Igonucleotides with only a single binding site, it suggests that either the two MidTbx monomers bound to a single oligonucleotide are not sufficiently stable to resolve on a gel, or that one monomer sterically hinders the binding of another, orIdentification of a Drosophila Tbx20 Binding SiteFigure 3. DNA motif selected by MidTbx. A) The sequence logo corresponding to the oligonucleotide selected by MidTbx after 4 rounds of selection. The 27 EMSA verified sequences and the flanking primer sequences for some were input into MEME. MEME was set to use each nucleotide once and to generate a motif with a maximum length of 26 nucleotides. Region 1 and 2 are underlined in black and blue respectively. B) Aligned sequences of the 27 oligonucleotides used to generate the motif in A. The 15 nucleotides present in A are colour coded according to the nucleotide. Flanking sequences are in black. Nucleotides within the random 26 bp core are 
in uppercase, while those found within the primer sequences are in lowercase. The second potential MidTbx binding site in oligonucleotides represented by clones 2, 8, 42, 74, and 75, has been underlined. C) An EMSA using the 15 base pair consensus motif identified in this study. The migration of oligonucleotides with wild-type Mid binding motif (CAAGGTGTCAAGGCG) is slowed in the presence of MidTbx. However, MidTbx does not appear to have an affinity for oligonucleotides where region 1 has been mutated (CACCCCCCCAAGGCG). Oligonucleotides mutant for region 2 (CAAGGTGTCAAGGAA) are still bound and retarded by MidTbx. doi:10.1371/journal.pone.0048176.gthat there is enough excess probe that the proteins always bind a unique probe. The sequences necessary for dimerization in other T-box factors are not conserved in Mid, which is also consistent with Mid binding as a monomer (Figure 4). Xbra homodimerizes Benzocaine biological activity through ?a relatively small interface of 250 A2 found near the centre of the T-box domain [30]. The small polar N129 residue in Xbra is replaced with a large hydrophobic F281 in Mid and F130 in Xbra is replaced by S282 in Mid. Likewise Xbra M85 is substituted with R235, and Xbra V173 corresponds to L326 in Mid. Overall, 4 of the 8 dimerization residues are not conserved in Drosophila Mid. 1081537 Furthermore, Tbx20 also differs from both Mid and Xbra at these same 4 positions (Figure 4). The crystal structure of Tbx3 bound to a palindromic T-site shows that the two monomers are rotated with respect to one another on the DNA strand and use different residues (238?41 on Tbx3) to contact one another [25]. These residues fall within a poorly conserved region of the T-box domain. Comparison to the corresponding Mid residues (327?30) shows that none of these amino acids are conserved (Figure 4). Similarly, only Tbx3 D239 is identical to the corresponding Tbx20 residue. The small monomer-monomer contacts defined in the Tbx3 crystal structure are thought to be insufficient to facilitate dimerization and as such, Tbx3 is believed to bind as a monomer [25]. Finally, the crystal structure of Tbx5 bound to a half-siteshows that the 76932-56-4 cost regions responsible for monomer-monomer contacts in Tbx3 have low electron density suggesting that these domains are conformationally flexible and thus are unlikely to be in involved in dimerization [32]. Taken together, the site selection data and the comparison of the Mid amino acid sequence with evidence from the crystal structures of the Xbra, Tbx3 and Tbx5 suggest that Mid binds DNA as a monomer. We have also.Igonucleotides with only a single binding site, it suggests that either the two MidTbx monomers bound to a single oligonucleotide are not sufficiently stable to resolve on a gel, or that one monomer sterically hinders the binding of another, orIdentification of a Drosophila Tbx20 Binding SiteFigure 3. DNA motif selected by MidTbx. A) The sequence logo corresponding to the oligonucleotide selected by MidTbx after 4 rounds of selection. The 27 EMSA verified sequences and the flanking primer sequences for some were input into MEME. MEME was set to use each nucleotide once and to generate a motif with a maximum length of 26 nucleotides. Region 1 and 2 are underlined in black and blue respectively. B) Aligned sequences of the 27 oligonucleotides used to generate the motif in A. The 15 nucleotides present in A are colour coded according to the nucleotide. Flanking sequences are in black. Nucleotides within the random 26 bp core are in uppercase, while those found within the primer sequences are in lowercase. The second potential MidTbx binding site in oligonucleotides represented by clones 2, 8, 42, 74, and 75, has been underlined. C) An EMSA using the 15 base pair consensus motif identified in this study. The migration of oligonucleotides with wild-type Mid binding motif (CAAGGTGTCAAGGCG) is slowed in the presence of MidTbx. However, 
MidTbx does not appear to have an affinity for oligonucleotides where region 1 has been mutated (CACCCCCCCAAGGCG). Oligonucleotides mutant for region 2 (CAAGGTGTCAAGGAA) are still bound and retarded by MidTbx. doi:10.1371/journal.pone.0048176.gthat there is enough excess probe that the proteins always bind a unique probe. The sequences necessary for dimerization in other T-box factors are not conserved in Mid, which is also consistent with Mid binding as a monomer (Figure 4). Xbra homodimerizes through ?a relatively small interface of 250 A2 found near the centre of the T-box domain [30]. The small polar N129 residue in Xbra is replaced with a large hydrophobic F281 in Mid and F130 in Xbra is replaced by S282 in Mid. Likewise Xbra M85 is substituted with R235, and Xbra V173 corresponds to L326 in Mid. Overall, 4 of the 8 dimerization residues are not conserved in Drosophila Mid. 1081537 Furthermore, Tbx20 also differs from both Mid and Xbra at these same 4 positions (Figure 4). The crystal structure of Tbx3 bound to a palindromic T-site shows that the two monomers are rotated with respect to one another on the DNA strand and use different residues (238?41 on Tbx3) to contact one another [25]. These residues fall within a poorly conserved region of the T-box domain. Comparison to the corresponding Mid residues (327?30) shows that none of these amino acids are conserved (Figure 4). Similarly, only Tbx3 D239 is identical to the corresponding Tbx20 residue. The small monomer-monomer contacts defined in the Tbx3 crystal structure are thought to be insufficient to facilitate dimerization and as such, Tbx3 is believed to bind as a monomer [25]. Finally, the crystal structure of Tbx5 bound to a half-siteshows that the regions responsible for monomer-monomer contacts in Tbx3 have low electron density suggesting that these domains are conformationally flexible and thus are unlikely to be in involved in dimerization [32]. Taken together, the site selection data and the comparison of the Mid amino acid sequence with evidence from the crystal structures of the Xbra, Tbx3 and Tbx5 suggest that Mid binds DNA as a monomer. We have also.
