Structure and function of 5'-flanking regions of Bombyx mori fibroin heavy chain gene: identification of a novel transcription enhancing element with a homeodomain protein-binding motif

RNA-Seq analysis of the blue light-emitting Orfelia fultoni (Diptera: Keroplatidae) suggest photoecological adaptations at the molecular level

Bioluminescence in Diptera is found in the Keroplatidae family, within Arachnocampininae and Keroplatinae subfamilies, with reported occurrences in Oceania, Eurasia, and Americas. Larvae of Orfelia fultoni, which inhabit stream banks in the Appalachian Mountains, emit the bluest bioluminescence among insects, using it for prey attraction, similarly to Arachnocampa spp. Although bioluminescence has a similar prey attraction function, the systems of Arachonocampininae and Keroplatinae subfamilies are morphologically/biochemically distinct, indicating different evolutionary origins. To identify the possible coding genes associated with physiological control, ecological adaptations, and origin/evolution of bioluminescence in the Keroplatinae subfamily, we performed the RNA-Seq analysis of O. fultoni larvae during day and night and compared it with the transcriptomes of Arachnocampa luminosa, and reanalyzed the previously published proteomic data of O. fultoni against the RNA-Seq dataset. The abundance of chaperones/heat-shock and hexamerin gene products at night and in luciferase enriched fractions supports their possible association and participation in bioluminescence. The low diversity of copies/families of opsins indicate a simpler visual system in O. fultoni. Noteworthy, gene products associated with silk protein biosynthesis in Orfelia were more similar to Lepidoptera than to the Arachnocampa, indicating that, similarly to the bioluminescent systems, at some point, the biochemical apparatus for web construction may have evolved independently in Orfelia and Arachnocampa.

Cocoon-Spinning Behavior and 20-Hydroxyecdysone Regulation of Fibroin Genes in Plutella xylostella

The diamondback moth Plutella xylostella is a serious pest of crucifers. It has high reproductive potential and is resistant to many insecticides. Typically, the last-instar larvae of P. xylostella, before pupation, move to the lower or outer plant leaves to make a loose silk cocoon and pupate inside for adult formation. To better understand this pivotal stage we studied the cocoon-spinning behavior of P. xylostella and measured three successive phases by video-recording, namely the selection of a pupation site, spinning a loose cocoon and padding the scaffold cocoon. Subsequently, we cloned three fibroin genes related to cocoon production, i.e., fibroin light chain (Fib-L), fibroin heavy chain (Fib-H), and glycoprotein P25. A spatio-temporal study of these three fibroin genes confirmed a high expression in the silk glands during the final larval instar silk-producing stage. In parallel, we did an exogenous treatment of the insect molting hormone 20-hydroxyecdysone (20E), and this suppressed fibroin gene expression, reduced the normal time needed for cocoon spinning, and we also observed a looser cocoon structure under the scanning electron microscope. Hence, we demonstrated that the expression levels of key genes related to the synthesis of 20E [the three Halloween genes Spook (Spo), Shadow (Sad), and Shade (Shd)] decreased significantly during spinning, the expression of the 20E receptor (EcR and USP) was significantly lower during spinning than before spinning, and that the expression levels of CYP18-A1 related to 20E degradation were significantly up-regulated during spinning. The significance of the cocoon and the effects of 20E on the cocoon-spinning behavior of P. xylostella are discussed.

Transgenic Ectopic Overexpression of Broad Complex (BrC-Z2) in the Silk Gland Inhibits the Expression of Silk Fibroin Genes of Bombyx mori

Bombyx mori silk protein genes are strictly turned on and off in different developmental stages under the hormone periodically change. The broad complex (BrC) is a transcription factor mediating 20-hydroxyecdysone action, which plays important roles during metamorphosis. Here, we observed that two isoforms of BmBrC (BmBrC-Z2 and BmBrC-Z4) exhibited contrasting expression patterns with fibroin genes (FibH, FibL and P25) in the posterior silk gland (PSG), suggesting that BmBrC may negatively regulate fibroin genes. Transgenic lines were constructed to ectopically overexpress BmBrC-Z2 in the PSG. The silk protein genes in the transgenic line were decreased to almost half of that in the wild type. The silk yield was decreased significantly. In addition, the expression levels of regulatory factors (BmKr-h1 and BmDimm) response to juvenile hormone (JH) signal were inhibited significantly. Then exogenous JH in the BmBrC-Z2 overexpressed lines can inhibit the expression of BmBrC-Z2 and activate the expression of silk protein genes and restore the silk yield to the level of the wild type. These results indicated that BmBrC may inhibit fibroin genes by repressing the JH signal pathway, which would assist in deciphering the comprehensive regulation mechanism of silk protein genes.

