotic factors in response to adducts formed from the metabolites of ethanol

Previous studies with alcohol-associated malondialdehyde-acetaldehyde (MAA)-modified proteins have demonstrated an increase in the expression of adhesion molecules, and the secretion of pro-inflammatory cytokines/chemokines by rat sinusoidal liver endothelial cells (SECs). However, no studies have been initiated to examine the effects of MAA-modified proteins on the expression of the extracellular matrix (ECM) protein, fibronectin and its isoforms. For these studies, SECs were isolated from the liver of normal rats, and exposed to MAA-modified bovine serum albumin (MAA-Alb). At selected time points, the total plasma and cellular fibronectin were determined by Western blot. Injection of rat liver via the mesenteric vein with MAA-Alb was performed in an effort to evaluate the potential in vivo role of MAA-modified proteins in the development of fibrosis. Expression of both plasma and cellular fibronectin was significantly increased over controls in the MAA-Alb stimulated SECs (>3-fold). Importantly, the isotype of fibronectin secreted was determined to be of the EIIIA variant and not EIIIB. These data were confirmed using RT-PCR procedures on liver tissue from; isolated SECs, and from an in vivo animal model wherein MAA-Alb was administered via the mesenteric vein. Thus, these studies demonstrate that MAA-modified proteins initiate a pro-fibrogenic response by initiating the expression of the fibronectin EIIIA isoform by SECs.

Scavenger receptors on sinusoidal liver endothelial cells are involved in the uptake of aldehyde-modified proteins

Scavenger receptors on sinusoidal liver endothelial cells (SECs) eliminate potentially harmful modified proteins circulating through the liver. It was shown recently that aldehyde-modified proteins bind to scavenger receptors and are associated with the development/progression of alcoholic liver diseases. For these studies, rat livers were perfused in situ with 125I-formaldehyde-bovine serum albumin (f-Alb) or 125I-malondialdehyde-acetaldehyde-bovine serum albumin (MAA-Alb) in the presence of known scavenger receptor ligands as inhibitors. Reverse transcription-polymerase chain reaction (RT-PCR) analysis and scavenger receptor Type A (SRA) knock-out mice were used to assess the role of these receptors in mediating immune responses. The degradation of 125I-f-Alb or 125I-MAA-Alb in whole livers and isolated SECs can be inhibited by known scavenger receptor ligands, including f-Alb, maleylated bovine albumin, and fucoidan. 125I-f-Alb could not be completely inhibited by MAA-Alb. In contrast, 125I-MAA-Alb was only partially inhibited with advanced glycosylated endproduct albumin. RT-PCR data show the presence of a number of scavenger receptors on SECs that may be responsible for the binding of MAA-modified proteins. SRA seems to be one of these receptors involved in the effects mediated by MAA-modified proteins. In a study using SRA knockout mice, it was shown that a decreased antibody response to MAA-Alb resulted. By RT-PCR, CD36, LOX-1, and SR-AI are the scavenger receptors most likely involved in the degradation of MAA-Alb.

Aldehydes in cigarette smoke react with the lipid peroxidation product malonaldehyde to form fluorescent protein adducts on lysines

Cigarette smoke is a risk factor for the development of several diseases, but the exact mechanism responsible has not been well-characterized. Because modification, or adducting, of biomolecules is thought to mediate the toxic effects observed from exposure to a wide variety of harmful chemicals, this study investigated the ability of cigarette smoke to produce specific adducts on a peptide to gain insight into the likely effect on cellular proteins. We describe the modification of the epsilon-amino group of lysine contained in a test peptide with stable fluorescent adducts derived from monofunctional aldehydes occurring in cigarette smoke and malonaldehyde, a product of lipid peroxidation. Utilizing high-performance liquid chromatography, fluorescent measurements, and matrix-assisted laser desorption/ionization time-of-flight mass spectroscopy, the 1,4-dihydropyridine-3,5-dicarbaldehyde and 4-methyl-1,4-dihydropyridine-3,5-dicarbaldehyde derivatives of lysine were identified as products of exposure to cigarette smoke extract and malonaldehyde. These data suggest that cigarette smoke may promote the modification of proteins, like those associated with oxidized low-density lipoprotein, and may contribute to smoking-related disease.

