The crystal structure of human forkhead box N1 in complex with DNA reveals the structural basis for forkhead box family specificity

Forkhead box N1 (FOXN1) is a member of the forkhead box family of transcription factors and plays an important role in thymic epithelial cell differentiation and development. FOXN1 mutations in humans and mice give rise to the “nude” phenotype, which is marked by athymia. FOXN1 belongs to a subset of the FOX family that recognizes an alternative forkhead-like (FHL) consensus sequence (GACGC) that is different from the more widely recognized forkhead (FKH) sequence RYAAAYA (where R is purine, and Y is pyrimidine). Here, we present the FOXN1 structure in complex with DNA containing an FHL motif at 1.6 Å resolution, in which the DNA sequence is recognized by a mixture of direct and water-mediated contacts provided by residues in an α-helix inserted in the DNA major groove (the recognition helix). Comparisons with the structure of other FOX family members revealed that the FKH and FHL DNA sequences are bound in two distinct modes, with partially different registers for the protein DNA contacts. We identified a single alternative rotamer within the recognition helix itself as an important determinant of DNA specificity and found protein sequence features in the recognition helix that could be used to predict the specificity of other FOX family members. Finally, we demonstrate that the C-terminal region of FOXN1 is required for high-affinity DNA binding and that FOXN1 has a significantly reduced affinity for DNA that contains 5′-methylcytosine, which may have implications for the role of FOXN1 in thymic involution.

The structural basis for forkhead box family specificity revealed by the crystal structure of human FOXN1 in complex with DNA

SignificanceFOXN1 is a transcription factor that is essential for the development of the thymus and the production of T-lymphocytes. It is a member of a large family of transcription factors which recognize DNA sequences through the conserved Forkhead (FH) domain. FOXN1 recognizes a DNA sequence that is different from the common consensus binding sequence of FH domains, although key binding residues are identical. We present crystal structures of the FH domain of FOXN1, free and DNA-bound, which shed light on the different binding specificities; the structure also revelas the basis of the immunocompromised nude mutation, as well as a preferential binding to non-methylated CpG motifs.AbstractFOXN1 is a member of the forkhead box (FOX) family of transcription factors, and plays an important role in thymic epithelial cell differentiation and function. FOXN1 mutations in humans and mice give rise to the "nude" phenotype which is marked by athymia. FOXN1 belongs to a subset of the FOX family that recognize an alternate consensus sequence (GACGC), which is different from the more widely-recognized canonical sequence consensus RYAAAYA. Here, we present the structure of FOXN1 in complex with DNA at 1.6 Å resolution, in which the DNA sequence is recognised by a mixture of direct and water-mediated contacts provided by residues in an a-helix inserted in the DNA major groove (the recognition helix). Comparisons with other FOX family structures reveal that the canonical and alternate DNA sequences are bound in two distinct modes, with partially different registers for the protein DNA contacts. We identify a single alternate rotamer within the recognition helix itself as an important determinant of DNA specificity, and indicate sequence features in the recognition helix that could be used to predict the specificity of other FOX family members. Finally we demonstrate that FOXN1 has a significantly reduced affinity for DNA containing 5-methylcytosine, which may have implications for the role of FOXN1 in thymic senescence.

333 ESTABLISHMENT OF NOVEL METHOD FOR REPEATED CONSTRUCTION OF ENGINEERED ZINC FINGER NUCLEASE

Zinc finger nucleases (ZFN), which are artificial restriction enzymes consisting of an engineered zinc-finger domain (ZF) and an endonuclease domain, can be used for the induction of site-directed mutation and the efficient generation of gene knockout animals. However, the repeated construction of various ZFN sequences is both expensive and time consuming. In this study, we attempted to establish a novel method for inexpensive and rapid ZFN construction. First, we constructed ZFN against mouse Rosa26 and original mouse Gli3 gene loci using short PCR primer sets (>30 bp), which contained 21 bp of the ZF recognition helix for a specific DNA triplet. We prepared 18 sets of such primers and PCR was performed using one of these primer sets and the partial ZF sequence as a template, which was obtained from the first to second DNA recognition helix of mouse Zif268. The PCR products were joined by overlap-PCR and nested PCR, and then inserted into a vector coding the endonuclease domain of FokI nuclease. By these steps, we successfully synthesised intended ZFN vectors containing 4 to 6 fingers. Next, we evaluated the functions of constructed ZFN. The mRNA of constructed ZFN were transcribed in vitro and injected into the cytoplasm of C57BL/6N zygotes. After 24 h of culture, 2-cell stage embryos were subjected to genomic PCR of the target locus, and the PCR products were directly sequenced. When ZFN mRNA for mouse Rosa26 was injected, 3- to 146-bp deletions were detected in 92.8% of injected embryos. This result was almost the same as previously reported for ZFN, indicating that our novel construction method can synthesise functional ZFN, which work as a site-directed nuclease, and that efficiency was comparable with those constructed by conventional PCR methods using long oligonucleotide sets (60 bp).