Mass spider silk production through targeted gene replacement in Bombyx mori

Spider silk is one of the best natural fibers and has superior mechanical properties. However, the large-scale harvesting of spider silk by rearing spiders is not feasible, due to their territorial and cannibalistic behaviors. The silkworm, Bombyx mori, has been the most well known silk producer for thousands of years and has been considered an ideal bioreactor for producing exogenous proteins, including spider silk. Previous attempts using transposon-mediated transgenic silkworms to produce spider silk could not achieve efficient yields, due to variable promoter activities and endogenous silk fibroin protein expression. Here, we report a massive spider silk production system in B. mori by using transcription activator-like effector nuclease-mediated homology-directed repair to replace the silkworm fibroin heavy chain gene (FibH) with the major ampullate spidroin-1 gene (MaSp1) in the spider Nephila clavipes We successfully replaced the ∼16-kb endogenous FibH gene with a 1.6-kb MaSp1 gene fused with a 1.1-kb partial FibH sequence and achieved up to 35.2% chimeric MaSp1 protein amounts in transformed cocoon shells. The presence of the MaSp1 peptide significantly changed the mechanical characteristics of the silk fiber, especially the extensibility. Our study provides a native promoter-driven, highly efficient system for expressing the heterologous spider silk gene instead of the transposon-based, random insertion of the spider gene into the silkworm genome. Targeted MaSp1 integration into silkworm silk glands provides a paradigm for the large-scale production of spider silk protein with genetically modified silkworms, and this approach will shed light on developing new biomaterials.

Genetic engineered color silk: fabrication of a photonics material through a bioassisted technology

Silk produced by the silkworm Bombyx mori is an attractive material because of its luster, smooth and soft texture, conspicuous mechanical strength, good biocompatibility, slow biodegradation, and carbon neutral synthesis. Silkworms have been domesticated and bred for production of better quality and quantity of silk, resulting in the development of sericulture and the textile industry. Silk is generally white, so dyeing is required to obtain colored fiber. However, the dyeing process involves harsh conditions and generates a large volume of waste water, which have environmentally and economically negative impacts. Although some strains produce cocoons that contain pigments derived from the mulberry leaves that they eat, the pigments are distributed in the sericin layer and are lost during gumming. In trials for production of colored silk by feeding silkworms on diets containing dyes, only limited species of dye molecules were incorporated into the silk threads. A method for the generation of transgenic silkworm was established in conjunction with the discovery of green fluorescent protein (GFP), and silkworms carrying the GFP gene spun silk threads that formed cocoons that glowed bright green and still retained the original properties of silk. A wide range of color variation of silk threads has been obtained by replacing the GFP gene with the genes of other fluorescent proteins chosen from the fluorescent protein palette. The genetically modified silk with photonic properties can be processed to form various products including linear threads, 2D fabrics, and 3D materials. The transgenic colored silk could be economically advantageous due to addition of a new value to silk and reduction of cost for water waste, and environmentally preferable for saving water. Here, I review the literature regarding the production methods of fluorescent silk from transgenic silkworms and present examples of genetically modified color silk.

Knockdown of Broad-Complex Gene Expression of Bombyx mori by Oligopyrrole Carboxamides Enhances Silk Production

Bombyx mori (B. mori) is important due to its major role in the silk production. Though DNA binding ligands often influence gene expression, no attempt has been made to exploit their use in sericulture. The telomeric heterochromatin of B. mori is enriched with 5'-TTAGG-3' sequences. These sequences were also found to be present in several genes in the euchromatic regions. We examined three synthetic oligopyrrole carboxamides that target 5'-TTAGG-3' sequences in controlling the gene expression in B. mori. The ligands did not show any defect or feeding difference in the larval stage, crucial for silk production. The ligands caused silencing of various isoforms of the broad-complex transcription factor and cuticle proteins which resulted in late pupal developmental defects. Furthermore, treatment with such drugs resulted in statistically enhanced cocoon weight, shell weight, and silk yield. This study shows for the first time use of oligopyrrole carboxamide drugs in controlling gene expression in B. mori and their long term use in enhancing silk production.

