Crystal Structure of a New Single-Stranded DNA-Binding Protein Encoded by <i>Staphylococcal</i> Cassette Chromosome Elements

Abstract

Methicillin resistant Staphylococcus aureus (MRSA) is a global public-health threat. Methicillin resistance is carried on mobile genetic elements belonging to the Staphylococcal Cassette Chromosome or SCC family. The molecular mechanisms that SCC elements exploit for stable maintenance and for horizontal transfer between strains are poorly understand. Previously, we identified and characterized several conserved SCC genes with putative functions in DNA replication, including lp1413, which we found encodes a single-stranded (ss) DNA binding protein. We report here the 2.18 A crystal structure of LP1413, which shows that it adopts a winged helix-turn-helix fold rather than the OB-fold normally seen in replication-related ssDNA binding proteins. However, conserved residues form a hydrophobic pocket not normally found in winged helixturn-helix domains. LP1413 also has a conserved but disordered C-terminal tail. As we find that deletion of the tail does not affect cooperative binding to ssDNA, we propose that it mediates interactions with other proteins. We discuss roles that LP1413 could play in vivo.