Domain analysis of the plant DNA-binding protein GT1a: requirement of four putative alpha-helices for DNA binding and identification of a novel oligomerization region

Comprehensive Genomic Analysis of Trihelix Family in Tea Plant (Camellia sinensis) and Their Putative Roles in Osmotic Stress

In plants, Trihelix transcription factors are responsible for regulating growth, development, and reaction to various abiotic stresses. However, their functions in tea plants are not yet fully understood. This study identified a total of 40 complete Trihelix genes in the tea plant genome, which are classified into five clades: GT-1 (5 genes), GT-2 (8 genes), GTγ (2 genes), SH4 (7 genes), and SIP1 (18 genes). The same subfamily exhibits similar gene structures and functional domains. Chromosomal mapping analysis revealed that chromosome 2 has the most significant number of trihelix family members. Promoter analysis identified cis-acting elements in C. sinensis trihelix (CsTH), indicating their potential to respond to various phytohormones and stresses. The expression analysis of eight representative CsTH genes from four subfamilies showed that all CsTHs were expressed in more tissues, and three CsTHs were significantly induced under ABA, NaCl, and drought stress. This suggests that CsTHs plays an essential role in tea plant growth, development, and response to osmotic stress. Furthermore, yeast strains have preliminarily proven that CsTH28, CsTH36, and CsTH39 can confer salt and drought tolerance. Our study provides insights into the phylogenetic relationships and functions of the trihelix transcription factors in tea plants. It also presents new candidate genes for stress-tolerance breeding.

Down-regulation of SlGT-26 gene confers dwarf plants and enhances drought and salt stress resistance in tomato

Plant architecture, an important agronomic trait closely associated with yield, is governed by a highly intricate molecular network. Despite extensive research, many mysteries surrounding this regulation remain unresolved. Trihelix transcription factor family plays a crucial role in the development of plant morphology and abiotic stresses. Here, we identified a novel trihelix transcription factor named SlGT-26, and its down-regulation led to significant alterations in plant architecture, including dwarfing, reduced internode length, smaller leaves, and shorter petioles. The dwarf phenotype of SlGT-26 silenced transgenic plants could be recovered after spraying exogenous GA3, and the GA3 content were decreased in the RNAi plants. Additionally, the expression levels of gibberellin-related genes were affected in the RNAi lines. These results indicate that the dwarf of SlGT-26-RNAi plants may be a kind of GA3-sensitive dwarf. SlGT-26 was response to drought and salt stress treatments. SlGT-26-RNAi transgenic plants demonstrated significantly enhanced drought resistance and salt tolerance in comparison to their wild-type tomato counterparts. SlGT-26-RNAi transgenic plants grew better, had higher relative water content and lower MDA and H2O2 contents. The expression of multiple stress-related genes was also up-regulated. In summary, we have discovered a novel gene, SlGT-26, which plays a crucial role in regulating plant architecture and in respond to drought and salt stress.

Identification and analysis of differentially expressed trihelix genes in maize (Zea mays) under abiotic stresses

Background

Trihelix transcription factors play important roles in triggering plant growth and imparting tolerance against biotic and abiotic stresses. However, a systematical analysis of the trihelix transcription factor family under heat and drought stresses in maize has not been reported.

Methods

PlantTFDB and TBtools were employed to identify the trihelix domain-containing genes in the maize genome. The heat-regulated transcriptome data for maize were obtained from NCBI to screen differentially expressed ZmTHs genes through statistical analysis. The basic protein sequences, chromosomal localization, and subcellular localization were analyzed using Maize GDB, Expasy, SOMPA, TBtools, and Plant-mPLoc. The conserved motifs, evolutionary relationships, and cis-elements, were analyzed by MEME, MEGA7.0 and PlantCARE software, respectively. The tissue expression patterns of ZmTHs and their expression profiles under heat and drought stress were detected using quantitative real-time PCR (qRT-PCR).

