The multifaceted role of Lon proteolysis in seedling establishment and maintenance of plant organelle function: living from protein destruction

Intracellular selective proteolysis is an important post-translational regulatory mechanism maintaining protein quality control by removing defective, damaged or even deleterious protein aggregates. The ATP-dependent Lon protease is a key component of protein quality control that is highly conserved across the kingdoms of living organisms. Major advancements have been made in bacteria and in non-plant organisms to understand the role of Lon in protection against protein oxidation, ageing and neurodegenerative diseases. This review presents the progress currently made in plants. The Lon gene family in Arabidopsis consists of four members that produce distinct protein isoforms localized in several organelles. Lon1 and Lon4 that potentially originate from a recent gene duplication event are dual-targeted to mitochondria and chloroplasts through distinct mechanisms revealing divergent evolution. Arabidopsis mutant analysis showed that mitochondria and peroxisomes biogenesis or maintenance of function is modulated by Lon1 and Lon2, respectively. Consequently, the lack of Lon selective proteolysis leading to growth retardation and impaired seedling establishment can be attributed to defects in the oil reserve mobilization pathway. The current progress in Arabidopsis research uncovers the role of Lon in the proteome homeostasis of plant organelles and stimulates biotechnology scenarios of plant tolerance against harsh abiotic conditions because of climate instability.

[Mutants of Burkholderia cenocepacia with a change in synthesis of N-acyl-homoserine lactones--signal molecules of Quorum Sensing regulation]

By means of plasposon mutagenesis, mutants of Burkholderia cenocepacia 370 with the change in production of N-acyl-homoserine lactones (AHL), signal molecules of the Quorum Sensing system of regulation, were obtained. To localize plasposon insertions in mutant strains, fragments of chromosomal DNA containing plasposons were cloned, adjacent DNA regions sequenced, and a search for homologous nucleotide sequences in the GeneBank was initiated. It has been shown that the insertion of plasposon into gene lon encoding lon proteinase drastically decreases AHL synthesis. Upon insertion of plasposon into gene pps encoding phosphoenolpyruvate-synthase, enhancement of AHL production is observed. In mutant carrying inactivated gene lon, a strong decline of extracellular protease activity, hemolytic, and chitinolytic activities was observed in comparison with the original strain; lipase activity was not changed in this mutant. Mutation in gene pps did not affect these properties of B. cenocepacia 370. Mutations in genes lon and pps reduced the virulence of bacteria upon infection of mice.

Negative regulation of pathogenesis in Pseudomonas syringae pv. tabaci 11528 by ATP-dependent Lon protease

Pseudomonas syringae pv. tabaci causes wildfire disease in tobacco plants. The hrp pathogenicity island (hrp PAI) of P. syringae pv. tabaci encodes a type III secretion system (TTSS) and its regulatory system, which are required for pathogenesis in plants. Three important regulatory proteins-HrpR, HrpS, and HrpL-have been identified to activate hrp PAI gene expression. The bacterial Lon protease regulates the expression of various genes. To investigate the regulatory mechanism of the Lon protease in P. syringae pv. tabaci 11528, we cloned the lon gene, and then a Δlon mutant was generated by allelic exchange. lon mutants showed increased UV sensitivity, which is a typical feature of such mutants. The Δlon mutant produced higher levels of tabtoxin than the wild-type. The lacZ gene was fused with hrpA promoter and activity of β-galactosidase was measured in hrp-repressing and hrp-inducing media. The Lon protease functioned as a negative regulator of hrp PAI under hrp-repressing conditions. We found that strains with lon disruption elicited the host defense system more rapidly and strongly than the wild-type strain, suggesting that the Lon protease is essential for systemic pathogenesis.

