HSP101 functions as a specific translational regulatory protein whose activity is regulated by nutrient status

FLOURY ENDOSPERM24, a heat shock protein 101 (HSP101), is required for starch biosynthesis and endosperm development in rice

Endosperm is the main storage organ in cereal grain and determines grain yield and quality. The molecular mechanisms of heat shock proteins in regulating starch biosynthesis and endosperm development remain obscure. Here, we report a rice floury endosperm mutant flo24 that develops abnormal starch grains in the central starchy endosperm cells. Map-based cloning and complementation test showed that FLO24 encodes a heat shock protein HSP101, which is localized in plastids. The mutated protein FLO24T296I dramatically lost its ability to hydrolyze ATP and to rescue the thermotolerance defects of the yeast hsp104 mutant. The flo24 mutant develops more severe floury endosperm when grown under high-temperature conditions than normal conditions. And the FLO24 protein was dramatically induced at high temperature. FLO24 physically interacts with several key enzymes required for starch biosynthesis, including AGPL1, AGPL3 and PHO1. Combined biochemical and genetic evidence suggests that FLO24 acts cooperatively with HSP70cp-2 to regulate starch biosynthesis and endosperm development in rice. Our results reveal that FLO24 acts as an important regulator of endosperm development, which might function in maintaining the activities of enzymes involved in starch biosynthesis in rice.

Inhibition of Cell-Free Translation and Replication of Tobacco Mosaic Virus RNA by Exogenously Added 5'-Proximal Fragments of the Genomic RNA

Replication proteins of tobacco mosaic virus (TMV), a positive-sense RNA virus, co-translationally bind to a 5′-proximal ~70-nucleotide (nt) region of the genomic RNA, referred to as the nuclease-resistant (NR) region for replication template selection. Therefore, disruption of the interaction between the viral replication proteins and viral genomic RNA is expected to inhibit the replication of TMV. In this study, we demonstrate that the addition of small RNA fragments (18–33 nts in length) derived from different regions within the NR region inhibit the binding of TMV replication proteins to viral RNA and TMV RNA replication in a cell-free system. Intriguingly, some of the small RNA fragments also inhibited the translation of mRNA in a sequence-nonspecific manner. These results highlight the pleiotropic roles of the 5′-proximal region of the TMV genome.

Genome-wide analysis of the HSP101/CLPB gene family for heat tolerance in hexaploid wheat

Heat Shock Protein 101 (HSP101), the homolog of Caseinolytic Protease B (CLPB) proteins, has functional conservation across species to play roles in heat acclimation and plant development. In wheat, several TaHSP101/CLPB genes were identified, but have not been comprehensively characterized. Given the complexity of a polyploid genome with its phenomena of homoeologous expression bias, detailed analysis on the whole TaCLPB family members is important to understand the genetic basis of heat tolerance in hexaploid wheat. In this study, a genome-wide analysis revealed thirteen members of TaCLPB gene family and their expression patterns in various tissues, developmental stages, and stress conditions. Detailed characterization of TaCLPB gene and protein structures suggested potential variations of the sub-cellular localization and their functional regulations. We revealed homoeologous specific variations among TaCLPB gene copies that have not been reported earlier. A study of the Chromosome 1 TaCLPB in four wheat genotypes demonstrated unique patterns of the homoeologous gene expression under moderate and extreme heat treatments. The results give insight into the strategies to improve heat tolerance by targeting one or some of the TaCLPB genes in wheat.

Concerted action of two 3' cap-independent translation enhancers increases the competitive strength of translated viral genomes

Several families of plant viruses evolved cap-independent translation enhancers (3'CITE) in the 3' untranslated regions of their genomic (g)RNAs to compete with ongoing cap-dependent translation of cellular mRNAs. Umbravirus Pea enation mosaic virus (PEMV)2 is the only example where three 3'CITEs enhance translation: the eIF4E-binding Panicum mosaic virus-like translational enhancer (PTE) and ribosome-binding 3' T-shaped structure (TSS) have been found in viruses of different genera, while the ribosome-binding kl-TSS that provides a long-distance interaction with the 5' end is unique. We report that the PTE is the key translation promoting element, but inhibits translation in cis and in trans in the absence of the kl-TSS by sequestering initiation factor eIF4G. PEMV2 strongly outcompeted a cellular mRNA mimic for translation, indicating that the combination of kl-TSS and PTE is highly efficient. Transferring the 3'-5' interaction from the kl-TSS to the PTE (to fulfill its functionality as found in other viruses) supported translationin vitro, but gRNA did not accumulate to detectable levels in protoplasts in the absence of the kl-TSS. It was shown that the PTE in conjunction with the kl-TSS did not markedly affect the translation initiation rate but rather increased the number of gRNAs available for translation. A model is proposed to explain how 3'CITE-based regulation of ribosome recruitment enhances virus fitness.

The heat-shock protein/chaperone network and multiple stress resistance

Crop yield has been greatly enhanced during the last century. However, most elite cultivars are adapted to temperate climates and are not well suited to more stressful conditions. In the context of climate change, stress resistance is a major concern. To overcome these difficulties, scientists may help breeders by providing genetic markers associated with stress resistance. However, multistress resistance cannot be obtained from the simple addition of single stress resistance traits. In the field, stresses are unpredictable and several may occur at once. Consequently, the use of single stress resistance traits is often inadequate. Although it has been historically linked with the heat stress response, the heat-shock protein (HSP)/chaperone network is a major component of multiple stress responses. Among the HSP/chaperone 'client proteins', many are primary metabolism enzymes and signal transduction components with essential roles for the proper functioning of a cell. HSPs/chaperones are controlled by the action of diverse heat-shock factors, which are recruited under stress conditions. In this review, we give an overview of the regulation of the HSP/chaperone network with a focus on Arabidopsis thaliana. We illustrate the role of HSPs/chaperones in regulating diverse signalling pathways and discuss several basic principles that should be considered for engineering multiple stress resistance in crops through the HSP/chaperone network.

TMV mutants with poly(A) tracts of different lengths demonstrate structural variations in 3'UTR affecting viral RNAs accumulation and symptom expression

The upstream pseudoknots domain (UPD) of Tobacco mosaic virus (TMV) is located at the 3'-untranslated region (UTR). It plays an important role in virus replication and translation. To determine the importance of UPD and 3'-UTR, and the effects of introduced RNA elements in TMV 3'-UTR, a series of TMV mutants with internal poly(A) tract upstream of UPD was constructed for structural analysis by selective 2'-hydroxyl acylation analyzed by primer extension (SHAPE). TMV(24A+UPD) and TMV(42A+UPD) formed a similar structure as that of TMV 3'-UTR, but TMV(62A+UPD) structures altered by the introduced poly(A) tract. In addition, TMV(24A+UPD) had a higher viral RNAs accumulation than TMV in N. benthamiana protoplasts, and induced lethal symptoms in the infected plants. TMV(62A+UPD) showed a drastically reduced accumulation, its coat protein was undetectable in protoplasts, and the inoculated plants remained symptomless. This study analyzed the structures of 3'-UTR of TMV and found that the longer poly(A) tract introduced upstream of UPD reduced viral RNAs accumulation and induced milder symptoms in N. benthamiana. In conclusion, different lengths of the internal poly(A) tract introduced into the TMV 3'UTR lead to structural variations that affect virus accumulation and symptom expression.