Results

A total of 44 trihelix family members were discovered, and members were distributed over 10 chromosomes in the maize genome. A total of 11 genes were identified that were regulated by heat stress; these were unevenly distributed on chromosomes 1, 2, 4, 5, and 10. ZmTHs encoded a total of 16 proteins, all of which were located in the nucleus; however, ZmTH04.1 was also distributed in the chloroplast. The protein length varied from 206 to 725 amino acids; the molecular weight ranged from 22.63 to 76.40 kD; and the theoretical isoelectric point (pI) ranged from 5.24 to 11.2. The protein's secondary structures were mainly found to be random coils and α-helices, with fewer instances of elongation chains and β-rotations. Phylogenetic relationship analysis showed that these can be divided into five sub-groups. The conserved domain of ZmTHs was GT1 or MyB_DNA-Bind_4. The protein and gene structure of ZmTHs differed greatly among the subfamilies, while the structures within the subfamilies were similar. The promoter of ZmTHs contained abundant tissue-specific expression cis-acting elements and abiotic stress response elements. qRT-PCR analysis showed that ZmTHs expression levels were significantly different in different tissues. Furthermore, the expression of ZmTH08 was dramatically up-regulated by heat stress, while the expression of ZmTH03, ZmTH04, ZmTH05, ZmTH06, ZmTH07, ZmTH09, ZmTH10, and ZmTH11 were down-regulated by heat stress. Upon PEG-simulated drought stress, ZmTH06 was significantly up-regulated, while ZmTH01 and ZmTH07 were down-regulated.

Conclusions

We performed a genome-wide, systematic identification and analysis of differentially expressed trihelix genes under heat and drought stresses in maize.

A reevaluation of the role of the ASIL trihelix transcription factors as repressors of the seed maturation program

Developmental transitions are typically tightly controlled at the transcriptional level. Two of these transitions involve the induction of the embryo maturation program midway through seed development and its repression during the vegetative phase of plant growth. Very little is known about the factors responsible for this regulation during early embryogenesis, and only a couple of transcription factors have been characterized as repressors during the postgerminative phase. Arabidopsis 6b-INTERACTING PROTEIN-LIKE1 (ASIL1), a trihelix transcription factor, has been proposed to repress maturation both embryonically and postembryonically. Preliminary data also suggested that its closest paralog, ASIL2, might play a role as well. We used a transcriptomic approach, coupled with phenotypical observations, to test the hypothesis that ASIL1 and ASIL2 redundantly turn off maturation during both phases of growth. Our results indicate that, contrary to what was previously published, neither of the ASIL genes plays a role in the regulation of maturation, at any point during plant development. Analyses of gene ontology (GO)-enriched terms and published transcriptomic datasets suggest that these genes might be involved in responses during the vegetative phase to certain biotic and abiotic stresses.

Identification and in Silico Characterization of GT Factors Involved in Phytohormone and Abiotic Stresses Responses in Brachypodium distachyon

GT factors play critical roles in plant growth and development and in response to various environmental stimuli. Considering the new functions of GT factors on the regulation of plant stress tolerance and seeing as few studies on Brachypodium distachyon were available, we identified GT genes in B. distachyon, and the gene characterizations and phylogenies were systematically analyzed. Thirty-one members of BdGT genes were distributed on all five chromosomes with different densities. All the BdGTs could be divided into five subfamilies, including GT-1, GT-2, GTγ, SH4, and SIP1, based upon their sequence homology. BdGTs exhibited considerably divergent structures among each subfamily according to gene structure and conserved functional domain analysis, but the members within the same subfamily were relatively structure-conserved. Synteny results indicated that a large number of syntenic relationship events existed between rice and B. distachyon. Expression profiles indicated that the expression levels of most of BdGT genes were changed under abiotic stresses and hormone treatments. Moreover, the co-expression network exhibited a complex regulatory network between BdGTs and BdWRKYs as well as that between BdGTs and BdMAPK cascade gene. Results showed that GT factors might play multiple functions in responding to multiple environmental stresses in B. distachyon and participate in both the positive and negative regulation of WRKY- or MAPK-mediated stress response processes. The genome-wide analysis of BdGTs and the co-regulation network under multiple stresses provide valuable information for the further investigation of the functions of BdGTs in response to environment stresses.

Genome-Wide Characterization and Identification of Trihelix Transcription Factor and Expression Profiling in Response to Abiotic Stresses in Rice (Oryza sativa L.)

