Iron-inducible transcription of the ap65-1 gene in Trichomonas vaginalis involves at least three Myb-like transcriptional factors (tvMyb1, tvMyb2 and tvMyb3) that differentially bind to two closely spaced promoter sites, MRE-1/MRE-2r and MRE-2f. Here, we defined a fragment of tvMyb2 comprising residues 40-156 (tvMyb240-156) as the minimum structural unit that retains near full binding affinity with the promoter DNAs. Like c-Myb in vertebrates, the DNA-free tvMyb240-156 has a flexible and open conformation. Upon binding to the promoter DNA elements, tvMyb240-156 undergoes significant conformational re-arrangement and structure stabilization. Crystal structures of tvMyb240-156 in complex with promoter element-containing DNA oligomers showed that 5′-a/gACGAT-3′ is the specific base sequence recognized by tvMyb240-156, which does not fully conform to that of the Myb binding site sequence. Furthermore, Lys 49, which is upstream of the R2 motif (amino acids 52-102) also participates in specific DNA sequence recognition. Intriguingly, tvMyb240-156 binds to the promoter elements in an orientation opposite to that proposed in the HADDOCK model of the tvMyb135-141/MRE-1-MRE-2r complex. These results shed new light on understanding the molecular mechanism of Myb-DNA recognition and provide a framework to study the molecular basis of transcriptional regulation of myriad Mybs in T. vaginalis.
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