Halothane potentiates the alcohol-adduct induced TNF-alpha release in heart endothelial cells

Background

The possibility exists for major complications to occur when individuals are intoxicated with alcohol prior to anesthetization. Halothane is an anesthetic that can be metabolized by the liver into a highly reactive product, trifluoroacetyl chloride, which reacts with endogenous proteins to form a trifluoroacetyl-adduct (TFA-adduct). The MAA-adduct which is formed by acetaldehyde (AA) and malondialdehyde reacting with endogenous proteins, has been found in both patients and animals chronically consuming alcohol. These TFA and MAA-adducts have been shown to cause the release of inflammatory products by various cell types. If both adducts share a similar mechanism of cell activation, receiving halothane anesthesia while intoxicated with alcohol could exacerbate the inflammatory response and lead to cardiovascular injury.

Methods

We have recently demonstrated that the MAA-adduct induces tumor necrosis factor-α (TNF-α) release by heart endothelial cells (HECs). In this study, pair and alcohol-fed rats were randomized to receive halothane pretreatments intra peritoneal. Following the pretreatments, the intact heart was removed, HECs were isolated and stimulated with unmodified bovine serum albumin (Alb), MAA-modified Alb (MAA-Alb), Hexyl-MAA, or lipopolysaccharide (LPS), and supernatant concentrations of TNF-α were measured by ELISA.

Results

Halothane pre-treated rat HECs released significantly greater TNF-α concentration following MAA-adduct and LPS stimulation than the non-halothane pre-treated in both pair and alcohol-fed rats, but was significantly greater in the alcohol-fed rats.

Conclusion

These results demonstrate that halothane and MAA-adduct pre-treatment increases the inflammatory response (TNF-α release). Also, these results suggest that halothane exposure may increase the risk of alcohol-induced heart injury, since halothane pre-treatment potentiates the HEC TNF-α release measured following both MAA-Alb and LPS stimulation.

Lipopolysaccharide is a cofactor for malondialdehyde-acetaldehyde adduct-mediated cytokine/chemokine release by rat sinusoidal liver endothelial and Kupffer cells

BACKGROUND

The nonparenchymal cells of the liver have been suggested to play a significant role in the inflammatory processes observed in the development and/or progression of alcoholic liver disease. Our laboratories have shown that malondialdehyde-acetaldehyde (MAA)-modified proteins can induce immune responses, cytokine/chemokine secretion, and antigen processing and presentation by liver sinusoidal endothelial cells (SECs). Another molecule that has been shown to induce similar types of responses in Kupffer cells (KCs) is lipopolysaccharide (LPS). Because these materials induce similar responses, it was the purpose of this study to investigate the relationship between LPS and MAA-modified proteins in the development of proinflammatory responses by SECs and KCs.

METHODS

For these studies, SECs and KCs were isolated from chow-fed, pair-fed, and ethanol-fed rats. Cells were stimulated with media alone, bovine serum albumin (Alb), or MAA-modified Alb (MAA-Alb) in the presence or absence of LPS 1 ng/ml, and the supernatants were assayed by enzyme-linked immunosorbent assay for tumor necrosis factor alpha, macrophage chemotactic protein 1, and macrophage inhibitory protein.

RESULTS

All three cytokines/chemokines were 3 to 5 times higher when SECs or KCs were stimulated by MAA-Alb in the presence of LPS, in contrast to cells stimulated with Alb or media in the presence of LPS. Chronic ethanol consumption (6 weeks) had variable effects on the secretion of these cytokines/chemokines but in general did not alter the increased secretion in response to MAA-Alb in the presence of LPS.

CONCLUSIONS

These studies strongly suggest that the sensitization of SECs and KCs by LPS plays a significant role in the development and/or progression of alcoholic liver disease, and the subsequent activation by MAA-modified proteins may be a mechanism by which proinflammatory processes are initiated.

