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 acetateions were demonstrated to facilitate the optimal binding of CI to cognate operator DNA, whereas K+, Li+, NH4+, Mg2+, carbonate2-, 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 improved thermal stability to CI. Furthermore, the intrinsic tryptophan 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 appropriate than Mg2+ for maintaining the biologically active conformation of CI needed for its optimal binding to operator DNA.