Functions of the 5'- and 3'-untranslated regions of tobamovirus RNA

The tobamovirus genome is a 5'-m(7)G-capped RNA that carries a tRNA-like structure at its 3'-terminus. The genomic RNA serves as the template for both translation and negative-strand RNA synthesis. The 5'- and 3'-untranslated regions (UTRs) of the genomic RNA contain elements that enhance translation, and the 3'-UTR also contains the elements necessary for the initiation of negative-strand RNA synthesis. Recent studies using a cell-free viral RNA translation-replication system revealed that a 70-nucleotide region containing a part of the 5'-UTR is bound cotranslationally by tobacco mosaic virus (TMV) replication proteins translated from the genomic RNA and that the binding leads the genomic RNA to RNA replication pathway. This mechanism explains the cis-preferential replication of TMV by the replication proteins. The binding also inhibits further translation to avoid a fatal ribosome-RNA polymerase collision, which might arise if translation and negative-strand synthesis occur simultaneously on a single genomic RNA molecule. Therefore, the 5'- and 3'-UTRs play multiple important roles in the life cycle of tobamovirus.

The length of an internal poly(A) tract of hibiscus latent Singapore virus is crucial for its replication

Hibiscus latent Singapore virus (HLSV) mutants were constructed to study roles of its internal poly(A) tract (IPAT) in viral replication and coat protein (CP) expression. Shortening of the IPAT resulted in reduced HLSV RNA accumulation and its minimal length required for HLSV CP expression in plants was 24 nt. Disruption of a putative long range RNA-RNA interacting structure between 5' and 3' untranslated regions of HLSV-22A and -24A resulted in reduced viral RNA and undetectable CP accumulation in inoculated leaves. Replacement of the IPAT in HLSV with an upstream pseudoknot domain (UPD) of Tobacco mosaic virus (TMV) or insertion of the UPD to the immediate downstream of a 24 nt IPAT in HLSV resulted in drastically reduced viral RNA replication. Plants infected with a TMV mutant by replacement of the UPD with 43 nt IPAT exhibited milder mosaic symptoms without necrosis. We have proposed a model for HLSV replication.

Intergenic sequence between Arabidopsis caseinolytic protease B-cytoplasmic/heat shock protein100 and choline kinase genes functions as a heat-inducible bidirectional promoter

In Arabidopsis (Arabidopsis thaliana), the At1g74310 locus encodes for caseinolytic protease B-cytoplasmic (ClpB-C)/heat shock protein100 protein (AtClpB-C), which is critical for the acquisition of thermotolerance, and At1g74320 encodes for choline kinase (AtCK2) that catalyzes the first reaction in the Kennedy pathway for phosphatidylcholine biosynthesis. Previous work has established that the knockout mutants of these genes display heat-sensitive phenotypes. While analyzing the AtClpB-C promoter and upstream genomic regions in this study, we noted that AtClpB-C and AtCK2 genes are head-to-head oriented on chromosome 1 of the Arabidopsis genome. Expression analysis showed that transcripts of these genes are rapidly induced in response to heat stress treatment. In stably transformed Arabidopsis plants harboring this intergenic sequence between head-to-head oriented green fluorescent protein and β-glucuronidase reporter genes, both transcripts and proteins of the two reporters were up-regulated upon heat stress. Four heat shock elements were noted in the intergenic region by in silico analysis. In the homozygous transfer DNA insertion mutant Salk_014505, 4,393-bp transfer DNA is inserted at position -517 upstream of ATG of the AtClpB-C gene. As a result, AtCk2 loses proximity to three of the four heat shock elements in the mutant line. Heat-inducible expression of the AtCK2 transcript was completely lost, whereas the expression of AtClpB-C was not affected in the mutant plants. Our results suggest that the 1,329-bp intergenic fragment functions as a heat-inducible bidirectional promoter and the region governing the heat inducibility is possibly shared between the two genes. We propose a model in which AtClpB-C shares its regulatory region with heat-induced choline kinase, which has a possible role in heat signaling.

Multiple different defense mechanisms are activated in the young transgenic tobacco plants which express the full length genome of the Tobacco mosaic virus, and are resistant against this virus

Previously described transgenic tobacco lines express the full length infectious Tobacco mosaic virus (TMV) genome under the 35S promoter (Siddiqui et al., 2007. Mol Plant Microbe Interact, 20: 1489-1494). Through their young stages these plants exhibit strong resistance against both the endogenously expressed and exogenously inoculated TMV, but at the age of about 7-8 weeks they break into TMV infection, with typical severe virus symptoms. Infections with some other viruses (Potato viruses Y, A, and X) induce the breaking of the TMV resistance and lead to synergistic proliferation of both viruses. To deduce the gene functions related to this early resistance, we have performed microarray analysis of the transgenic plants during the early resistant stage, and after the resistance break, and also of TMV-infected wild type tobacco plants. Comparison of these transcriptomes to those of corresponding wild type healthy plants indicated that 1362, 1150 and 550 transcripts were up-regulated in the transgenic plants before and after the resistance break, and in the TMV-infected wild type tobacco plants, respectively, and 1422, 1200 and 480 transcripts were down-regulated in these plants, respectively. These transcriptome alterations were distinctly different between the three types of plants, and it appears that several different mechanisms, such as the enhanced expression of the defense, hormone signaling and protein degradation pathways contributed to the TMV-resistance in the young transgenic plants. In addition to these alterations, we also observed a distinct and unique gene expression alteration in these plants, which was the strong suppression of the translational machinery. This may also contribute to the resistance by slowing down the synthesis of viral proteins. Viral replication potential may also be suppressed, to some extent, by the reduction of the translation initiation and elongation factors eIF-3 and eEF1A and B, which are required for the TMV replication complex.

Clp chaperones and proteases are central in stress survival, virulence and antibiotic resistance of Staphylococcus aureus

Intracellular proteolysis carried out by energy-dependent proteases is one of the most conserved biological processes. In all cells proteolysis maintains and shapes the cellular proteome by ridding the cell of damaged proteins and by regulating abundance of functional proteins such as regulatory proteins. The ATP-dependent ClpP protease is highly conserved among eubacteria and in the chloroplasts and mitochondria of eukaryotic cells. In the serious human pathogen, Staphylococcus aureus inactivation of clpP rendered the bacterium avirulent emphasizing the central role of proteolysis in virulence. The contribution of the Clp proteins to virulence is likely to occur at multiple levels. First of all, both Clp ATPases and the Clp protease are central players in stress responses required to cope with the adverse conditions met in the host. The ClpP protease has a dual role herein, as it both eliminates stress-damaged proteins as well as ensures the timely degradation of major stress regulators such as Spx, LexA and CtsR. Additionally, as we will summarize in this review, Clp proteases and Clp chaperones impact on such central processes as virulence gene expression, cell wall metabolism, survival in stationary phase, and cell division. These observations together with recent findings that Clp proteins contribute to adaptation to antibiotics highlights the importance of this interesting proteolytic machinery both for understanding pathogenicity of the organism and for treating staphylococcal infections.