Immunologic mechanisms of alcoholic liver injury

Clinically, the association of alcoholic liver disease (ALD) with circulating autoantibodies, hypergammaglobulinemia, antibodies to unique hepatic proteins, and cytotoxic lymphocytes reacting against autologous hepatocytes strongly suggests altered immune regulation with an increased activity toward normal self-proteins (loss of tolerance). Experimentally, there are several immune responses generated specifically recognizing self-proteins that are modified by metabolites of alcohol. These data strongly suggest that immune reactions may play a significant role in inducing and sustaining an inflammatory cascade of tissue damage to the liver. Additional support for this comes from the observation that the histological appearance of livers with ALD is that of a chronic active hepatitis-like inflammatory disease. Therefore, the hypothesis that immune mechanisms are involved in recurrent alcoholic hepatitis, although not summarily proven, is reasonable, supported by clinical and experimental evidence, and the subject of this article.

N-(methylamino)isobutyric acid inhibits proliferation of CFSC-2C hepatic stellate cells

Activation of hepatic stellate cells (HSCs) involves the induction of ECM protein synthesis and rapid cell proliferation. Thus, agents that interfere with either process could potentially mitigate the development of liver disease by reducing the synthesis of proteins associated with fibrosis or by reducing the number of activated HSC. Previously, we described that the non-metabolizable amino acid analog N-(methylamino)isobutyric acid (MeAIB) reduced hepatic collagen content of rats in a model of CCl(4)-induced liver injury, and in vitro studies using CFSC-2G cells indicated that MeAIB directly reduced collagen synthesis. However, the MeAIB-mediated reduction of hepatic collagen, in vivo, following liver injury was associated with a decrease in hepatic alpha-smooth muscle actin (alpha-SMA) which suggested that MeAIB also inhibited the activation of HSCs. Because HSC activation is inseparable from proliferation, the purpose of this study was to examine the effect of MeAIB treatment on the proliferation of HSCs in an in vitro model utilizing CFSC-2G cell cultures. In these studies, MeAIB effectively inhibited the proliferation of CFSC-2G cells by interfering with the progression of the cells through the G(1)-phase of the cell cycle which delayed entry into S-phase. MeAIB prevented the phosphorylation of p70S6 kinase (p70S6K) at Thr389 and reduced the phosphorylation at Thr421/Ser424. Because p70S6K is required for G(1)-cell cycle progression and is known to be regulated by nutrient availability, this correlates well with MeAIB interfering with the proliferation of CFSC-2G HSCs. In addition, the rate of protein synthesis was reduced by MeAIB treatment following mitogenic stimulation, which agrees with a p70S6K-mediated reduction in translation. These data are consistent with MeAIB inhibiting the proliferation of CFSC-2G cells by altering the mitogen activated pathway(s) leading to phosphorylation of p70S6K by a yet to be described mechanism.

Mechanisms of alcohol liver damage: aldehydes, scavenger receptors, and autoimmunity

While most of the investigations into the causative events in the development of alcoholic liver disease (ALD) have been focused on multiple factors, increasing interest has centered around the possible role of immune mechanisms in the pathogenesis and perpetuation of ALD. This is because many of the clinical features of ALD suggest that immune effector mechanisms may be contributing to liver tissue damage, as evidenced by the detection of circulating autoantibodies, and the presence of CD4+ and CD8+ lymphoid cells in the livers of patients with ALD. One mechanism that has been associated with the development of autoimmune responses is the modification (haptenation or adduction) of liver proteins with aldehydes or other products of oxidative stress. This is because it has been shown that these adducted proteins can induce specific immune responses, to the adduct, the adduct plus protein (conformational antigens), as well as the unmodified parts of the protein. More importantly, it is possible to demonstrate that adducted self-proteins can induce reactivity to the normal self-protein and thereby induce autoimmune responses. Therefore, it is the purpose of this manuscript to outline the mechanism(s) by which these modified self proteins can induce autoimmune reactivity, and thus play a role in the development and/or progression of ALD.

