*Communication* **Direct Single-Molecule Observation of Sequential DNA Bending Transitions by the Sox2 HMG Box**

**Mahdi Muhammad Moosa, Phoebe S. Tsoi, Kyoung-Jae Choi, Allan Chris M. Ferreon \* and Josephine C. Ferreon \***

Department of Pharmacology and Chemical Biology, Baylor College of Medicine, Houston, TX 77030, USA; Mahdi.Moosa@bcm.edu (M.M.M.); Phoebe.Tsoi@bcm.edu (P.S.T.); Kyoungjae.Choi@bcm.edu (K.-J.C.)

**\*** Correspondence: Allan.Ferreon@bcm.edu (A.C.M.F.); Josephine.Ferreon@bcm.edu (J.C.F.);

Tel.: +1-713-798-1754 (A.C.M.F.); +1-713-798-1756 (J.C.F.)

Received: 2 October 2018; Accepted: 30 November 2018; Published: 4 December 2018

**Abstract:** Sox2 is a pioneer transcription factor that initiates cell fate reprogramming through locus-specific differential regulation. Mechanistically, it was assumed that Sox2 achieves its regulatory diversity via heterodimerization with partner transcription factors. Here, utilizing single-molecule fluorescence spectroscopy, we show that Sox2 alone can modulate DNA structural landscape in a dosage-dependent manner. We propose that such stoichiometric tuning of regulatory DNAs is crucial to the diverse biological functions of Sox2, and represents a generic mechanism of conferring functional plasticity and multiplicity to transcription factors.

**Keywords:** transcription factors; DNA-protein interactions; Sox2 sequential DNA loading; smFRET; DNA conformational landscape; sequential DNA bending; transcription factor dosage