The immediate upstream region of the 5'-UTR from the AUG start codon has a pronounced effect on the translational efficiency in Arabidopsis thaliana

The nucleotide sequence around the translational initiation site is an important cis-acting element for post-transcriptional regulation. However, it has not been fully understood how the sequence context at the 5′-untranslated region (5′-UTR) affects the translational efficiency of individual mRNAs. In this study, we provide evidence that the 5′-UTRs of Arabidopsis genes showing a great difference in the nucleotide sequence vary greatly in translational efficiency with more than a 200-fold difference. Of the four types of nucleotides, the A residue was the most favourable nucleotide from positions −1 to −21 of the 5′-UTRs in Arabidopsis genes. In particular, the A residue in the 5′-UTR from positions −1 to −5 was required for a high-level translational efficiency. In contrast, the T residue in the 5′-UTR from positions −1 to −5 was the least favourable nucleotide in translational efficiency. Furthermore, the effect of the sequence context in the −1 to −21 region of the 5′-UTR was conserved in different plant species. Based on these observations, we propose that the sequence context immediately upstream of the AUG initiation codon plays a crucial role in determining the translational efficiency of plant genes.

SUPPRESSOR OF MORE AXILLARY GROWTH2 1 controls seed germination and seedling development in Arabidopsis

Abiotic chemical signals discovered in smoke that are known as karrikins (KARs) and the endogenous hormone strigolactone (SL) control plant growth through a shared MORE AXILLARY GROWTH2 (MAX2)-dependent pathway. A SL biosynthetic pathway and candidate KAR/SL receptors have been characterized, but signaling downstream of MAX2 is poorly defined. A screen for genetic suppressors of the enhanced seed dormancy phenotype of max2 in Arabidopsis (Arabidopsis thaliana) led to identification of a suppressor of max2 1 (smax1) mutant. smax1 restores the seed germination and seedling photomorphogenesis phenotypes of max2 but does not affect the lateral root formation, axillary shoot growth, or senescence phenotypes of max2. Expression of three transcriptional markers of KAR/SL signaling, D14-LIKE2, KAR-UP F-BOX1, and INDOLE-3-ACETIC ACID INDUCIBLE1, is rescued in smax1 max2 seedlings. SMAX1 is a member of an eight-gene family in Arabidopsis that has weak similarity to HEAT SHOCK PROTEIN 101, which encodes a caseinolytic peptidase B chaperonin required for thermotolerance. SMAX1 and the SMAX1-like (SMXL) homologs are differentially expressed in Arabidopsis tissues. SMAX1 transcripts are most abundant in dry seed, consistent with its function in seed germination control. Several SMXL genes are up-regulated in seedlings treated with the synthetic SL GR24. SMAX1 and SMXL2 transcripts are reduced in max2 seedlings, which could indicate negative feedback regulation by KAR/SL signaling. smax1 seed and seedling growth mimics the wild type treated with KAR/SL, but smax1 seedlings are still responsive to 2H-furo[2,3-c]pyran-2-one (KAR2) or GR24. We conclude that SMAX1 is an important component of KAR/SL signaling during seed germination and seedling growth but is not necessary for all MAX2-dependent responses. We hypothesize that one or more SMXL proteins may also act downstream of MAX2 to control the diverse developmental responses to KARs and SLs.

Interplay between heat shock proteins HSP101 and HSA32 prolongs heat acclimation memory posttranscriptionally in Arabidopsis

Heat acclimation improves the tolerance of organisms to severe heat stress. Our previous work showed that in Arabidopsis (Arabidopsis thaliana), the "memory" of heat acclimation treatment decayed faster in the absence of the heat-stress-associated 32-kD protein HSA32, a heat-induced protein predominantly found in plants. The HSA32 null mutant attains normal short-term acquired thermotolerance but is defective in long-term acquired thermotolerance. To further explore this phenomenon, we isolated Arabidopsis defective in long-term acquired thermotolerance (dlt) mutants using a forward genetic screen. Two recessive missense alleles, dlt1-1 and dlt1-2, encode the molecular chaperone heat shock protein101 (HSP101). Results of immunoblot analyses suggest that HSP101 enhances the translation of HSA32 during recovery after heat treatment, and in turn, HSA32 retards the decay of HSP101. The dlt1-1 mutation has little effect on HSP101 chaperone activity and thermotolerance function but compromises the regulation of HSA32. In contrast, dlt1-2 impairs the chaperone activity and thermotolerance function of HSP101 but not the regulation of HSA32. These results suggest that HSP101 has a dual function, which could be decoupled by the mutations. Pulse-chase analysis showed that HSP101 degraded faster in the absence of HSA32. The autophagic proteolysis inhibitor E-64d, but not the proteasome inhibitor MG132, inhibited the degradation of HSP101. Ectopic expression of HSA32 confirmed its effect on the decay of HSP101 at the posttranscriptional level and showed that HSA32 was not sufficient to confer long-term acquired thermotolerance when the HSP101 level was low. Taken together, we propose that a positive feedback loop between HSP101 and HSA32 at the protein level is a novel mechanism for prolonging the memory of heat acclimation.

Regulation of Translation Initiation under Biotic and Abiotic Stresses

Plants have developed versatile strategies to deal with the great variety of challenging conditions they are exposed to. Among them, the regulation of translation is a common target to finely modulate gene expression both under biotic and abiotic stress situations. Upon environmental challenges, translation is regulated to reduce the consumption of energy and to selectively synthesize proteins involved in the proper establishment of the tolerance response. In the case of viral infections, the situation is more complex, as viruses have evolved unconventional mechanisms to regulate translation in order to ensure the production of the viral encoded proteins using the plant machinery. Although the final purpose is different, in some cases, both plants and viruses share common mechanisms to modulate translation. In others, the mechanisms leading to the control of translation are viral- or stress-specific. In this paper, we review the different mechanisms involved in the regulation of translation initiation under virus infection and under environmental stress in plants. In addition, we describe the main features within the viral RNAs and the cellular mRNAs that promote their selective translation in plants undergoing biotic and abiotic stress situations.

Cellular chaperones and folding enzymes are vital contributors to membrane bound replication and movement complexes during plant RNA virus infection

Cellular chaperones and folding enzymes play central roles in the formation of positive-strand and negative-strand RNA virus infection. This article examines the key cellular chaperones and discusses evidence that these factors are diverted from their cellular functions to play alternative roles in virus infection. For most chaperones discussed, their primary role in the cell is to ensure protein quality control. They are system components that drive substrate protein folding, complex assembly or disaggregation. Their activities often depend upon co-chaperones and ATP hydrolysis. During plant virus infection, Hsp70 and Hsp90 proteins play central roles in the formation of membrane-bound replication complexes for certain members of the tombusvirus, tobamovirus, potyvirus, dianthovirus, potexvirus, and carmovirus genus. There are several co-chaperones, including Yjd1, RME-8, and Hsp40 that associate with the bromovirus replication complex, pomovirus TGB2, and tospovirus Nsm movement proteins. There are also examples of plant viruses that rely on chaperone systems in the endoplasmic reticulum (ER) to support cell-to-cell movement. TMV relies on calreticulin to promote virus intercellular transport. Calreticulin also resides in the plasmodesmata and plays a role in calcium sequestration as well as glycoprotein folding. The pomovirus TGB2 interacts with RME-8 in the endosome. The potexvirus TGB3 protein stimulates expression of ER resident chaperones via the bZIP60 transcription factor. Up-regulating factors involved in protein folding may be essential to handling the load of viral proteins translated along the ER. In addition, TGB3 stimulates SKP1 which is a co-factor in proteasomal degradation of cellular proteins. Such chaperones and co-factors are potential targets for antiviral defense.

