Enhanced conformational flexibility of the histone-like (HU) protein from Mycoplasma gallisepticum

The histone-like (HU) protein is one of the major nucleoid-associated proteins involved in DNA supercoiling and compaction into bacterial nucleoid as well as in all DNA-dependent transactions. This small positively charged dimeric protein binds DNA in a non-sequence specific manner promoting DNA super-structures. The majority of HU proteins are highly conserved among bacteria; however, HU protein from Mycoplasma gallisepticum (HUMgal) has multiple amino acid substitutions in the most conserved regions, which are believed to contribute to its specificity to DNA targets unusual for canonical HU proteins. In this work, we studied the structural dynamic properties of the HUMgal dimer by NMR spectroscopy and MD simulations. The obtained all-atom model displays compliance with the NMR data and confirms the heterogeneous backbone flexibility of HUMgal. We found that HUMgal, being folded into a dimeric conformation typical for HU proteins, has a labile alpha-helical body with protruded beta-stranded arms forming DNA-binding domain that are highly flexible in the absence of DNA. The amino acid substitutions in conserved regions of the protein are likely to affect the conformational lability of the HUMgal dimer that can be responsible for complex functional behavior of HUMgal in vivo, e.g. facilitating its spatial adaptation to non-canonical DNA-targets.