Trihelix transcription factors play a role in plant growth, development and various stress responses. Here, we identified 41 trihelix family genes in the rice genome. These OsMSLs (Myb/SANT-LIKE) were located on twelve chromosomes. Synteny analysis indicated only six duplicated gene pairs in the rice trihelix family. Phylogenetic analysis of these OsMSLs and the trihelix genes from other species divided them into five clusters. OsMSLs from different groups significantly diverged in terms of gene structure and conserved functional domains. However, all OsMSLs contained the same five cis-elements. Some of these were responsive to light and dehydration stress. All OsMSLs expressed in four tissues and six developmental stages of rice but with different expression patterns. Quantitative real-time PCR analysis revealed that the OsMSLs responded to abiotic stresses including drought and high salt stress and stress signal molecule including ABA (abscisic acid), hydrogen peroxide. OsMSL39 were simultaneously expressed under all treatments, while OsMSL28 showed high expression under hydrogen peroxide, drought, and high salt treatments. Moreover, OsMSL16/27/33 displayed significant expression under ABA and drought treatments. Nevertheless, their responses were regulated by light. The expression levels of the 12 chosen OsMSLs differed between light and dark conditions. In conclusion, our results helped elucidate the biological functions of rice trihelix genes and provided a theoretical basis for further characterizing their biological roles in responding to abiotic stresses.

Functional domains of the PETAL LOSS protein, a trihelix transcription factor that represses regional growth in Arabidopsis thaliana

PETAL LOSS (PTL) is a trihelix transcription factor that represses growth, especially between sepal primordia. As one of 30 trihelix proteins in Arabidopsis, it falls in the GT2 clade with duplicated trihelix DNA-binding domains and a long α-helical central domain. PTL orthologs occur in all angiosperm genomes examined except grasses, and sequence comparisons reveal that there are two further short conserved domains at each end. GT2 itself carries two nuclear localization sequences, but PTL has an additional nuclear localization sequence (NLS). We show that PTL can act as a transcriptional activator in yeast and in planta, with the latter tested by two different functional assays. Specific deletions revealed that the activation region is C-terminal. Site-directed mutagenesis of the DNA-binding domains has shown that a conserved tryptophan and two downstream acidic amino acids in the second trihelix, predicted to promote folding, are each required for PTL function. Also, three basic residues in the third helix, near the DNA interaction sites, support its function. PTL was found to dimerize in yeast. This was confirmed and extended by jointly expressing differentially tagged forms of PTL in a transient expression system in Nicotiana benthamiana leaves. Cytoplasmic PTL (with mutant NLS sequences) was carried into the nucleus upon binding with nuclear-localized PTL, providing each partner carried intact central domains. As this 90-amino acid domain is conserved in most trihelix family members, it seems likely that they all function in dimeric form.

The trihelix family of transcription factors--light, stress and development

GT factors are the founding members of the trihelix transcription factor family. They bind GT elements in light regulated genes, and their nature was uncovered in a burst of activity in the 1990s. Study of the trihelix family then slowed. However, interest is now re-awakening. Genomic studies have revealed 30 members of this family in Arabidopsis and 31 in rice, falling into five clades. Newly discovered functions involve responses to salt and pathogen stresses, the development of perianth organs, trichomes, stomata and the seed abscission layer, and the regulation of late embryogenesis. Thus the time is ripe for a review of the genomic and functional information now emerging for this neglected family.

Chloroplast RNase J compensates for inefficient transcription termination by removal of antisense RNA

Ribonuclease J is an essential enzyme, and the Bacillus subtilis ortholog possesses both endoribonuclease and 5' → 3' exoribonuclease activities. Chloroplasts also contain RNase J, which has been postulated to participate, as both an exo- and endonuclease, in the maturation of polycistronic mRNAs. Here we have examined recombinant Arabidopsis RNase J and found both 5' → 3' exoribonuclease and endonucleolytic activities. Virus-induced gene silencing was used to reduce RNase J expression in Arabidopsis and Nicotiana benthamiana, leading to chlorosis but surprisingly few disruptions in the cleavage of polycistronic rRNA and mRNA precursors. In contrast, antisense RNAs accumulated massively, suggesting that the failure of chloroplast RNA polymerase to terminate effectively leads to extensive symmetric transcription products that are normally eliminated by RNase J. Mung bean nuclease digestion and polysome analysis revealed that this antisense RNA forms duplexes with sense strand transcripts and prevents their translation. We conclude that a major role of chloroplast RNase J is RNA surveillance to prevent overaccumulation of antisense RNA, which would otherwise exert deleterious effects on chloroplast gene expression.