Hsp90 interacts specifically with viral RNA and differentially regulates replication initiation of Bamboo mosaic virus and associated satellite RNA

Host factors play crucial roles in the replication of plus-strand RNA viruses. In this report, a heat shock protein 90 homologue of Nicotiana benthamiana, NbHsp90, was identified in association with partially purified replicase complexes from BaMV-infected tissue, and shown to specifically interact with the 3' untranslated region (3' UTR) of BaMV genomic RNA, but not with the 3' UTR of BaMV-associated satellite RNA (satBaMV RNA) or that of genomic RNA of other viruses, such as Potato virus X (PVX) or Cucumber mosaic virus (CMV). Mutational analyses revealed that the interaction occurs between the middle domain of NbHsp90 and domain E of the BaMV 3' UTR. The knockdown or inhibition of NbHsp90 suppressed BaMV infectivity, but not that of satBaMV RNA, PVX, or CMV in N. benthamiana. Time-course analysis further revealed that the inhibitory effect of 17-AAG is significant only during the immediate early stages of BaMV replication. Moreover, yeast two-hybrid and GST pull-down assays demonstrated the existence of an interaction between NbHsp90 and the BaMV RNA-dependent RNA polymerase. These results reveal a novel role for NbHsp90 in the selective enhancement of BaMV replication, most likely through direct interaction with the 3' UTR of BaMV RNA during the initiation of BaMV RNA replication.

The absence of heat shock protein HSP101 affects the proteome of mature and germinating maize embryos

Maize heat shock protein HSP101 accumulates during embryo maturation and desiccation and persists at high levels during the first 24 h following kernel imbibition in the absence of heat stress. This protein has a known function in disaggregation of high molecular weight complexes and has been proposed to be a translational regulator of specific mRNAs. Here, a global proteomic approach was used to identify changes in the maize proteome due to the absence of HSP101 in embryos from mature-dry or 24 h-imbibed kernels. A total of 26 protein spots from the mature dry embryo exhibited statistically significant expression changes in the L10 inbred hsp101 mutant (hsp101-m5::Mu1/hsp101-m5::Mu1) line as compared to the corresponding wild type (Hsp101/Hsp101). Additional six spots reproducibly showed qualitative changes between the mutant and wild-type mature and germinating embryos. Several chaperones, translation-related proteins, actin, and enzymes participating in cytokinin metabolism were identified in these spots by tandem mass-spectrometry (MS). The proteomic changes partially explain the altered root growth and architecture observed in young hsp101 mutant seedlings. In addition, specific protein de novo synthesis was altered in the 24 h-imbibed mutant embryos indicating that maize HSP101 functions as both chaperone and translational regulator during germination. Supporting this, HSP101 was found as part of Cap-binding and translation initiation complexes during early kernel imbibition. Overall, these findings expose the relevance of maize HSP101 for protein synthesis and balance mechanisms during germination.

Cellular chaperones and folding enzymes are vital contributors to membrane bound replication and movement complexes during plant RNA virus infection

Cellular chaperones and folding enzymes play central roles in the formation of positive-strand and negative-strand RNA virus infection. This article examines the key cellular chaperones and discusses evidence that these factors are diverted from their cellular functions to play alternative roles in virus infection. For most chaperones discussed, their primary role in the cell is to ensure protein quality control. They are system components that drive substrate protein folding, complex assembly or disaggregation. Their activities often depend upon co-chaperones and ATP hydrolysis. During plant virus infection, Hsp70 and Hsp90 proteins play central roles in the formation of membrane-bound replication complexes for certain members of the tombusvirus, tobamovirus, potyvirus, dianthovirus, potexvirus, and carmovirus genus. There are several co-chaperones, including Yjd1, RME-8, and Hsp40 that associate with the bromovirus replication complex, pomovirus TGB2, and tospovirus Nsm movement proteins. There are also examples of plant viruses that rely on chaperone systems in the endoplasmic reticulum (ER) to support cell-to-cell movement. TMV relies on calreticulin to promote virus intercellular transport. Calreticulin also resides in the plasmodesmata and plays a role in calcium sequestration as well as glycoprotein folding. The pomovirus TGB2 interacts with RME-8 in the endosome. The potexvirus TGB3 protein stimulates expression of ER resident chaperones via the bZIP60 transcription factor. Up-regulating factors involved in protein folding may be essential to handling the load of viral proteins translated along the ER. In addition, TGB3 stimulates SKP1 which is a co-factor in proteasomal degradation of cellular proteins. Such chaperones and co-factors are potential targets for antiviral defense.

Role of HSP101 in the stimulation of nodal root development from the coleoptilar node by light and temperature in maize (Zea mays L.) seedlings

Nodal roots (NRs) constitute the prevalent root system of adult maize plants. NRs emerge from stem nodes located below or above ground, and little is known about their inducing factors. Here, it is shown that precocious development of NRs at the coleoptilar node (NRCNs) occurred in maize seedlings when: (i) dark grown and stimulated by the concurrent action of a single light shock of low intensity white light (2 μmol m(-2) s(-1)) and a single heat shock; (ii) grown under a photoperiod of low intensity light (0.1 μmol m(-2) s(-1)); or (iii) grown in the dark under a thermoperiod (28 °C/34 °C). The light shock effects were synergistic with heat shock and with the photoperiod, whereas the thermoperiodical and photoperiodical effects were additive. Dissection of the primary root or the root cap, to mimic the fatal consequences of severe heat shock, caused negligible effects on NRCN formation, indicating that the shoot is directly involved in perception of the heat shock-inducible signal that triggered NRCN formation. A comparison between hsp101-m5::Mu1/hsp101-m5::Mu1 and Hsp101/Hsp101 seedlings indicated that the heat shock protein 101 (HSP101) chaperone inhibited NRCN formation in the light and in the dark. Stimulation of precocious NRCN formation by light and heat shocks was affected by genetic background and by the stage of seedling development. HSP101 protein levels increased in the coleoptilar node of induced wild-type plants, particularly in the procambial region, where NRCN formation originated. The adaptive relevance of development of NRCNs in response to these environmental cues and hypothetical mechanisms of regulation by HSP101 are discussed.