Expression of nuclear-encoded genes involved in mitochondrial biogenesis and dynamics in experimentally denervated muscle

The abundance, morphology, and functional properties of mitochondria become altered in response to denervation. To gain insight into the regulation of this process, mitochondrial enzyme activities and gene expression involved in mitochondrial biogenesis and dynamics in mouse gastrocnemius muscle was investigated. Sciatic nerve transactions were performed on mice, and then gastrocnemius muscles were isolated at days 5 and 30 after surgery. Muscle weight was decreased significantly by 15% and 62% at days 5 and 30 after surgery, respectively. The activity of citrate synthase, a marker of oxidative enzyme, was reduced significantly by 31% and 53% at days 5 and 30, respectively. Enzyme histochemical analysis revealed that subsarcolemmal mitochondria were largely lost than intermyofibrillar mitochondria at day 5, and this trend was further progressed at day 30 after surgery. Expression levels of peroxisome proliferator-activated receptor, γ coactivator 1 (PGC-1)α, estrogen-related receptor α (ERRα), and mitofusin 2 were down-regulated throughout the experimental period, whereas those of PGC-1β, PRC, nuclear respiratory factor (NRF)-1, NRF-2, TFAM, and Lon protease were down-regulated at day 30 after surgery. These results suggest that PGC-1α, ERRα, and mitofusin 2 may be important factors in the process of denervation-induced mitochondrial adaptation. In addition, other PGC-1 family of transcriptional coactivators and DNA binding transcription factors may also contribute to mitochondrial adaptation after early response to denervation.

Mitochondrial proteases and cancer

Mitochondria are a major source of intracellular reactive oxygen species, the production of which increases with cancer. The deleterious effects of reactive oxygen species may be responsible for the impairment of mitochondrial function observed during various pathophysiological states associated with oxidative stress and cancer. These organelles are also targets of oxidative damage (oxidation of mitochondrial DNA, lipids, protein). An important factor for protein maintenance in the presence of oxidative stress is enzymatic reversal of oxidative modifications and/or protein degradation. Failure of these processes is likely a critical component of the cancer process. Mitochondrial proteases degrade misfolded and non-assemble polypeptides, thus performing quality control surveillance in the organelle. Mitochondrial proteases may be directly involved in cancer development as recently shown for HtrA2/Omi or may regulate crucial mitochondrial molecule such as cytochrome c oxidase 4 a subunit of the cytochrome c oxidase complex degraded by the Lon protease. Thus, the role of mitochondrial proteases is further addressed in the context of oxidative stress and cancer.

The Lon protease from the haloalkaliphilic archaeon Natrialba magadii is transcriptionally linked to a cluster of putative membrane proteases and displays DNA-binding activity

The ATP-dependent Lon protease is universally distributed in bacteria, eukaryotic organelles and archaea. In comparison with bacterial and eukaryal Lon proteases, the biology of the archaeal Lon has been studied to a limited extent. In this study, the gene encoding the Lon protease of the alkaliphilic haloarchaeon Natrialba magadii (Nmlon) was cloned and sequenced, and the genetic organization of Nmlon was examined at the transcriptional level. Nmlon encodes a 84 kDa polypeptide with a pI of 4.42 which contains the ATPase, protease and membrane targeting domains of the archaeal-type LonB proteases. Nmlon is part of an operon that encodes membrane proteases and it is transcribed as a polycistronic mRNA in N. magadii cells at different growth stages. Accordingly, NmLon was detected in cell membranes of N. magadii throughout growth by Western blot analysis using specific anti-NmLon antibodies. Interestingly, in electrophoretic mobility shift assays, purified NmLon bound double stranded as well as single stranded DNA in the presence of elevated salt concentrations. This finding shows that DNA-binding is conserved in the LonA and LonB subfamilies and suggests that Lon-DNA interaction may be relevant for its function in haloarchaea.

[Proteolytic control of expression of Vibrio fischeri lux-operon genes in Escherichia coli cells]

The key elements of the regulatory system activating expression of the lux-operon genes in the sea bacteria Vibrio fischeri are the LuxR protein (an activator oftranscription) and N-(3-oxohexanoyl) L-homoserine lactone (an autoinducer, AI). It is shown that the ATP-dependent proteases ClpXP and Lon take part in the negative control of expression of the lux-operon genes and that AI protects the LuxR protein from proteolysis.