Systematic analysis of GT factor family of rice reveals a novel subfamily involved in stress responses

GT factors constitute a plant-specific transcription factor family with a conserved trihelix DNA-binding domain. In this study, comprehensive sequence analysis suggested that 26 putative GT factors exist in rice. Phylogenetic analysis revealed three distinctive subfamilies (GTalpha, GTbeta, and GTgamma) of plant GT factors and each subfamily has a unique composition of predicted motifs. We characterized the OsGTgamma-1 gene, a typical member of the GTgamma subfamily in rice. This gene encodes a protein containing a conserved trihelix domain, and the OsGTgamma-1:GFP fusion protein was targeted to nuclei of rice cells. The transcript level of OsGTgamma-1 was strongly induced by salt stress and slightly induced by drought and cold stresses and abscisic acid treatment. Two other members of the GTgamma subfamily, OsGTgamma-2 and OsGTgamma-3, were also induced by most of the abiotic stresses. These results suggested that the genes of the GTgamma subfamily in rice may be involved in stress responses. A homozygous mutant osgtgamma-1 (with T-DNA inserted in the promoter region of OsGTgamma-1) showed more sensitive to salt stress than wild-type rice. Overexpression of OsGTgamma-1 in rice enhanced salt tolerance at the seedling stage. This evidence suggests that the OsGTgamma subfamily may participate in the regulation of stress tolerance in rice.

Establishing glucose- and ABA-regulated transcription networks in Arabidopsis by microarray analysis and promoter classification using a Relevance Vector Machine

Establishing transcriptional regulatory networks by analysis of gene expression data and promoter sequences shows great promise. We developed a novel promoter classification method using a Relevance Vector Machine (RVM) and Bayesian statistical principles to identify discriminatory features in the promoter sequences of genes that can correctly classify transcriptional responses. The method was applied to microarray data obtained from Arabidopsis seedlings treated with glucose or abscisic acid (ABA). Of those genes showing >2.5-fold changes in expression level, approximately 70% were correctly predicted as being up- or down-regulated (under 10-fold cross-validation), based on the presence or absence of a small set of discriminative promoter motifs. Many of these motifs have known regulatory functions in sugar- and ABA-mediated gene expression. One promoter motif that was not known to be involved in glucose-responsive gene expression was identified as the strongest classifier of glucose-up-regulated gene expression. We show it confers glucose-responsive gene expression in conjunction with another promoter motif, thus validating the classification method. We were able to establish a detailed model of glucose and ABA transcriptional regulatory networks and their interactions, which will help us to understand the mechanisms linking metabolism with growth in Arabidopsis. This study shows that machine learning strategies coupled to Bayesian statistical methods hold significant promise for identifying functionally significant promoter sequences.

Harbinger transposons and an ancient HARBI1 gene derived from a transposase

In this study we report main properties of Harbinger DNA transposons identified in protists, plants, insects, worms, and vertebrates. This is the first superfamily of eukaryotic DNA transposons where all autonomous transposons, even those that are hosted by species from different kingdoms, encode two proteins: a superfamily-specific transposase and a DNA-binding protein characterized by the presence of the conserved SANT/myb/trihelix motif. The last motif is known to be important for the DNA binding by different transcription regulators. Therefore, we suggest that this protein is necessary for coordinated expression of the Harbinger transposase. Although mammalian genomes are free of recognizable remnants of Harbingers, we identified a widely expressed HARBI1 gene encoding a 350-aa protein entirely derived from a Harbinger transposase some 450-500 million years ago. The HARBI1 proteins are conserved in humans, rats, mice, cows, pigs, chickens, frogs, and various bony fish, as well as other extremely important proteins, including RAG1 and RAG2. Conserved motifs detected in the Harbinger transposases are also well preserved in the HARBI1 proteins. Therefore, the HARBI1 proteins are expected to be nucleases important for functioning of bony vertebrates. We also found that the protein most similar to HARBI1 is encoded by an autonomous Harbinger 3_DR transposon that was transpositionally active in the zebrafish genome a few million years ago. Nonautonomous transposons derived from Harbinger3_DR are characterized by a striking preference for a 17-bp target site never seen previously in any other DNA transposon. Based on this observation, we suggest that the hypothetical HARBI1 nucleases are also characterized by a strong DNA-target specificity.

Transcript abundance of rml1, encoding a putative GT1-like factor in rice, is up-regulated by Magnaporthe grisea and down-regulated by light

We isolated and sequenced both genomic DNA and cDNA clones, which encoded a putative GT1-like protein with 385 amino acids, from cultivated rice (Oryza sativa ssp. indica). This protein shows significant amino acid sequence similarities with trihelix DNA-binding GT-1a/B2F and GT-1 factors that were identified in dicot plants. Northern blotting analysis indicated that the transcript of the rice GT-1 factor in seedling was up-regulated by the rice blast fungus Magnaporthe grisea, down-regulated by various continuous light conditions and expressed rhythmically in light/dark cycles. This GT1-like factor gene was therefore designated as rml1 (rice gene regulated by M. grisea and light). The putative RML1 protein, encoded by this single copy gene, is thus identified as a new member of the plant-specific GT family of transcription factors in rice.