Chapter 2. Development of key technologies for high-throughput cell-free protein production with the extract from wheat embryos

The cell-free translation system from wheat embryos had been considered to be inefficient as compared with the E. coli cell-based and cell-free protein production methods. However, it was revealed that the extract from extensively washed wheat embryo particles can provide a very productive cell-free protein synthesis system. Since then, the method has been improved, so that it fits the postgenomic researches. New mRNA configurations enabled us to synthesize many different proteins in parallel and to prepare large amounts of proteins, which fits the need for screening of suitable proteins for structural and functional analyses before large-scale production. The new reaction formats promoted the developments of new machines that perform highly parallel and highly productive protein synthesis reactions automatically. It was revealed that, by parallel synthesis of many proteins, much more multidomain proteins are produced in soluble forms in the wheat system than in the prokaryotic systems. The wheat system provides a rapid and cost-effective method for stable isotope labeling of proteins for NMR analyses. Selenomethionine substitution of proteins for X-ray crystallography through the cell-free synthesis was also achieved. Synthesis of some families of proteins that were difficult to be produced by conventional methods has been tested. At least, cytotoxic restriction enzymes were readily produced in a large amount. Some multisubunit proteins and cofactor-binding proteins could be synthesized by the method and were characterized successfully. Membrane proteins have also been tested, and a transporter was synthesized in an active form. Although some issues remains to be solved, we expect that the wheat cell-free protein synthesis system can contribute to the structural and functional genomics and to the future understanding of life.

A Simple, Highly Efficient Method for Heterologous Expression in Mammalian Primary Neurons Using Cationic Lipid-mediated mRNA Transfection

Expression of heterologous proteins in adult mammalian neurons is a valuable technique for the study of neuronal function. The post-mitotic nature of mature neurons prevents effective DNA transfection using simple, cationic lipid-based methods. Adequate heterologous protein expression is often only achievable using complex techniques that, in many cases, are associated with substantial toxicity. Here, a simple method for high efficiency transfection of mammalian primary neurons using in vitro transcribed mRNA and the cationic lipid transfection reagent Lipofectamine™ 2000 is described. Optimal transfection conditions were established in adult mouse dissociated dorsal root ganglion (DRG) neurons using a 96-well based luciferase activity assay. Using these conditions, a transfection efficiency of 25% was achieved in DRG neurons transfected with EGFP mRNA. High transfection efficiencies were also obtained in dissociated rat superior cervical ganglion (SCG) neurons and mouse cortical and hippocampal cultures. Endogenous Ca²⁺ currents in EGFP mRNA-transfected SCG neurons were not significantly different from untransfected neurons, which suggested that this technique is well suited for heterologous expression in patch clamp recording experiments. Functional expression of a cannabinoid receptor (CB1R), a G protein inwardly rectifying K⁺ channel (GIRK4) and a dominant-negative G protein α-subunit mutant (G_oA G203T) indicate that the levels of heterologous protein expression attainable using mRNA transfection are suitable for most functional protein studies. This study demonstrates that mRNA transfection is a straightforward and effective method for heterologous expression in neurons and is likely to have many applications in neuroscience research.

Plant Hsp100/ClpB-like proteins: poorly-analyzed cousins of yeast ClpB machine

ClpB/Hsp100 proteins act as chaperones, mediating disaggregation of denatured proteins. Recent work shows that apart from cytoplasm, these proteins are localized to nuclei, chloroplasts, mitochondria and plasma membrane. While ClpB/Hsp100 genes are essentially stress-induced (mainly heat stress) in vegetative organs of the plant body, expression of ClpB/Hsp100 proteins is noted to be constitutive in plant reproductive structures like pollen grains, developing embryos, seeds etc. With global warming looming large on the horizon, ways to genetically engineer plants against high temperature stress are urgently needed. Yeast mutants unable to synthesize active ClpB/Hsp100 protein show a clear thermosensitive phenotype. ClpB/Hsp100 proteins are implicated in high temperature stress tolerance in plants. We herein highlight the selected important facets of this protein family in plants.

The chaperone ClpX stimulates expression of Staphylococcus aureus protein A by Rot dependent and independent pathways

The Clp ATPases (Hsp100) constitute a family of closely related proteins that have protein reactivating and remodelling activities typical of molecular chaperones. In Staphylococcus aureus the ClpX chaperone is essential for virulence and for transcription of spa encoding Protein A. The present study was undertaken to elucidate the mechanism by which ClpX stimulates expression of Protein A. For this purpose, we prepared antibodies directed against Rot, an activator of spa transcription, and demonstrated that cells devoid of ClpX contain three-fold less Rot than wild-type cells. By varying Rot expression from an inducible promoter we showed that expression of Protein A requires a threshold level of Rot. In the absence of ClpX the Rot content is reduced below this threshold level, hence, explaining the substantially reduced Protein A expression in the clpX mutant. Experiments addressed at pinpointing the role of ClpX in Rot synthesis revealed that ClpX is required for translation of Rot. Interestingly, translation of the spa mRNA was, like the rot mRNA, enhanced by ClpX. These data demonstrate that ClpX performs dual roles in regulating Protein A expression, as ClpX stimulates transcription of spa by enhancing translation of Rot, and that ClpX additionally is required for full translation of the spa mRNA. The current findings emphasize that ClpX has a central role in fine-tuning virulence regulation in S. aureus.

Lipid-mediated delivery of RNA is more efficient than delivery of DNA in non-dividing cells

The design of appropriate gene delivery systems is essential for the successful application of gene therapy to clinical medicine. Cationic lipid-mediated delivery is a viable alternative to viral vector-mediated gene delivery in applications where transient gene expression is desirable. However, cationic lipid-mediated delivery of DNA to post-mitotic cells such as neurons is often reported to be of low efficiency, due to the presumed inability of the DNA to translocate to the nucleus. Lipid-mediated delivery of RNA is an attractive alternative to non-viral DNA delivery in some clinical applications, because transit across the nuclear membrane is not necessary. Here we report a comparative investigation of cationic lipid-mediated delivery of RNA versus DNA vectors encoding the reporter gene green fluorescent protein (GFP) in Chinese Hamster Ovary (CHO) and NIH3T3 cells following chemical inhibition of proliferation, and in primary mixed neuronal cell cultures. Using optimized formulations and transfection procedures, we assess gene expression by flow cytometry to specifically address some of the advantages and disadvantages of lipid-mediated RNA and DNA gene transfer. Despite inhibition of cell proliferation, over 45% of CHO cells express GFP after lipid-mediated transfection with RNA vectors. Transfection efficiency of DNA encoding GFP in proliferation-inhibited CHO cells was less than 5%. Detectable expression after RNA transfection occurs at least 3h earlier than after DNA transfection, but DNA transfection eventually produces a mean level of per cell GFP expression (as assayed by flow cytometry) that is higher than after RNA transfection. Transfection of proliferation-inhibited NIH3T3 cells and primary mixed neuronal cultures produced similar results, with RNA encoded GFP expression in 2-4 times the number of cells as after DNA encoded GFP expression. These results demonstrate the increased efficiency of RNA transfection relative to DNA transfection in non-dividing cells. We used firefly luciferase encoded by RNA and DNA vectors to investigate the time course of gene expression after delivery of RNA or DNA to primary neuronal cortical cells. Delivery of mRNA resulted in rapid onset (within 1h) of luciferase expression after transfection, a peak in expression 5-7h after transfection, and a return to baseline within 12h after transfection. After DNA delivery significant luciferase activity did not appear until 7h after transfection, but peak luciferase expression was always at least one order of magnitude higher than after RNA delivery. The peak expression after luciferase-expressing DNA delivery occurred 36-48 h after transfection and remained at a significant level for at least one week before dropping to baseline. This observation is consistent with our in vivo delivery results, which are shown as well. RNA delivery may therefore be more suitable for short-term transient gene expression due to rapid onset, shorter duration of expression and greater efficiency, particularly in non-dividing cells. Higher mean levels of expression per cell obtained following DNA delivery and the longer duration of expression confirm a continuing role for DNA gene delivery in clinical applications that require longer term transient gene expression.