[The C-terminal domain of the Vibrio fischeri transcription activator LuxR is not essential for degradation by Lon protease]

The Vibrio fischer luxICDABEG genes are activated by autoinducer N-(3-oxohexanoyl)homoserine lactone and the LuxR protein. The LuxR contains 250 aa and consists of two domains. The C-domain, that extends from around residue 162 to the C-terminus, is thought to bind lux regulatory DNA and activate transcription of the luxICDABEG genes. The N-terminal domain, which binds the autoinducer, consists of about 70% residues of LuxR. In E. coli C-terminal domain can activate the lux genes in the absence of autoinducer. Previously it was shown that the ATP-dependent Lon protease of E. coli takes part in the negative regulation of the transcription of the V. fischeri lux operon and that LuxR is a target of Lon protease. Comparative analysis of effects of Lon protease on the V. fischeri luxICDABEG genes expression was made. Special constructed hybrid plasmids which permit the regulation of luxR, luxR 5'-deletion mutation were used and luICDABEG genes were activated independently and quantitatively. We show that the full length LuxR, but not C-terminal domain is a target protein for Lon protease. The transcription activity by full length LuxR protein isobserved when its intracellular concentration is about two order lower than that of its C-terminal domain.

Evidence that two ATP-dependent (Lon) proteases in Borrelia burgdorferi serve different functions

The canonical ATP-dependent protease Lon participates in an assortment of biological processes in bacteria, including the catalysis of damaged or senescent proteins and short-lived regulatory proteins. Borrelia spirochetes are unusual in that they code for two putative ATP-dependent Lon homologs, Lon-1 and Lon-2. Borrelia burgdorferi, the etiologic agent of Lyme disease, is transmitted through the blood feeding of Ixodes ticks. Previous work in our laboratory reported that B. burgdorferi lon-1 is upregulated transcriptionally by exposure to blood in vitro, while lon-2 is not. Because blood induction of Lon-1 may be of importance in the regulation of virulence factors critical for spirochete transmission, the clarification of functional roles for these two proteases in B. burgdorferi was the object of this study. On the chromosome, lon-2 is immediately downstream of ATP-dependent proteases clpP and clpX, an arrangement identical to that of lon of Escherichia coli. Phylogenetic analysis revealed that Lon-1 and Lon-2 cluster separately due to differences in the NH₂-terminal substrate binding domains that may reflect differences in substrate specificity. Recombinant Lon-1 manifested properties of an ATP-dependent chaperone-protease in vitro but did not complement an E. coli Lon mutant, while Lon-2 corrected two characteristic Lon-mutant phenotypes. We conclude that B. burgdorferi Lons -1 and -2 have distinct functional roles. Lon-2 functions in a manner consistent with canonical Lon, engaged in cellular homeostasis. Lon-1, by virtue of its blood induction, and as a unique feature of the Borreliae, may be important in host adaptation from the arthropod to a warm-blooded host.

Lyme disease, caused by the spirochete Borrelia burgdorferi, is the most prevalent arthropod-borne disease in North America. In nature, the bacterium oscillates between its tick vector host (Ixodes spp.) and small rodents (Peromyscus spp.). B. burgdorferi is able to persist in these two very different host environments by modulating the expression of surface lipoproteins proteins, or other proteins, in response to host factors or environmental cues such as temperature and pH. Our interest in this process led to the identification of a homolog of the E. coli ATP-dependent lon protease (lon-1) in B. burgdorferi that was upregulated in response to blood. The prototypical Lon of E. coli is a conserved protease important for the destruction of abnormal and short-lived proteins. B. burgdorferi is unusual in that it also codes for a second lon homolog, lon-2, that was not upregulated in response to blood. In this study, we sought to clarify the roles for Lon-1 and Lon-2 in B. burgdorferi. We present evidence that Lon-1 is an ATP- and Mg²⁺-dependent protease but does not function in a manner consistent with a prototypical Lon. Lon-2, however, functionally complemented Lon in E. coli. Thus, Lon-1 and Lon-2 appear to have distinct roles in B. burgdorferi; Lon-1 by virtue of its blood induction may be important in host adaptation, while Lon-2 is the functional homolog of E. coli Lon.