Analysis of GT-3a identifies a distinct subgroup of trihelix DNA-binding transcription factors inArabidopsis

Trihelix DNA-binding factors (or GT factors) bind to GT elements found in the promoters of many plant genes. Although the binding specificity and the transcriptional activity of some members (e.g. GT-1 and GT-2) have been studied, the regulatory function of this family of transcription factors remains largely unknown. In this work, we have characterised a new GT factor, namely GT-3a, and a closely related member, GT-3b. We show that (1) they can form either homo- or heterodimers but do not interact with GT-1; (2) they are predominantly expressed in floral buds and roots; (3) GT-3a cannot bind to the binding sites of GT-1 or GT-2, but binds to the cab2 and rbcS-1A gene promoters via the 5'-GTTAC sequence, which has been previously shown to be the core of the Site 1 type of GT elements. These results suggest that GT-3a and GT-3b belong to a distinct subgroup of GT factors and that each subgroup of GT factors binds to a functionally distinct type of cis-acting GT elements.

Intermolecular and intramolecular interactions regulate catalytic activity of myotonic dystrophy kinase-related Cdc42-binding kinase alpha

Myotonic dystrophy kinase-related Cdc42-binding kinase (MRCK) is a Cdc42-binding serine/threonine kinase with multiple functional domains. We had previously shown MRCKalpha to be implicated in Cdc42-mediated peripheral actin formation and neurite outgrowth in HeLa and PC12 cells, respectively. Here we demonstrate that native MRCK exists in high-molecular-weight complexes. We further show that the three independent coiled-coil (CC) domains and the N-terminal region preceding the kinase domain are responsible for intermolecular interactions leading to MRCKalpha multimerization. N terminus-mediated dimerization and consequent transautophosphorylation are critical processes regulating MRCKalpha catalytic activities. A region containing the two distal CC domains (CC2 and CC3; residues 658 to 930) was found to interact intramolecularly with the kinase domain and negatively regulates its activity. Its deletion also resulted in an active kinase, confirming a negative autoregulatory role. We provide evidence that the N terminus-mediated dimerization and activation of MRCK and the negative autoregulatory kinase-distal CC interaction are two mutually exclusive events that tightly regulate the catalytic state of the kinase. Disruption of this interaction by a mutant kinase domain resulted in increased kinase activity. MRCK kinase activity was also elevated when cells were treated with phorbol ester, which can interact directly with a cysteine-rich domain next to the distal CC domain. We therefore suggest that binding of phorbol ester to MRCK releases its autoinhibition, allowing N-terminal dimerization and subsequent kinase activation.

Mutation of GT-1 binding sites in the Pr-1A promoter influences the level of inducible gene expression in vivo

Infection of Nicotiana tabacum Samsun NN with tobacco mosaic virus (TMV) results in a hypersensitive plant response and leads to systemic acquired resistance (SAR). The induction of SAR is mediated by the plant hormone salicylic acid (SA) and is accompanied by the induced expression of a number of genes including the pathogenesis-related (PR) gene 1a. Previously, it has been found that TMV infection and SA treatment resulted in a reduction of binding of nuclear protein GT-1 to far-upstream regions (-902 to -656) of the PR-1a gene. To test if GT-1 is a negative regulator of PR-1a gene expression, the effects of mutations in the seven putative GT-1 binding sites in this region were studied in vitro using dimethyl sulfate interference footprinting and band shift assays. This showed that at least one of the seven sites is indeed a GT-1 binding site. However, when tested in transgenic plants, the mutations did not result in constitutive expression of the chimeric PR-1a/GUS transgene, while inducible expression after SA treatment was decreased. The results suggest that binding of GT-1-like proteins to far-upstream PR-1a promoter regions indeed influences gene expression. A possible model for GT-1's mode of action in PR-1a gene expression is discussed.