Chronic ethanol consumption impairs receptor-mediated endocytosis of MAA-modified albumin by liver endothelial cells

Alcoholic liver disease has been associated with abnormalities in receptor-mediated endocytosis (RME) which results in abnormal degradation of metabolically altered proteins. Model systems using formaldehyde-modified albumin (f-Alb) have shown an impairment in RME following chronic alcohol consumption utilizing both in situ perfused rat livers and isolated rat liver endothelial cells (LECs). The discovery that alcohol metabolite derived aldehydes can modify proteins prompted a study to determine if malondialdehyde-acetaldehyde-modified albumin (MAA-Alb) would be degraded similar to that reported for f-Alb, and whether ethanol-fed rats would demonstrate an impaired RME with respect to this ligand which occurs as a consequence of chronic ethanol consumption. MAA-Alb was degraded slightly more than f-Alb in both in situ perfused livers and at the single cell level. This degradation was completely inhibited with 100x unlabeled f-Alb, which suggests the use of a similar receptor. Following alcohol consumption there was a 50-60% decrease in MAA-Alb degradation in whole livers and isolated LECs. Utilizing isolated LECs it was determined that impairment in internalization was the most likely mechanism for the decrease in the amount of MAA-Alb that was degraded. These data show that chronic alcohol consumption by rats does in fact impair RME of alcohol metabolite-derived adducted proteins, and this impairment is due to a defect in the post-internalization step rather than the binding or degradation of the modified protein.

ATA2-mediated amino acid uptake following partial hepatectomy is regulated by redistribution to the plasma membrane

System A, the Na(+)-dependent amino acid transport activity, is encoded by the ATA2 gene and up-regulated following partial hepatectomy (PH), and its competitive inhibition interferes with liver regeneration. Rabbit polyclonal antibody was raised against a portion of the ATA2 gene product followed by immunodetection of ATA2 in isolated liver plasma membrane and lysate. The level of ATA2 increased in the plasma membrane following PH, while the relatively high quantity of ATA2 found in liver lysate remained constant. We also have shown that Northern analysis of steady-state ATA2 mRNA revealed no significant change following PH. These data show that ATA2-mediated transport is not regulated by the steady-state level of ATA2 mRNA but is regulated by the amount of ATA2 and redistribution to the plasma membrane. We hypothesize that ATA2 activity is regulated by recruitment of ATA2 protein from an intracellular compartment. In addition, the pattern of expression of System A activity in oocytes, transport kinetics, and sensitivity to chemical modification indicate the presence of a second System A isoform in liver that differs substantially from ATA2.

Inhibition of hepatic stellate cell collagen synthesis by N-(methylamino)isobutyric acid

The increased deposition of extracellular matrix by hepatic stellate cells following liver injury, in a process known as activation, is considered a key mechanism for increased collagen content of liver during the development of liver fibrosis. We report that N-(methylamino)isobutyric acid (MeAIB), a specific inhibitor of System A-mediated amino acid uptake, reduces the accumulation of collagen in CFSC-2G hepatic stellate cell cultures and in a rat model of liver injury and fibrosis. Rat CFSC-2G cells were cultured in 0-5mM MeAIB, and the accumulation and synthesis of collagen were measured by binding to Sirius red F3B and pulse-labeling with [3H]-proline, respectively. The effect of MeAIB on collagen accumulation in vivo was evaluated utilizing a rat model of hepatic fibrosis. MeAIB inhibited collagen accumulation in CFSC-2G cultures in a concentration-dependent manner with 5mM MeAIB reducing collagen 44.6+/-1.2% compared with the control. In CFSC-2G cultures, MeAIB selectively inhibited the incorporation of proline into cellular macromolecules by 43+/-4%, while the synthesis of proteins containing leucine was not affected. In vivo, oral administration of 160mg MeAIB/kg body weight per day to rats significantly reduced the hepatic collagen accumulation in response to 1 week of CCl(4)-induced liver injury. MeAIB reduces the accumulation of collagen in CFSC-2G hepatic stellate cell cultures and in a CCl(4)-induced rat model of liver injury and fibrosis.