Regulation and quality control by Lon-dependent proteolysis

After their first discovery in Escherichia coli, Lon homologues were found to be widely distributed among prokaryotes to eukaryotes. The ATP-dependent Lon protease belongs to the AAA(+) (ATPases associated with a variety of cellular activities) superfamily, and is involved in both general quality control by degrading abnormal proteins and in the specific control of several regulatory proteins. As such, this enzyme has a pivotal role in quality control and cellular physiology. This review focuses on mechanisms of degradation both from the protease and substrate points of view, and discusses the role of Lon in global regulation, stress response and virulence.

Mitochondrial Lon protease is a human stress protein

The targeted removal of damaged proteins by proteolysis is crucial for cell survival. We have shown previously that the Lon protease selectively degrades oxidized mitochondrial proteins, thus preventing their aggregation and cross-linking. We now show that the Lon protease is a stress-responsive protein that is induced by multiple stressors, including heat shock, serum starvation, and oxidative stress. Lon induction, by pretreatment with low-level stress, protects against oxidative protein damage, diminished mitochondrial function, and loss of cell proliferation induced by toxic levels of hydrogen peroxide. Blocking Lon induction with Lon siRNA also blocks this induced protection. We propose that Lon is a generalized stress-protective enzyme whose decline may contribute to the increased levels of protein damage and mitochondrial dysfunction observed in aging and age-related diseases.

Role of Lon1 protease in post-germinative growth and maintenance of mitochondrial function in Arabidopsis thaliana

Maintenance of protein quality control and turnover is essential for cellular homeostasis. In plant organelles this biological process is predominantly performed by ATP-dependent proteases. Here, a genetic screen was performed that led to the identification of Arabidopsis thaliana Lon1 protease mutants that exhibit a post-embryonic growth retardation phenotype. Translational fusion to yellow fluorescent protein revealed AtLon1 subcellular localization in plant mitochondria, and the AtLon1 gene could complement the respiratory-deficient phenotype of the yeast PIM1 gene homolog. AtLon1 is highly expressed in rapidly growing plant organs of embryonic origin, including cotyledons and primary roots, and in inflorescences, which have increased mitochondria numbers per cell to fulfill their high energy requirements. In lon1 mutants, the expression of both mitochondrial and nuclear genes encoding respiratory proteins was normal. However, mitochondria isolated from lon1 mutants had a lower capacity for respiration of succinate and cytochrome c via complexes II and IV, respectively. Furthermore, the activity of key enzymes of the tricarboxylic acid (TCA) cycle was significantly reduced. Additionally, mitochondria in lon1 mutants had an aberrant morphology. These results shed light on the developmental mechanisms of selective proteolysis in plant mitochondria and suggest a critical role for AtLon1 protease in organelle biogenesis and seedling establishment.

[Influence of mutations at genes of global transcriptional regulators on production of autoinducer AI-2 Quorum Sensing in the system of Escherichia coli]

The control of gene expression in response to an increase in the bacterial population density (Quorum Sensing) involves low-molecular-weight signal molecules (autoinducers, AI). AI-2 and synthase LuxS mediating its synthesis are widely distributed in Gram-negative and Gram-positive bacteria. In this work, the data were obtained on the role of global regulators of gene expression in AI-2 synthesis in Escherichia coli cells. The mutation inactivating gene rpoS (encodes sigma S subunit of RNA polymerase) was shown to drastically decrease an amount of active AI-2 in the culture medium. Mutations at gene rpoN that encodes sigma N subunit of RNA polymerase and also at gene lon, which encodes Lon proteinase, on the contrary, increase an amount of active AI-2 in supernatants of cultures. Mutant strains lacking histone-like proteins H-NS and StpA accumulate a slightly higher amount of AI-2 than the isogenic wild-type strain: however, an amount of AI-2 decreased in the culture medium of the double mutant devoid of both these proteins.