Modulation of GT-1 DNA-binding activity by calcium-dependent phosphorylation

The analysis of pea rbcS-3A promoter sequence showed that BoxII was necessary for the control of rbcS-3A gene expression by light. GT-1, a DNA-binding protein that interacts with BoxII in vitro, is a good candidate for being a light-modulated molecular switch controlling gene expression. However, the relationship between GT-1 activity and light-responsive gene activation still remains hypothetical. Because no marked de novo synthesis was detected after light treatment, light may induce post-translational modifications of GT-1 such as phosphorylation or dephosphorylation. Here, we show that recombinant GT-1 (hGT-1) of Arabidopsis can be phosphorylated by various mammalian kinase activities in vitro. Whereas phosphorylation by casein kinase II had no apparent effect on hGT-1 DNA binding, phosphorylation by calcium/calmodulin kinase II (CaMKII) increased the binding activity 10-20-fold. Mass spectrometry analyses of the phosphorylated hGT-1 showed that amongst the 6 potential phosphorylatable residues (T86, T133, S175, T179, S198 and T278), only T133 and S198 are heavily modified. Analyses of mutants altered at T86, T133, S175, T179, S198 and T278 demonstrated that phosphorylation of T133 can account for most of the stimulation of DNA-binding activity by CaMKII, indicating that this residue plays an important role in hGT-1/BoxII interaction. We further showed that nuclear GT-1 DNA-binding activity to BoxII was reduced by treatment with calf intestine phosphatase in extracts prepared from light-grown plants but not from etiolated plants. Taken together, our results suggest that GT-1 may act as a molecular switch modulated by calcium-dependent phosphorylation and dephosphorylation in response to light signals.

The trihelix DNA-binding motif in higher plants is not restricted to the transcription factors GT-1 and GT-2

GT-2 is a plant transcriptional activator that contains two separate, but similar, trihelix DNA-binding domains. GT-1 is similar to GT-2, although it contains only one of such domains. cDNAs that encode GT-2 were isolated from rice (OS-GT2) and Arabidopsis (AT-GT2). Evidence is presented for the existence of an Arabidopsis gene family that is structurally related to AT-GT2. Two members of this GT2-like family, AT-GTL1 and AT-GTL2, have been isolated and characterized. Their sequences suggest that they evolved by a recent gene duplication event. Both AT-GT2 and AT-GTL genes contain an intron in the amino-terminal trihelix motif, indicating that this DNA-binding domain resulted from exon shuffling. RNA gel blot analysis using AT-GTL1 as a probe revealed four transcripts in the aerial part of the plant. All mRNA levels were significantly higher in siliques, suggesting that this gene family may function in fruit and/or seed development. To date, DNA-binding proteins characterized by the trihelix motif have been described only in plants, and may therefore be involved in plant-specific processes. Our results show that in Arabidopsis thaliana, the trihelix motif is not restricted to the GT-1 and GT-2 DNA-binding proteins.

Dof DNA-binding domains of plant transcription factors contribute to multiple protein-protein interactions

Dof proteins are a family of plant transcription factors that have a strongly conserved DNA-binding domain, designated the Dof domain. This domain has the potential to form a single zinc finger. This report describes the self-association of a maize Dof protein, Dof1 (previously designated MNB1a). Affinity chromatography revealed that Dof1 also interacted with another maize Dof protein, Dof2, as well as with high-mobility-group (HMG) protein 1. Results of mapping of the region required for the protein-protein interactions of Dof1 suggested that these interactions may be mediated by the Dof domain. When gel mobility shift assays were performed with purified recombinant Dof proteins, homomeric and heteromeric complexes of Dof proteins on DNA were detected. It seems possible that formation of complexes of different Dof proteins through direct protein-protein interactions might be involved in the regulation of transcription. Evidence is also presented that HMG1 has an effect on the binding of Dof1 to DNA. Therefore, it appears that the Dof domain is a multifunctional domain that is involved not merely in binding to DNA but also in multiple protein-protein interactions.

Analysis of protein aggregates by combination of cross-linking reactions and chromatographic separations

Chemical cross-linking provides a method that covalently bridges near-neighbour associations within proteins and protein aggregates. Combined with chromatographic separations and protein-chemical methods, it may be used to localize and to investigate three-dimensional relations as present under natural conditions. This paper reviews the chemistry and application of cross-linking reagents and the development of combination experimental approaches in view of chromatographic separations and cross-linking reactions. Investigations of homooligomeric and heterooligomeric protein associations as well as conformational analysis are presented.

Light regulated transcription in higher plants

Studies on the function of plant promoters have demonstrated the presence of regulatorycis-acting elements that mediate developmental or environmental signals. Analysis of many light-responsive genes showed thatcis-acting elements responsible for light regulated transcription are located within the 5' upstream region. Numerous light responsivecis-acting elements andtrans-acting factors have been identified and characterized. The present article reviews the recent advances in studies of light regulated transcriptional regulation and signal transduction.