Phosphoenolpyruvate carboxykinase is induced in growth-arrested hepatoma cells

Phosphoenolpyruvate carboxykinase (PEPCK) mRNA is elevated in H4IIEC3 rat hepatoma cells cultured at high density, suggesting that PEPCK expression and growth arrest may be coordinately regulated. Induction of growth arrest either by contact inhibition (high culture density) or by serum deprivation correlated with significant increases in PEPCK protein and its mRNA. The observation that PEPCK mRNA was induced by contact inhibition in the presence of serum indicates that the effect of high density is independent of insulin or any other serum component. The magnitudes of the changes in PEPCK expression during growth arrest were greatly enhanced in KRC-7 cells, an H4IIEC3 subclone that is much more sensitive to growth arrest than its parental cell line. Restimulation of proliferation in growth-arrested KRC-7 cells, either by addition of serum or insulin to serum-deprived cells or by replating contact-inhibited cells at low density, caused a rapid decrease in PEPCK expression. However, PEPCK mRNA is not always reduced in proliferating cells since treatment of serum-starved cells with epidermal growth factor stimulated entry into the cell cycle but did not affect PEPCK mRNA levels. Finally, dexamethasone induction of PEPCK mRNA was blunted in cells cultured at high density but was unaffected by the presence or absence of serum. Collectively, these data suggest the possibility of cross-talk between the control of PEPCK expression and growth arrest in KRC-7 cells.

Posttranscriptional regulation of ATA2 transport during liver regeneration

The recent cloning of ATA2, a cDNA displaying characteristics identical to the System A transporter, has provided the first molecular tool for study of System A-mediated amino acid transport in liver. Despite the 233 +/- 9 and 472 +/- 11% increase in System A transport activity following partial hepatectomy at 6 and 12 h, respectively, the steady-state level of ATA2 mRNA did not show a corresponding marked increase. Examination of the kinetic properties of System A following partial hepatectomy revealed a K(m) of 0.26 +/- 0.04 mM which is consistent with the reported K(m) for ATA2. These results indicate that a System A transporter present in regenerating liver and ATA2 are identical, but that the increase in System A activity following partial hepatectomy does not result from an increase in steady-state levels of ATA2 mRNA. These observations suggest that ATA2-mediated transport of amino acids is regulated at the posttranscriptional level.

Complications of fluoroscopically guided transforaminal lumbar epidural injections

OBJECTIVES

To assess the incidence of complications of fluoroscopically guided lumbar transforaminal epidural injections.

DESIGN

A retrospective cohort design study. Patients presenting with radiculopathy, caused by either lumbar spinal stenosis or herniated nucleus pulposus confirmed by magnetic resonance imaging or computed tomography scanning, received transforaminal epidural steroid injections as part of a conservative care treatment plan.

SETTING

A multidisciplinary spine care center.

INTERVENTION

All injections were performed consecutively over a 4-month period by five physiatrists. An independent observer reviewed medical charts, which included a 24-hour postprocedure telephone call by an ambulatory surgery center nurse who had asked a standardized questionnaire about complications following the injections. Physician follow-up office notes 1 to 3 weeks after the injection, along with epiduragrams, were also reviewed.

RESULTS

Two hundred seven patients who received 322 injections were reviewed. Complications per injection seen included 10 transient nonpositional headaches that resolved within 24 hours (3.1%), 8 increased back pain (2.4%), 2 increased leg pain (0.6%), 4 facial flushing (1.2%), 1 vasovagal reaction (0.3%), 1 increased blood sugar (258 mg/dL) in an insulin-dependent diabetic (0.3%), and 1 intraoperative hypertension (0.3%). No dural punctures occurred.

CONCLUSIONS

There were no major complications. The incidence of minor complications was 9.6% per injection. All reactions resolved without morbidity, and no patient required hospitalization.