Nuclear hormone receptor BmFTZ-F1 is involved in regulating the fibroin heavy chain gene in the silkworm, Bombyx mori

BACKGROUND

The synthesis of silk protein is controlled by hormones. The expression of the nuclear hormone Bmftz-f1 in the posterior silk gland (PSG) is induced by 20-hydroxyecdysone in vivo and in vitro. However, whether Bmftz-f1 regulates silk protein expression is unknown.

METHODS

In our study, western blotting and quantitative polymerase chain reactions were conducted to detect the expression of FTZ-F1 in the PSG. Electrophoretic mobility shift, chromatin immunoprecipitation, far-western blotting, bimolecular fluorescence complementation, and dual luciferase reporter assays were performed to investigate the effect of FTZ-F1 on the fibH promoter.

RESULTS

(1) The expression of the hormone receptor BmFTZ-F1 was opposite to that of fibH. It was highly expressed in the PSG during the fourth molting stage and the beginning of the fifth instar, and then its expression decreased gradually until it disappeared at the end of the fifth instar and the wandering stage. (2) We identified a FTZ-F1 response element 390bp upstream of the transcription initiation site of the fibH promoter. (3) BmFTZ-F1 interacted with the basic helix-loop-helix transcription factor Bmdimm. (4) BmFTZ-F1 down-regulated fibH promoter activity and counteracted the effect of Bmdimm on fibH expression.

CONCLUSIONS

Integrating these results, we conclude that BmFTZ-F1 regulates the transcription of fibH by binding to the FTZ-F1 response element in the fibH promoter and counteracts the effect of Bmdimm on fibH expression.

GENERAL SIGNIFICANCE

These findings provide new insights into the mechanism of regulation of the silk protein gene.

Regulation of Silk Genes by Hox and Homeodomain Proteins in the Terminal Differentiated Silk Gland of the Silkworm Bombyx mori

The silk gland of the silkworm Bombyx mori is a long tubular organ that is divided into several subparts along its anteroposterior (AP) axis. As a trait of terminal differentiation of the silk gland, several silk protein genes are expressed with unique regional specificities. Most of the Hox and some of the homeobox genes are also expressed in the differentiated silk gland with regional specificities. The expression patterns of Hox genes in the silk gland roughly correspond to those in embryogenesis showing "colinearity". The central Hox class protein Antennapedia (Antp) directly regulates the expression of several middle silk gland-specific silk genes, whereas the Lin-1/Isl-1/Mec3 (LIM)-homeodomain transcriptional factor Arrowhead (Awh) regulates the expression of posterior silk gland-specific genes for silk fiber proteins. We summarize our results and discuss the usefulness of the silk gland of Bombyx mori for analyzing the function of Hox genes. Further analyses of the regulatory mechanisms underlying the region-specific expression of silk genes will provide novel insights into the molecular bases for target-gene selection and regulation by Hox and homeodomain proteins.

High yield exogenous protein HPL production in the Bombyx mori silk gland provides novel insight into recombinant expression systems

The silk gland of Bombyx mori (BmSG) has gained significant attention by dint of superior synthesis and secretion of proteins. However, the application of BmSG bioreactor is still a controversial issue because of low yields of recombinant proteins. Here, a 3057 bp full-length coding sequence of Hpl was designed and transformed into the silkworm genome, and then the mutant (Hpl/Hpl) with specific expression of Hpl in posterior BmSG (BmPSG) was obtained. In the mutants, the transcription level of Fib-L and P25, and corresponding encoding proteins, did not decrease. However, the mRNA level of Fib-H was reduced by 71.1%, and Fib-H protein in the secreted fibroin was decreased from 91.86% to 71.01%. The mRNA level of Hpl was 0.73% and 0.74% of Fib-H and Fib-L, respectively, while HPL protein accounted for 18.85% of fibroin and 15.46% of the total amount of secreted silk protein. The exogenous protein was therefore very efficiently translated and secreted. Further analysis of differentially expressed gene (DEG) was carried out in the BmPSG cells and 891 DEGs were detected, of which 208 genes were related to protein metabolism. Reduced expression of endogenous silk proteins in the BmPSG could effectively improve the production efficiency of recombinant exogenous proteins.