The 5' untranslated region of the VR-ACS1 mRNA acts as a strong translational enhancer in plants

The structure and function of untranslated mRNA leader sequences and their role in controlling gene expression remains poorly understood. Previous research has suggested that the 5' untranslated region (5'UTR) of the Vigna radiata aminocyclopropane-1-carboxylate synthase synthase (VR-ACS1) gene may function as a translational enhancer in plants. To test such hypothesis we compared the translation enhancing properties of three different 5'UTRs; those from the VR-ACS1, the chlorophyll a/b binding gene from petunia (Cab22L; a known translational enhancer) and the Vigna radiata pectinacetylesterase gene (PAE; used as control). Identical constructs in which the coding region of the beta-glucuronidase (GUS) gene was fused to each of the three 5'UTRs and placed under the control of the cauliflower mosaic virus 35S promoter were prepared. Transient expression assays in tobacco cell cultures and mung bean leaves showed that the VR-ACS1 and Cab22L 5'UTRs directed higher levels of GUS activity than the PAE 5'UTR. Analysis of transgenic Arabidopsis thaliana seedlings, as well as different tissues from mature plants, confirmed that while transcript levels were equivalent for all constructs, the 5'UTRs from the VR-ACS1 and Cab22L genes can increase GUS activity twofold to fivefold compared to the PAE 5'UTR, therefore confirming the translational enhancing properties of the VR-ACS1 5'UTR.

The 5' cap of tobacco mosaic virus (TMV) is required for virion attachment to the actin/endoplasmic reticulum network during early infection

Almost nothing is known of the earliest stages of plant virus infections. To address this, we microinjected Cy3 (UTP)-labelled tobacco mosaic virus (TMV) into living tobacco trichome cells. The Cy3-virions were infectious, and the viral genome trafficked from cell to cell. However, neither the fluorescent vRNA pool nor the co-injected green fluorescent protein (GFP) left the injected trichome, indicating that the synthesis of (unlabelled) progeny viral (v)RNA is required to initiate cell-to-cell movement, and that virus movement is not accompanied by passive plasmodesmatal gating. Cy3-vRNA formed granules that became anchored to the motile cortical actin/endoplasmic reticulum (ER) network within minutes of injection. Granule movement on actin/ER was arrested by actin inhibitors indicating actin-dependent RNA movement. The 5' methylguanosine cap was shown to be required for vRNA anchoring to the actin/ER. TMV vRNA lacking the 5' cap failed to form granules and was degraded in the cytoplasm. Removal of the 3' untranslated region or replicase both inhibited replication but did not prevent granule formation and movement. Dual-labelled TMV virions in which the vRNA and the coat protein were highlighted with different fluorophores showed that both fluorescent signals were initially located on the same ER-bound granules, indicating that TMV virions may become attached to the ER prior to uncoating of the viral genome.

Identification and validation of reference genes for quantitative RT-PCR normalization in wheat

BACKGROUND

Usually the reference genes used in gene expression analysis have been chosen for their known or suspected housekeeping roles, however the variation observed in most of them hinders their effective use. The assessed lack of validated reference genes emphasizes the importance of a systematic study for their identification. For selecting candidate reference genes we have developed a simple in silico method based on the data publicly available in the wheat databases Unigene and TIGR.

RESULTS

The expression stability of 32 genes was assessed by qRT-PCR using a set of cDNAs from 24 different plant samples, which included different tissues, developmental stages and temperature stresses. The selected sequences included 12 well-known HKGs representing different functional classes and 20 genes novel with reference to the normalization issue. The expression stability of the 32 candidate genes was tested by the computer programs geNorm and NormFinder using five different data-sets. Some discrepancies were detected in the ranking of the candidate reference genes, but there was substantial agreement between the groups of genes with the most and least stable expression. Three new identified reference genes appear more effective than the well-known and frequently used HKGs to normalize gene expression in wheat. Finally, the expression study of a gene encoding a PDI-like protein showed that its correct evaluation relies on the adoption of suitable normalization genes and can be negatively affected by the use of traditional HKGs with unstable expression, such as actin and alpha-tubulin.

CONCLUSION

The present research represents the first wide screening aimed to the identification of reference genes and of the corresponding primer pairs specifically designed for gene expression studies in wheat, in particular for qRT-PCR analyses. Several of the new identified reference genes outperformed the traditional HKGs in terms of expression stability under all the tested conditions. The new reference genes will enable more accurate normalization and quantification of gene expression in wheat and will be helpful for designing primer pairs targeting orthologous genes in other plant species.

Identification of an Arabidopsis thaliana protein that binds to tomato mosaic virus genomic RNA and inhibits its multiplication

The genomic RNAs of positive-strand RNA viruses carry RNA elements that play positive, or in some cases, negative roles in virus multiplication by interacting with viral and cellular proteins. In this study, we purified Arabidopsis thaliana proteins that specifically bind to 5' or 3' terminal regions of tomato mosaic virus (ToMV) genomic RNA, which contain important regulatory elements for translation and RNA replication, and identified these proteins by mass spectrometry analyses. One of these host proteins, named BTR1, harbored three heterogeneous nuclear ribonucleoprotein K-homology RNA-binding domains and preferentially bound to RNA fragments that contained a sequence around the initiation codon of the 130K and 180K replication protein genes. The knockout and overexpression of BTR1 specifically enhanced and inhibited, respectively, ToMV multiplication in inoculated A. thaliana leaves, while such effect was hardly detectable in protoplasts. These results suggest that BTR1 negatively regulates the local spread of ToMV.

The 5'-untranslated region of the Oryza sativa alcohol dehydrogenase gene functions as a translational enhancer in monocotyledonous plant cells

The 5'-untranslated region (5'-UTR) functions as a translational enhancer in monocotyledonous plant cells is necessary to express a foreign gene efficiently. Here, we show that the 5'-UTR of the rice alcohol dehydrogenase gene contributes to efficient translation in not only dicotyledonous plant cells, but also in monocotyledonous rice cells.

The effect of drought and heat stress on reproductive processes in cereals

As the result of intensive research and breeding efforts over the last 20 years, the yield potential and yield quality of cereals have been greatly improved. Nowadays, yield safety has gained more importance because of the forecasted climatic changes. Drought and high temperature are especially considered as key stress factors with high potential impact on crop yield. Yield safety can only be improved if future breeding attempts will be based on the valuable new knowledge acquired on the processes determining plant development and its responses to stress. Plant stress responses are very complex. Interactions between plant structure, function and the environment need to be investigated at various phases of plant development at the organismal, cellular as well as molecular levels in order to obtain a full picture. The results achieved so far in this field indicate that various plant organs, in a definite hierarchy and in interaction with each other, are involved in determining crop yield under stress. Here we attempt to summarize the currently available information on cereal reproduction under drought and heat stress and to give an outlook towards potential strategies to improve yield safety in cereals.

