Antagonistic effects Na＋ and Mg2＋ on the structure, function, and stability of mycobacteriophage L1 repressor

Abstract

Temperate mycobacteriophage L1 encodes an unusual repressor (CI) for regulating its lytic-lysogenic switching and, in contrast to the repressors of most temperate phages, it binds to multiple asymmetric operator DNAs. Here, ions like Na＋, Cl−, and ace tate− ions were demonstrated to facilitate the optimal binding of CI to cognate operator DNA, whereas K＋, Li＋, NH4 ＋, Mg2＋, car bonate2−, and citrate3− ions significantly affected its operator binding activity. Of these ions, Mg2＋ unfolded CI most severely at room temperature and, compared to Mg2＋, Na＋ provided im proved thermal stability to CI. Furthermore, the intrinsic trypto phan fluorescence of CI was changed notably upon replacing Na＋ with Mg2＋ and these opposing effects of Mg2＋ and Na＋ were also noticed in their actions on the C-terminal fragment (CTD) of CI. Taken together, Na＋ appeared to be more appro priate than Mg2＋ for maintaining the biologically active con formation of CI needed for its optimal binding to operator DNA. [BMB reports 2009; 42(5): 293-298]