Limited proteolysis of E. coli ATP-dependent protease Lon - a unified view of the subunit architecture and characterization of isolated enzyme fragments

We carried out chymotryptic digestion of multimeric ATP-dependent Lon protease from Escherichia coli. Four regions sensitive to proteolytic digestion were located in the enzyme and several fragments corresponding to the individual structural domains of the enzyme or their combinations were isolated. It was shown that (i) unlike the known AAA(+) proteins, the ATPase fragment (A) of Lon has no ATPase activity in spite of its ability to bind nucleotides, and it is monomeric in solution regardless of the presence of any effectors; (ii) the monomeric proteolytic domain (P) does not display proteolytic activity; (iii) in contrast to the inactive counterparts, the AP fragment is an oligomer and exhibits both the ATPase and proteolytic activities. However, unlike the full-length Lon, its AP fragment oligomerizes into a dimer or a tetramer only, exhibits the properties of a non-processive protease, and undergoes self-degradation upon ATP hydrolysis. These results reveal the crucial role played by the non-catalytic N fragment of Lon (including its coiled-coil region), as well as the contribution of individual domains to creation of the quaternary structure of the full-length enzyme, empowering its function as a processive protease.

Effects of acclimation temperature and cadmium exposure on mitochondrial aconitase and LON protease from a model marine ectotherm, Crassostrea virginica

Temperature and heavy metals such as cadmium (Cd) are important stressors which can strongly affect physiology of marine ectotherms in polluted estuaries. Mitochondria are among the key intracellular targets for these stressors, but the mechanisms of Cd-induced mitochondrial damage are not fully understood. In this study we determined the effects of acclimation temperature (12, 20 and 28 degrees C) and Cd exposure (0 or 50 microg L(-1) Cd) in vivo on activity and mRNA expression of a key mitochondrial enzyme, aconitase, which is known as a sensitive marker of oxidative stress, and on mRNA expression of LON protease involved in the degradation of oxidatively damaged mitochondrial proteins, in eastern oysters Crassostrea virginica. Sensitivity of mitochondrial aconitase to exposure to Cd in vitro (0 or 50 microM) was also determined in oysters acclimated to different temperatures and Cd levels. Acclimation at 28 degrees C resulted in a strong decrease in activity of mitochondrial aconitase as well as mRNA expression of aconitase and LON protease suggesting mitochondrial dysfunction at elevated temperatures. Exposure of isolated mitochondria to 50 microM Cd in vitro resulted in a 20-25% inhibition of mitochondrial aconitase reflecting oxidative damage of this enzyme. However, long-term (3-6 weeks) exposure of whole oysters to Cd had no effect on mitochondrial aconitase activity suggesting that this enzyme is well protected against Cd-induced oxidative stress in vivo. Aconitase mRNA expression was positively correlated with the enzyme activity within control and Cd-exposed groups; however, this correlation was strikingly different when compared between control and Cd-exposed oysters. The level of aconitase transcript was considerably lower (3-13-fold) in Cd-exposed oysters while the specific aconitase activities were similar in control and Cd-exposed oysters indicating regulation at the post-transcriptional level. LON protease expression was upregulated by 2-4-fold in Cd-exposed oysters suggesting an increase in mitochondrial protein degradation as a novel protective mechanism against Cd-induced mitochondrial stress. Our data indicate that mitochondrial aconitase is not a good biomarker for Cd-induced oxidative stress in oysters in vivo, because of its complex regulation at transcriptional and post-transcriptional levels, low sensitivity to Cd effects in vivo but high sensitivity to acclimation temperature that can potentially mask effects of other stressors under the field conditions.

[Regulation of biofilm formation in Escherichia coli K12: effect of mutations in HNS, StpA, lon, and rpoN genes]

More than 99% of bacteria exist in natural ecosystems as specifically organized biofilms adhering to solid surfaces. Biofilms have a typical architecture and are enclosed in exopolymeric matrix. Bacteria living in biofilms are extremely resistant to antibacterial factors. In this work we studied the role of some global regulators of gene expression on biofilm formation by Escherichia coli K12. The Histone-like proteins H-NS and StpA were shown to play an essential role in the regulation of biofilm formation. Mutant strains deficient in HNS or StpA had lower levels of biofilm formation than the wild-type isogenic strain. A double mutant deficient in the two proteins was virtually incapable of forming the biofilms. The mutations in the rpoN gene encoding for the sigma N subunit of RNA-polymerase and in lon gene encoding for Lon-proteinase induced a 40-60% increase in the biofilm formation.