Improvement of the pBI121 plant expression vector by leader replacement with a sequence combining a poly(CAA) and a CT motif

To improve expression levels of recombinant proteins in plants, a new leader sequence was designed. Several elements known to enhance gene translation and/or transcription were considered, including the CaMV 35S Inr site, a CT-rich motif often shared by highly expressed plant genes and a poly(CAA) region widespread in tobamovirus and plant leaders. The effect of the synthetic leader on gusA expression was evaluated in genetically modified tobacco plants by measuring the beta-glucuronidase activity and the mRNA level. When compared to the gusA leader of pBI121, the new sequence determined a 8.6-fold and a 12.5-fold increase of enzyme concentration taking into account the whole plant population or the above-average expressors, respectively. Since most pCAMBIA vectors harbour a very short 5'-UTR, identical to a fragment of the pBI121 leader, leader replacement with the sequence herein described is strongly suggested.

Complexity of rice Hsp100 gene family: lessons from rice genome sequence data

Elucidation of genome sequence provides an excellent platform to understand detailed complexity of the various gene families. Hsp100 is an important family of chaperones in diverse living systems. There are eight putative gene loci encoding for Hsp100 proteins in Arabidopsis genome. In rice, two full-length Hsp100 cDNAs have been isolated and sequenced so far. Analysis of rice genomic sequence by in silico approach showed that two isolated rice Hsp100 cDNAs correspond to Os05g44340 and Os02g32520 genes in the rice genome database. There appears to be three additional proteins (encoded by Os03g31300, Os04g32560 and Os04g33210 gene loci) that are variably homologous to Os05g44340 and Os02g32520 throughout the entire amino acid sequence. The above five rice Hsp100 genes show significant similarities in the signature sequences known to be conserved among Hsp100 proteins. While Os05g44340 encodes cytoplasmic Hsp100 protein, those encoded by the other four genes are predicted to have chloroplast transit peptides.

A multiprotein complex that mediates translational enhancement in Drosophila

Modulating the efficiency of translation plays an important role in a wide variety of cellular processes and is often mediated by trans-acting factors that interact with cis-acting sequences within the mRNA. Here we show that a cis-acting element, the Hsp83 degradation element (HDE), within the 3'-untranslated region of the Drosophila Hsp83 mRNA functions as a translational enhancer. We show that this element is bound by a multiprotein complex, and we identify components using a novel affinity-based method called tandem RNA affinity purification tagging. Three proteins (DDP1, Hrp48, and poly(A)-binding protein) are components of the HDE-binding complex and function in translational enhancement. Our data support a model whereby the HDE is composed of several cis-acting subelements that represent binding sites for trans-acting factors, and the combined action of these trans-acting factors underlies the ability of the HDE to stimulate translation.

Four members of the HSP101 gene family are differently regulated in Triticum durum Desf

Heat shock proteins play an essential role in preventing deleterious effects of high temperatures. In many plants, HSP101 has a central role in heat stress survival. We report the isolation and characterization of four cDNAs corresponding to different members of the durum wheat HSP101 gene family. Expression analysis revealed differences in their induction. Accordingly, durum wheat HSP101 genes are differently regulated, therefore having distinct roles in stress response and thermotolerance acquisition. These findings are important for further dissection of the molecular mechanisms underlying the stress response and for understanding the functions of the HSP101 family members. This information could be important for the exploitation of specific alleles in marker assisted selection for abiotic stress resistance.

Transactivation of protein expression by rice HSP101 in planta and using Hsp101 as a selection marker for transformation

Plant HSP101 has dual activities, first, in conferring thermotolerance, and secondly, in serving as a translational activator. In this study, we introduced Oryza sativa Hsp101 (osHsp101) cDNA into tobacco by Agrobacterium-mediated transformation. Stable integration and expression of the transgene into the tobacco genome was demonstrated by Southern and Western blot analysis. Overexpression of osHSP101 had no noticeable effect on growth or development of the transgenic plants. Homozygous T(2) transgenic plants with overexpressed osHSP101 survived heat treatment better than untransformed control plants. In addition, taking advantage of conferring basal thermotolerance by plant HSP101, we were able to demonstrate the feasibility of using osHsp101 as a selection marker and select the transformants under high temperature in tobacco leaf disc transformation mediated by Agrobacterium. Furthermore, transgenic tobacco plants with overexpressed osHSP101 were able to enhance luciferase expression up to 2.9-fold more than untransformed plants in the progeny of reciprocally crossed with omega-luciferase reporter lines.

Transcriptional response of Citrus aurantifolia to infection by Citrus tristeza virus

Changes in gene expression of Mexican lime plants in response to infection with a severe (T305) or a mild (T385) isolate of Citrus tristeza virus (CTV) were analyzed using a cDNA microarray containing 12,672 probes to 6875 different citrus genes. Statistically significant (P<0.01) expression changes of 334 genes were detected in response to infection with isolate T305, whereas infection with T385 induced no significant change. Induced genes included 145 without significant similarity with known sequences and 189 that were classified in seven functional categories. Genes related with response to stress and defense were the main category and included 28% of the genes induced. Selected transcription changes detected by microarray analysis were confirmed by quantitative real-time RT-PCR. Changes detected in the transcriptome upon infecting lime with T305 may be associated either with symptom expression, with a strain-specific defense mechanism, or with a general response to stress.

The leader sequence of tobacco mosaic virus RNA devoid of Watson-Crick secondary structure possesses a cooperatively melted, compact conformation

The 5'-untranslated region (5'-UTR) of RNA of tobacco mosaic virus (TMV), called omega sequence, is known as an mRNA leader promoting efficient initiation of translation. The central part of the sequence consists of many CAA repeats, which were reported to be mainly responsible for the enhancing activity of the omega leader. In this work we synthesized the polyribonucleotides containing either the natural omega sequence or the regular (CAA)(n) sequence, and studied them using UV spectrophotometry and analytical ultracentrifugation methods. It was demonstrated that the polyribonucleotides manifest significant hypochromicity, cooperative melting of their structures upon heating, high melting temperature, and the sedimentation coefficients typical of compactly folded RNAs of this size. Thus, the omega leader and its core (CAA)(n) repeat sequence devoid of secondary structure of the Watson-Crick type seem to be well structured elements of mRNA.

Testing of internal translation initiation via dicistronic constructs in yeast is complicated by production of extraneous transcripts

Introduction of sequences of interest into an intercistronic spacer (ICS) of dual reporter plasmids is the main experimental set-up used to identify and study internal ribosome entry sites (IRESs). We studied internal initiation of translation in yeast using the dicistronic approach. Three viral sequences and a polylinker-derived reference sequence were inserted into the ICS of a dual reporter plasmid upstream of the firefly luciferase gene, luc. LUC expression was taken as a putative indication of internal translation initiation from the studied sequences. Interestingly, all sequences mediated 3' LUC expression. However, northern blot analysis revealed that in addition to the dicistronic mRNAs, transcripts containing only the LUC-encoding sequences were produced from the plasmids. Electroporation studies with in vitro synthesized mRNAs showed that expression from the 3' cistron of the dicistronic mRNAs was below the level of detection. This suggested that the observed LUC expression from yeast transformed with the dicistronic expression plasmids did not originate from dicistronic messages. Deletion of the promoter increased 3' LUC expression. Similarly, repression of transcription prevented 5' cistron expression whereas 3' LUC expression was stimulated. These results suggested that the observed LUC expression did not result from partially degraded or spliced dicistronic RNAs but rather from transcripts synthesized from cryptic promoters. Despite high LUC activity, northern blot analysis detected few transcripts from yeast transformed with the promoterless constructs. Therefore, our data indicate that the functional assay is more revealing than RNA analysis in the case of very sensitive reporter genes generally used in IRES studies. Furthermore, our studies show that there is a clear need for detailed analysis prior to concluding the mechanism of 3' cistron expression.