Silk Gland Gene Expression during Larval-Pupal Transition in the Cotton Leaf Roller Sylepta derogata (Lepidoptera: Pyralidae)

The cotton leaf roller, Sylepta derogata, is a silk-producing insect pest. While young larvae feed on the underside of leaves, the older ones roll cotton leaves and feed on the leaf edges, which defoliates cotton plants. The larvae produce silk to stabilize the rolled leaf and to balloon from used to new leaves. Despite the significance of silk in the biology of pest insect species, there is virtually no information on the genes involved in their silk production. This is a substantial knowledge gap because some of these genes may be valuable targets for developing molecular pest management technologies. We addressed the gap by posing the hypothesis that silk gland gene expression changes during the transition from larvae to pupae. We tested our hypothesis using RNA-seq to investigate changes in silk gland gene expression at three developmental stages, 5th instar larvae (silk producing; 15,445,926 clean reads), prepupae (reduced silk producing; 13,758,154) and pupae (beyond silk producing; 16,787,792). We recorded 60,298 unigenes and mapped 50,158 (larvae), 48,415 (prepupae) and 46,623 (pupae) of them to the NCBI database. Most differentially expressed genes in the 5th instar larvae/prepupae libraries were relevant to nucleotide synthesis and maintenance of silk gland function. We identified down-regulated transcriptional factors and several genes involved in silk formation in the three libraries and verified the expression pattern of eight genes by qPCR. The developmental- and tissue-specific expression patterns of the fibroin light chain gene showed it was highly expressed during the larval silk-producing stage. We recorded highest expression of this gene in the larval silk gland, compared to other tissues, including midgut, hindgut, epidermis, Malpighian tubes, hemolymph and fat body. These data are a genetic resource to guide selection of key genes that may be targeted for in planta and other gene-silencing technologies for sustainable cotton agriculture.

The 5'-UTR intron of the midgut-specific BmAPN4 gene affects the level and location of expression in transgenic silkworms

Introns are important for regulating gene expression. BmAPN4, which has a 5'-UTR upstream intron (5 UI), is specifically expressed in the entire silkworm midgut. In our previous study, the promoter region upstream of the 5 UI of BmAPN4 was cloned and identified as the P3 promoter (P3P) with activity only in the anterior midgut. In this study, the sequence consisting of the P3P and the 5 UI was cloned and named as P3P+5 UI. A transgenic vector was constructed in which EGFP was controlled by P3P+5 UI. Transgenic P3+5 UI silkworms were generated by embryo microinjection. RT-PCR showed P3P+5 UI activity throughout the larval stage. Intense green fluorescence was seen only in the entire midgut of P3+5 UI silkworms and expression was confirmed by RT-PCR. qPCR revealed that expression of EGFP in the anterior midgut of P3+5 UI silkworms was 64% higher than in P3 silkworms, indicating the 5 UI sustained intron-mediated enhancement of gene expression. These results suggested that the BmAPN4 5 UI affected the level and site of expression. The 5 UI was cloned and added behind P2P, another specific promoter with activity only in the anterior midgut of silkworm, to construct the P2P+5 UI and transgenic P2+5 UI silkworms. Expression patterns were the same for P2P+5 UI and P2P, suggesting that the 5UI of BmAPN4 did not affect P2P. This study found that the BmAPN4 5 UI affected the amount and location of gene expression. Its influence appeared to be dependent on a specific promoter.

LIM-homeodomain transcription factor Awh is a key component activating all three fibroin genes, fibH, fibL and fhx, in the silk gland of the silkworm, Bombyx mori

In the silkworm Bombyx mori, three fibroin genes, fibroin-heavy-chain (fibH), fibroin-light-chain (fibL) and fibrohexamerin (fhx), are coexpressed only in the posterior silk gland (PSG) cells, while the sericin genes encoding silk glue proteins are expressed in the middle silk gland (MSG) cells. Silk gland factor-2 (SGF-2) is a PSG-specific activator complex of fibH, composed of a LIM-homeodomain protein, Awh, and its cofactors, Ldb and Lcaf. We investigated whether SGF-2 can activate other fibroin genes using transgenic silkworms. The genes for Ldb and Lcaf were expressed ubiquitously in various tissues, while the gene for Awh was expressed strictly specific in PSG of the wild type silkworms. Misexpression of Awh in transgenic silkworms induced ectopic expression of fibL and fhx as well as fibH in MSG. Coincidently with the induction of fibL and fhx by Awh, binding of SGF-2 to the promoter of fibL and fhx was detected in vitro, and SGF-2 binds directly to the fhx core promoter. Ectopic expression of the fibroin genes was observed at high levels in the middle part of MSG. Moreover, fibL and fhx were induced in the anterior silk gland (ASG) of the transgenic silkworms, but fibH was not. These results indicate that Awh is a key activator of all three fibroin genes, and the activity is probably regulated in conjunction with additional factors.