Characterization of three putative Lon proteases of Thermus thermophilus HB27 and use of their defective mutants as hosts for production of heterologous proteins

In the genome of a thermophilic bacterium, Thermus thermophilus HB27, three genes, TTC0418, TTC0746 and TTC1975, were annotated as ATP-dependent protease La (Lon). Sequence comparisons indicated that TTC0418 and TTC0746 showed significant similarities to bacterial LonA-type proteases, such as Escherichia coli Lon protease, especially in regions corresponding to domains for ATP-binding and hydrolysis, and for proteolysis, but TTC1975 exhibited a similarity only at the C-terminal proteolytic domain. The enzymatic analyses, using purified recombinant proteins produced by E. coli, revealed that TTC0418 and TTC0746 exhibited peptidase and protease activities against two synthetic peptides and casein, respectively, in an ATP-dependent manner, and at the same time, both the enzymes had significant ATPase activities in the presence of substrates. On the other hand, TTC1975 possessed a protease activity against casein, but addition of ATP did not enhance this activity. Moreover, a T. thermophilus mutant deficient in both TTC0418 and TTC0746 showed a similar growth characteristic to an E. coli lon mutant, i.e., a growth defect lag after a nutritional downshift. These results indicate that TTC0418 and TTC0746 are actually members of bacterial LonA-type proteases with different substrate specificities, whereas TTC1975 should not be classified as a Lon protease. Finally, the effects of mutations deficient in these proteases were assessed on production of several heterologous gene products from Pyrococcus horikoshii and Geobacillus stearothermophilus. It was shown that TTC0746 mutation was more effective in improving production than the other two mutations, especially for production of P. horikoshii alpha-mannosidase and G. stearothermophilus alpha-amylase, indicating that the TTC0746 mutant of T. thermophilus HB27 may be useful for production of heterologous proteins from thermophiles and hyperthermophiles.

Role of lon, an ATP-dependent protease homolog, in resistance of Pseudomonas aeruginosa to ciprofloxacin

With few novel antimicrobials in the pharmaceutical pipeline, resistance to the current selection of antibiotics represents a significant therapeutic challenge. Microbial persistence in subinhibitory antibiotic environments has been proposed to contribute to the development of resistance. Pseudomonas aeruginosa cultures pretreated with subinhibitory concentrations of ciprofloxacin were found to exhibit an adaptive resistance phenotype when cultures were subsequently exposed to suprainhibitory ciprofloxacin concentrations. Microarray experiments revealed candidate genes involved in such adaptive resistance. Screening of 10,000 Tn5-luxCDABE mutants identified several mutants with increased or decreased ciprofloxacin susceptibilities, including mutants in PA1803, a close homolog of the ATP-dependent lon protease, which were found to exhibit > or = 4-fold-increased susceptibilities to ciprofloxacin and other fluoroquinolones, but not to gentamicin or imipenem, as well as a characteristic elongated morphology. Complementation of the lon mutant restored wild-type antibiotic susceptibility and cell morphology. Expression of the lon mutant, as monitored through a luciferase reporter fusion, was found to increase over time in the presence of subinhibitory ciprofloxacin concentrations. The data are consistent with the hypothesis that the induction of Lon by ciprofloxacin is involved in adaptive resistance.

Lon protease promotes survival of Escherichia coli during anaerobic glucose starvation

In Escherichia coli, Lon is an ATP-dependent protease which degrades misfolded proteins and certain rapidly-degraded regulatory proteins. Given that oxidatively damaged proteins are generally degraded rather than repaired, we anticipated that Lon deficient cells would exhibit decreased viability during aerobic, but not anaerobic, carbon starvation. We found that the opposite actually occurs. Wild-type and Lon deficient cells survived equally well under aerobic conditions, but Lon deficient cells died more rapidly than the wild-type under anaerobiosis. Aerobic induction of the Clp family of ATP-dependent proteases could explain these results, but direct quantitation of Clp protein established that its level was not affected by Lon deficiency and overexpression of Clp did not rescue the cells under anaerobic conditions. We conclude that the Lon protease supports survival during anaerobic carbon starvation by a mechanism which does not depend on Clp.

