A novel member of the bacterial-archaeal regulator family is a nonspecificDNA-binding protein and induces positive supercoiling

In hyperthermophilic Archaea genomic DNA is from relaxed to positively supe rcoiled in vivo because of the action of the enzyme reverse gyrase, and thi s peculiarity is believed to be related to stabilization of DNA against den aturation. We report the identification and characterization of Smj12, a no vel protein of Sulfolobus solfataricus, which is homologous to members of t he so-called Bacterial-Archaeal family of regulators, found in multiple cop ies in Eubacteria and Archaea. Whereas other members of the family are sequ ence-specific DNA-binding proteins and have been implicated in transcriptio nal regulation, Smj12 is a nonspecific DNA-binding protein that stabilizes the double helix and induces positive supercoiling. Smj12 is not abundant, suggesting that it is not a general architectural protein, but rather has a specialized function and/or localization. Smj12 is the first protein with the described features identified in Archaea and might participate in contr ol of superhelicity during DNA transactions.