Structural studies of two nucleoid-associated proteins : histone-like nucleoid-structuring protein H-NS and α-hemolysin expression-modulating protein Hha

Abstract

In prokaryotic cells, the nucleoid contains almost all the genetic materials as well as a number of nucleoid structuring factors. The nucleoid-associated proteins (NAPs) are known to have low molecular weight and the ability to form dimer or oligomer, and most of them can bind to DNA for regulation of gene expression. The Histone-like nucleoid structuring protein H-NS, well studied as one of the NAPs, acts as a global transcriptional repressor. It has independent functional N-terminal domain for oligomerization and C-terminal domain for DNA binding, joined by a flexible linker. H-NS contributes to horizontal genes transfer and responses to environmental factors like temperature or pH, which would influence the oligomerization ability of H-NS and DNA binding. The α-hemolysin expression-modulating protein Hha is a member of the Hha-YmoA family, expressed only in Gram-negative Enterobacteriaceae as a modulator of virulence factors expression. In E. coli, the binding of Hha to H-NS can modulate the expression of α-hemolysin operon, which is essential for the H-NS-regulated gene expression. In this study, both Hha and the oligomerization domain of H-NS (H-NS64) were expressed in E. coli and the purified proteins were crystallized. The Hha crystals diffracted to 2.2 Å; and the HhA/H-NS complex crystals diffracted to 1.8 Å. Both structures were successfully determined by molecular replacement method. Comparisons were carried out between the published apo Hha and H-NS structures and our complex structures. The structures showed the binding details between H-NS and Hha and also conformational changes of each protein, which may indicate how Hha regulates gene expressions through H-NS.