New insight into the mechanism underlying fibroin secretion in silkworm, Bombyx mori

In order to investigate the role of different parts of the fibroin heavy chain (H-chain) in the secretion of fibroin in the silk gland of the silkworm (Bombyx mori) in vivo, two enhanced green fluorescent protein (EGFP)/H-chain fusion genes with deduced protein sequences containing an identical N-terminal region and different C-terminal regions of the H-chain were introduced into the B. mori genome using a piggyBac-mediated germline transformation. EGFP fluorescence and molecular analysis showed the products of two different EGFP/H-chain fusion proteins were secreted into the posterior silk gland lumen and aggregated in the middle silk gland and spun into cocoons. The results revealed that only the non-repetitive N terminus of the H-chain is essential for secretion of the H-chain into the posterior silk gland lumen. In addition, our results also indicated that the most likely post-translational modification of the H-chain is at the C-terminal domain. Here, our results not only provide a theoretical basis for the genetic modification of silk fiber as a functional biomaterial but also are of great significance to establishing a new silk gland bioreactor to mass-produce exogenous proteins in an active form.

Hox transcription factor Antp regulates sericin-1 gene expression in the terminal differentiated silk gland of Bombyx mori

Hox genes are well-known master regulators in developmental morphogenesis along the anteroposterior axis of animals. However, the molecular mechanisms by which Hox proteins regulate their target genes and determine cell fates are not fully understood. The silk gland of Bombyx mori is a tubular tissue divided into several subparts along the anteroposterior axis, and the silk genes are expressed with specific patterns. The sericin-1 gene (ser1) is expressed in the middle silk gland (MSG) with sublocal specificity. Here we show that the Hox protein Antp is a component of the middle silk gland-specific complex, MIC (MSG-intermolt-specific complex), binds to the essential promoter element of ser1, and activates its expression. Ectopic expression of Antp in transgenic silkworms induced the expression of ser1 in the posterior silk gland (PSG), but not in the anterior part of MSG (MSG-A). Correspondingly, a MIC-like complex was formed by the addition of recombinant Antp in extracts from PSG with its cofactors Exd and Hth, but not in extracts from MSG-A. Splicing patterns of ser1 mRNA induced by the ectopic expression of Antp in PSG were almost the same as those in MSG at the fifth instar and altered depending on the induction timing of Antp. Other Hox genes were expressed with sublocal specificity in the silk gland. The Bombyx silk gland might provide a useful system for understanding how Hox proteins select and regulate their target genes.

Silk gland factor-2, involved in fibroin gene transcription, consists of LIM homeodomain, LIM-interacting, and single-stranded DNA-binding proteins

SGF-2 binds to promoter elements governing posterior silk gland-specific expression of the fibroin gene in Bombyx mori. We purified SGF-2 and showed that SGF-2 contains at least four gene products: the silkworm orthologues of LIM homeodomain protein Awh, LIM domain-binding protein (Ldb), a sequence-specific single-stranded DNA-binding protein (Lcaf), and the silk protein P25/fibrohexamerin (fhx). Using co-expression of these factors in Sf9 cells, Awh, Ldb, and Lcaf proteins were co-purified as a ternary complex that bound to the enhancer sequence in vitro. Lcaf interacts with Ldb as well as Awh through the conserved regions to mediate transcriptional activation in yeast. Misexpression of Awh in transgenic silkworms induces ectopic expression of the fibroin gene in the middle silk glands, where Ldb and Lcaf are expressed. Taken together, this study demonstrates that SGF-2 is a multisubunit activator complex containing Awh. Moreover, our results suggest that the Ldb·Lcaf protein complex serves as a scaffold to facilitate communication between transcriptional control elements.