EPAS1 trans-activation during hypoxia requires p42/p44 MAPK

Hypoxia is a common environmental stress that regulates gene expression and cell function. A number of hypoxia-regulated transcription factors have been identified and have been shown to play critical roles in mediating cellular responses to hypoxia. One of these is the endothelial PAS-domain protein 1 (EPAS1/HIF2-alpha/HLF/HRF). This protein is 48% homologous to hypoxia-inducible factor 1-alpha (HIF1-alpha). To date, virtually nothing is known about the signaling pathways that lead to either EPAS1 or HIF1-alpha activation. Here we show that EPAS1 is phosphorylated when PC12 cells are exposed to hypoxia and that p42/p44 MAPK is a critical mediator of EPAS1 activation. Pretreatment of PC12 cells with the MEK inhibitor, PD98059, completely blocked hypoxia-induced trans-activation of a hypoxia response element (HRE) reporter gene by transfected EPAS1. Likewise, expression of a constitutively active MEK1 mimicked the effects of hypoxia on HRE reporter gene expression. However, pretreatment with PD98059 had no effect on EPAS1 phosphorylation during hypoxia, suggesting that MAPK targets other proteins that are critical for the trans-activation of EPAS1. We further show that hypoxia-induced trans-activation of EPAS1 is independent of Ras. Finally, pretreatment with calmodulin antagonists nearly completely blocked both the hypoxia-induced phosphorylation of MAPK and the EPAS1 trans-activation of HRE-Luc. These results demonstrate that the MAPK pathway is a critical mediator of EPAS1 activation and that activation of MAPK and EPAS1 occurs through a calmodulin-sensitive pathway and not through the GTPase, Ras. These results are the first to identify a specific signaling pathway involved in EPAS1 activation.

Inhibition of system A amino acid transport and hepatocyte proliferation following partial hepatectomy in the rat

System A, the sodium-dependent neutral amino acid transport activity, has a 3-fold increase in its initial uptake velocity into hepatocytes following partial hepatectomy (PH) in the rat. The purpose of this study was to examine the effect of inhibition of System A-mediated amino acid transport on hepatocyte proliferation and liver regeneration. We describe the in vivo competitive inhibition of System A activity following PH by the nonmetabolizable, System A-specific substrate, alpha-(methylamino)isobutyric acid (MeAIB). Administration of MeAIB 60 minutes before PH decreased the incorporation of [(3)H]thymidine into DNA by 45% +/- 5% and 76% +/- 17% at 24 and 36 hours, respectively. The readministration of MeAIB every 12 hours further decreased DNA synthesis by 92% +/- 18% and 82% +/- 11% at 24 and 36 hours. The recovery of liver mass of rats receiving MeAIB was decreased by 46.4% +/- 5.1% at 24 hours after PH. In vitro, 5 mmol/L MeAIB inhibited proliferation of primary hepatocytes by 56% +/- 4% and 61% +/- 12% 48 hours after incubation with 10% fetal calf serum or epidermal growth factor (5 ng/mL), respectively. Thus, MeAIB inhibition of System A transport activity decreased both in vivo and in vitro inducement of hepatocyte proliferation. Treatment with MeAIB did not significantly change the incorporation of [(3)H]leucine into total liver protein, but changes in serum amino acids and hepatocyte cell volume were observed, suggesting System A transport activity during hepatocyte proliferation functions primarily to provide amino acids to fuel liver-specific biochemical pathways and to increase cell volume.

The exuT gene of Erwinia chrysanthemi EC16: nucleotide sequence, expression, localization, and relevance of the gene product

Galacturonic acid (GalUA) is a major component of pectin and polygalacturonic acid in the plant cell wall. In the phytopathogen Erwinia chrysanthemi, the uptake of molecules derived from degradation of these polymers is an important early step in the events preceding induction of pectinases, ultimately leading to plant tissue maceration. Uptake systems for GalUA and dimers of GalUA have been described and shown to be inducible in E. chrysanthemi. The GalUA uptake gene (exuT) was cloned and sequenced. Nucleotide sequence analysis identified an open reading frame encoding a 345-amino-acid polypeptide with a calculated mass of 37,825 Da. This polypeptide is predicted to be an integral membrane protein based on its high nonpolar amino acid content and hydropathic profile. Localization studies with the labeled polypeptide in the T7-RNA polymerase system also suggest that ExuT is a membrane protein. This evidence is further supported by the observation of hybrid ExuT-PhoA proteins in the bacterial cytoplasmic membrane following immunoblot analysis. Northern (RNA) analysis indicated that the gene is inducible in the presence of the monomer, GalUA. A targeted mutation in the exuT gene affected the utilization of GalUA as a role carbon source for growth. Maceration of potato tuber tissue by this mutant was delayed and reduced, when compared with the parental strain EC16.