Lon protease degrades transfer-messenger RNA-tagged proteins

Bacterial trans translation is activated when translating ribosomes are unable to elongate or terminate properly. Small protein B (SmpB) and transfer-messenger RNA (tmRNA) are the two known factors required for and dedicated to trans translation. tmRNA, encoded by the ssrA gene, is a bifunctional molecule that acts both as a tRNA and as an mRNA during trans translation. The functions of tmRNA ensure that stalled ribosomes are rescued, the causative defective mRNAs are degraded, and the incomplete polypeptides are marked for targeted proteolysis. We present in vivo and in vitro evidence that demonstrates a direct role for the Lon ATP-dependent protease in the degradation of tmRNA-tagged proteins. In an endogenous protein tagging assay, lon mutants accumulated excessive levels of tmRNA-tagged proteins. In a reporter protein tagging assay with lambda-CI-N, the protein product of a nonstop mRNA construct designed to activate trans translation, lon mutant cells efficiently tagged the reporter protein, but the tagged protein exhibited increased stability. Similarly, a green fluorescent protein (GFP) construct containing a hard-coded C-terminal tmRNA tag (GFP-SsrA) exhibited increased stability in lon mutant cells. Most significantly, highly purified Lon preferentially degraded the tmRNA-tagged forms of proteins compared to the untagged forms. Based on these results, we conclude that Lon protease participates directly in the degradation of tmRNA-tagged proteins.

Mutation of Lon protease differentially affects the expression of Pseudomonas syringae type III secretion system genes in rich and minimal media and reduces pathogenicity

The bacterial Lon protease participates in a variety of biological processes. In Pseudomonas syringae, mutation of lon is known to activate hrpL and a few hrpL-regulated genes in rich medium. The elevated expression of hrpL and hrpL-regulated genes results from increased stability of HrpR, the transcriptional activator of hrpL, in the lon mutant. Here, we conducted a microarray analysis to identify genes that are differentially expressed in a lon- mutant of P. syringae pv. tomato DC3000 grown in the rich medium King's B (KB). Most genes induced in the lon- mutant belong to the HrpL regulon or are related to transcription, protein synthesis, and energy metabolism. A major group of genes reduced in the lon- mutant are related to cell wall biogenesis. The HrpL-regulated genes exhibit different induction patterns in the lon- mutant, suggesting that additional regulators other than HrpL are likely to be involved in regulation of these genes. Compared with the wild-type bacteria, lon- mutants of P. syringae pv. tomato DC3000 and P. syringae pv. phaseolicola NPS3121 strains exhibit elevated hrpL expression in KB medium, but reduced hrpL expression in minimal medium (MM). The reduced hrpL RNA is correlated with reduced hrpR and hrpS RNAs, suggesting that the Lon-mediated regulation of hrpL involves different mechanisms in KB and MM. The lon- mutation also reduced bacterial pathogenicity.

[Effect of RNA interference for Lon gene silencing on growth and apoptosis of human breast cancer MCF7 cells]

OBJECTIVE

To study the role of Lon gene in tumor cell proliferation, apoptosis and cell stress response.

METHODS

Small interfering RNAs (smRNAs) for Lon gene were designed using Ambion software and synthesized. The recombinant plasmid pSilencer U6 2.1/Lon was constructed with the smRNAs and pSilencer U6 2.1, followed by transfection into MCF7 cells via Lipofectamine(TM) 2000. The positive cLones were detected by RT-PCR 24 h after cell transfection. The transfected MCF7 cells were then subjected to cisplatin treatment, ultraviolet (UV) exposure and heat stress, respectively, after which the cells growth was tested with MTT assay and the measurements were plotted against time or concentration depending on the treatment administered. Apoptosis of MCF7 cells following the treatments was measured with flow cytometry.