Silkworms transformed with chimeric silkworm/spider silk genes spin composite silk fibers with improved mechanical properties

The development of a spider silk-manufacturing process is of great interest. However, there are serious problems with natural manufacturing through spider farming, and standard recombinant protein production platforms have provided limited progress due to their inability to assemble spider silk proteins into fibers. Thus, we used piggyBac vectors to create transgenic silkworms encoding chimeric silkworm/spider silk proteins. The silk fibers produced by these animals were composite materials that included chimeric silkworm/spider silk proteins integrated in an extremely stable manner. Furthermore, these composite fibers were, on average, tougher than the parental silkworm silk fibers and as tough as native dragline spider silk fibers. These results demonstrate that silkworms can be engineered to manufacture composite silk fibers containing stably integrated spider silk protein sequences, which significantly improve the overall mechanical properties of the parental silkworm silk fibers.

Novel enhancer and promoter elements indispensable for the tissue-specific expression of the sericin-1 gene of the silkworm Bombyx mori

Sericins are glue proteins produced specifically in the middle silk gland (MSG) of the silkworm Bombyx mori, while the silk fiber protein, fibroin, is produced in the posterior silk gland (PSG). These silk proteins are expected to be useful biomaterials in medical technology as well as biotechnology. In this study, we analyzed promoter elements of the sericin-1 gene (ser1) in vivo by introducing reporter constructs into silk glands via gene gun technology. The region from -1602 to +47 was sufficient to induce MSG-specific expression. The 5' deletion mutants showed a three-step decrease in promoter activity with the key sequences located between -1362 and -1250, -201 and -116, and -115 and -37. We detected a tissue- and stage-specific factor complex (MSG-intermolt-specific complex: MIC) bound to the sequence elements around the -1350, -320, -180, and -70 regions. A mutation in the -70 region, which inhibits MIC-binding, diminished almost all promoter activity, while another mutation that did not inhibit MIC-binding showed no effect on promoter activity. The results suggest that the binding of MIC to the above elements is intrinsic for the spatiotemporal specificity of ser1 in vivo.

betaFTZ-F1 and Broad-Complex positively regulate the transcription of the wing cuticle protein gene, BMWCP5, in wing discs of Bombyx mori

The present study was undertaken to clarify the mechanism regulating cuticle protein gene expression. Expression of BMWCP5 was strong at around pupation and weak at the mid-pupal stage in wing tissues of Bombyx mori. We analyzed the upstream region of the BMWCP5 gene using a transient reporter assay with a gene gun system to identify the regulatory elements responsible for its unique expression pattern. We identified two betaFTZ-F1 binding sites to be important cis-acting elements for the transcription activation of the luciferase reporter gene by an ecdysone pulse. Site-directed mutagenesis of these sites, followed by introduction into wing discs, significantly decreased the reporter activity. We also found that the regions carrying the binding sites for the ecdysone-responsive factor BR-C Z4 (BR-Z4) were responsible for the hormonal enhancement of the reporter gene activity in wing discs. Mutation of the BR-Z4 binding sites decreased the reporter activity. The nuclear proteins that bound to these betaFTZ-F1 and BR-Z4 sites were identified by an electrophoretic mobility shift assay (EMSA). The results demonstrate for the first time that the BR-Z4 isoform can bind to the upstream region of the cuticle protein gene, BMWCP5, and activate its expression. The results also suggest that the BMWCP5 transcription is primarily regulated by the ecdysone pulse through betaFTZ-F1, and the stage-specific enhancement is brought about through BR-Z4.

DNA-binding property of the novel DNA-binding domain STPR in FMBP-1 of the silkworm Bombyx mori

The STPR domain is a novel DNA-binding domain composed of repeats of 23 amino-acid-long peptide found in the fibroin-modulator-binding protein-1 (FMBP-1) of the silkworm Bombyx mori. Theoretical proteins having the STPR domain are highly conserved, particularly in vertebrates, but the functions are mostly unknown. In this study, the DNA-binding property of the STPR domain in FMBP-1 was examined. Use of reagents selecting the DNA groove and an oligonucleotide in which the dA:dT pairs of the probe were replaced with dI:dC pairs in mobility shift assay demonstrated that FMBP-1 approaches DNA from the major groove. Permutation electrophoresis using probes of the same length but containing the FMBP-1-binding site at different positions showed that FMBP-1 bends DNA through its binding. To induce the sharp bend of DNA, the STPR domain alone was insufficient and the long N-terminal extending region was necessary. Moreover, the basic region extending from the N-terminus of the STPR domain stabilized the DNA binding of the STPR domain. These results suggested that DNA-binding properties of the STPR domain are affected strongly by the structure of the flanking regions in the STPR domain-containing proteins.