# Coupled Heterogeneity to Dimeric Site-Specific Binding by the POU-Family Transcription Factor OCT2

## TL;DR

This study explores how the POU-family transcription factor OCT2 binds to DNA in a dimeric form, revealing complex interactions that influence gene regulation.

## Contribution

The paper introduces new insights into the thermodynamic and structural heterogeneity of OCT2 dimeric DNA binding.

## Key findings

- OCT2 forms secondary complexes that are dependent on site-specific binding and influence affinity.
- Salt and poly[d(I-C)] modulate nonspecific and specific DNA binding through intermolecular transfer.
- Molecular dynamics simulations support a mechanism of DNA cross-linking by OCT2 subdomains.

## Abstract

POU-family transcription factors regulate metazoan gene
expression
via a bipartite DNA-binding domain consisting of two covalently linked
helix-turn-helix subdomains, POUS and POUH.
POU factors bind as dimers to DNA half-sites to form complexes with
a variable quaternary structure. To enhance the knowledge of the physical
chemistry of dimeric POU/DNA recognition, we carried out a crystallographic
and titration analysis of the cooperative homodimer formed by the
POU factor OCT2 and an optimized palindromic DNA site known as MORE.
The data evidence strong heterogeneity in the binding and formation
of secondary complexes in site-specific DNA recognition by OCT2 at
thermodynamic equilibrium. These secondary complexes are strictly
contingent to the site-specific complex, detectable at subsaturating
OCT2 concentrations, and cooperate with nonspecific binding to guide
the affinity of the site-specific complex. Modulation with salt and
poly[d(I-C)] unmasks the compensation between nonspecific DNA depleting
unbound OCT2 on the one hand while driving specific binding by intermolecular
transfer of OCT2 via secondary complexes on the other. Molecular dynamics
simulations extend a mechanism, previously proposed for POU monomers,
in which the two subdomains dynamically cross-link DNA strands to
form supramolecular dimeric POU/DNA complexes at equilibrium.