RESULTS

The mRNA of Lon gene was downregulated in cells transfected with the recombinant plasmid pSilencer U6 2.1-Lon, and RT-PCR fail to detect the specific band of Lon as could be detected in untransfected and mock-transfected MCF7 cells. MTT assay showed that pSilencer U6 2.1-Lon transfection resulted in reduced cell proliferation capacity. Stress response test revealed that MCF7 cells with Lon gene down-regulation enhanced cell sensitivity for UV and cisplatin, which was not observed for non-transfected or mock transfection group. The same changes were also observed for heat stress exposure at 41 degrees Celsius;, but not at 43 degrees Celsius; or 45 degrees Celsius;. Increased cell apoptosis rate from (1.14-/+0.79)% to (22.47-/+3.15)% occurred following pSilencer U6 2.1-Lon transfection of the cells.

CONCLUSIONS

Lon gene can be significantly downregulated by introduction of siRNA in MCF7 cells to result in enhanced sensitivity of MCF7 cells to UV, cisplatin and heat stress.

Cellular levels and activity of the flagellar sigma factor FliA of Escherichia coli are controlled by FlgM-modulated proteolysis

In Escherichia coli the flagellar regulon consists of more than 60 genes organized in three hierarchically and temporally regulated transcriptional classes. The flagellar sigma factor FliA (sigma(28)) is responsible for class 3 expression and, in the early phase of flagellar assembly, is inhibited by its anti-sigma factor FlgM. The flagellar hook basal body forms a type III secretion system capable of secreting both flagellar subunits and FlgM. This results in release and therefore activation of FliA and class 3 expression. Here we demonstrate that FliA is also subject to proteolysis which mainly depends on Lon protease. FlgM not only acts as an anti-sigma factor but also protects FliA from being degraded. Based on quantitative measurements over time upon experimental induction of the flagellar cascade as well as during the growth cycle of a motile strain, we show that FliA proteolysis increases in parallel to class 3 expression, i.e. correlates with FlgM secretion and the phase of highest activity of FliA. Thus, when FlgM is not available due to secretion or mutation, and with RNA polymerase interaction being only transient during the transcription initiation cycle, the proteases can degrade FliA. Experiments with a lon mutant indicate that Lon protease and FliA degradation maintain appropriate FliA : FlgM stoichiometry upon induction of the flagellar system and thereby contribute to timely shut-off of this system.

HIF-1 regulates cytochrome oxidase subunits to optimize efficiency of respiration in hypoxic cells

O(2) is the ultimate electron acceptor for mitochondrial respiration, a process catalyzed by cytochrome c oxidase (COX). In yeast, COX subunit composition is regulated by COX5a and COX5b gene transcription in response to high and low O(2), respectively. Here we demonstrate that in mammalian cells, expression of the COX4-1 and COX4-2 isoforms is O(2) regulated. Under conditions of reduced O(2) availability, hypoxia-inducible factor 1 (HIF-1) reciprocally regulates COX4 subunit expression by activating transcription of the genes encoding COX4-2 and LON, a mitochondrial protease that is required for COX4-1 degradation. The effects of manipulating COX4 subunit expression on COX activity, ATP production, O(2) consumption, and reactive oxygen species generation indicate that the COX4 subunit switch is a homeostatic response that optimizes the efficiency of respiration at different O(2) concentrations. Thus, mammalian cells respond to hypoxia by altering COX subunit composition, as previously observed in yeast, but by a completely different molecular mechanism.

Translational regulation of the Escherichia coli stress factor RpoS: a role for SsrA and Lon

Escherichia coli cell viability during starvation is strongly dependent on the expression of the rpoS gene, encoding the RpoS sigma subunit of RNA polymerase. RpoS abundance has been reported to be regulated at many levels, including transcription initiation, translation, and protein stability. The regulatory RNA SsrA (or tmRNA) has both tRNA and mRNA activities, relieving ribosome stalling and cotranslationally tagging proteins. We report here that SsrA is needed for the correct high-level translation of RpoS. The ATP-dependent protease Lon was also found to negatively affect RpoS translation, but only at low temperature. We suggest that SsrA may indirectly improve RpoS translation by limiting ribosome stalling and depletion of some component of the translation machinery.