Use of in vitro translation extract depleted in specific initiation factors for the investigation of translational regulation

Regulation of gene expression often involves the control of translation mediated through one or more initiation factors that are required for the translation of eukaryotic mRNAs. Genetic and molecular biological approaches can be highly useful in the initial identification of translational regulation, but the use of in vitro translation lysates can be essential in elucidating the details of translational regulatory mechanisms. Wheat germ lysate has long been used for in vitro translation studies. The noncompetitive conditions that prevail in this lysate as it is normally produced, however, preclude the translational regulatory analysis of many mRNAs involving the preferential recruitment of initiation factors. The development of lysate depleted in specific translation initiation factors converts wheat germ lysate from a noncompetitive system to one that is competitive in a fast and simple procedure that enables it to be used in the analysis of many more translational regulatory mechanisms than is currently possible with unfractionated lysate.

The involvement of chloroplast HSP100/ClpB in the acquired thermotolerance in tomato

The chloroplast HSP100/ClpB is a newly documented member of the ClpB family, but little was known about its role in imparting thermotolerance to cells. A cDNA coding for a HSP100/ClpB homolog has been cloned from Lycopersicon esculentum and termed as Lehsp100/ClpB (the cDNA sequence of Lehsp100/ClpB has been submitted to the GenBank database under accession number: AB219939). The protein encoded by the cDNA was most similar to the putative chloroplast HSP100/ClpBs in higher plants and the ClpB from Cyanobacterium Synechococcus sp. A 97 kDa protein, which matched the predicted size of mature LeHSP100/ClpB, was immunologically detected in chloroplast isolated from heat-treated tomato plants. In addition, the fusion protein, combining the transit sequence of LeHSP100/ClpB and GFP, was found to be located in chloroplast based on the observations of fluorescent microscope images. These results indicated the chloroplast-localization of LeHSP100/ClpB. Both the transcript and the protein of Lehsp100/ClpB were not detected under normal growth conditions, but they were induced by increasingly higher temperatures. An antisense Lehsp100/ClpB cDNA fragment was introduced into the tomato by Agrobacterium-mediated transformation. Antisense lines exhibited an extreme repression of heat-induced expression of Lehsp100/ClpB. The levels of chloroplast HSP60 and small HSP in antisense lines were identical to those of the control plants. After plants preconditioned at 38 degrees C for 2 h were exposed to a lethal heat shock at 46 degrees C for 2 h, the antisense lines were greatly impaired and withered in 21 days of the recovery phase, whereas the untransformed control plants and the vector-transformed plants survived. Furthermore, chlorophyll fluorescence measurements showed that PS II in antisense lines were more susceptible to the thermal irreversible inactivation than the untransformed and vector-transformed control plants. This work provides the first example that induction of chloroplast LeHSP100/ClpB contributes to the acquisition of thermotolerance in higher plants.

An Arabidopsis chloroplast-targeted Hsp101 homologue, APG6, has an essential role in chloroplast development as well as heat-stress response

Analysis of albino or pale-green (apg) mutants is important for identifying nuclear genes responsible for chloroplast development and pigment synthesis. We have identified 38 apg mutants by screening 11 000 Arabidopsis Ds-tagged lines. One mutant, apg6, contains a Ds insertion in a gene encoding APG6 (ClpB3), a homologue of the heat-shock protein Hsp101 (ClpB1). We isolated somatic revertants and identified two Ds-tagged and one T-DNA-tagged mutant alleles of apg6. All three alleles gave the same pale-green phenotype. These results suggest that APG6 is important for chloroplast development. The APG6 protein contains a transit peptide and is localized in chloroplasts. The plastids of apg6 pale-green cells were smaller than those of the wild type, and contained undeveloped thylakoid membranes. APG6 mRNA accumulated in response to heat shock in various organs, but not in response to other abiotic stresses. Under normal conditions, APG6 is constitutively expressed in the root tips, the organ boundary region, the reproductive tissues of mature plants where plastids exist as proplastids, and slightly in the stems and leaves. In addition, constitutive overexpression of APG6 in transgenic plants inhibited chloroplast development and resulted in a mild pale-green phenotype. The amounts of chloroplast proteins related to photosynthesis were markedly decreased in apg6 mutants. These results suggest that APG6 functions as a molecular chaperone involved in plastid differentiation mediating internal thylakoid membrane formation and conferring thermotolerance to chloroplasts during heat stress. The APG6 protein is not only involved in heat-stress response in chloroplasts, but is also essential for chloroplast development.

Tobamovirus infection is independent of HSP101 mRNA induction and protein expression

Heat shock protein 101 (HSP101) has been implicated in tobamovirus infections by virtue of its ability to enhance translation of mRNAs possessing the 5'Omega-leader of Tobacco mosaic virus (TMV). Enhanced translation is mediated by HSP101 binding to a CAA-repeat motif in TMV Omega leader. CAA repeat sequences are present in the 5' leaders of other tobamoviruses including Oilseed rape mosaic virus (ORMV), which infects Arabidopsis thaliana. HSP101 is one of eight HSP100 gene family members encoded by the A. thaliana genome, and of these, HSP101 and HSP98.7 are predicted to encode proteins localized to the cytoplasm where they could potentially interact with TMV RNA. Analysis of the expression of the HSP100s showed that only HSP101 mRNA transcripts were induced significantly by ORMV in A. thaliana. The induction of HSP101 mRNA was also correlated with an increase in its protein levels and was independent of defense-related signaling pathways involving salicylic acid, jasmonic acid, or ethylene. A. thaliana mutants lacking HSP101, HSP98.7, or both supported wild-type levels of ORMV replication and movement. Similar results were obtained for TMV infection in Nicotiana benthamiana plants silenced for HSP101, demonstrating that HSP101 is not necessary for efficient tobamovirus infection.

Functional diversity of the eukaryotic translation initiation factors belonging to eIF4 families

Protein synthesis in eukaryotic cells is fundamental for gene expression. This process involves the binding of an mRNA molecule to the small ribosomal subunit in a group of reactions catalyzed by eukaryotic translation initiation factors (eIF) eIF4. To date, the role of each of the four eIF4, i.e. eIF4E, eIF4G, eIF4A and eIF4B, is well established. However, with the advent of genome-wide sequencing projects of various organisms, families of genes for each translation initiation factor have been identified. Intriguingly, recent studies have now established that certain eIF4 proteins can promote or inhibit translation of specific mRNAs, and also that some of them are active in processes other than translation. In addition, there is evidence of tissue- and developmental-stage-specific expression for some of these proteins. These new findings point to an additional level of complexity in the translation initiation process. In this review, we analyze the latest advances concerning the functionality of members of the eIF4 families in eukaryotic organisms and discuss the implications of this in the context of our current understanding of regulation of the translation initiation process.