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

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 acetate(-) ions were demonstrated to facilitate the optimal binding of CI to cognate operator DNA, whereas K(+), Li(+), NH4(+), Mg(2+), carbonate(2-), and citrate(3-) ions significantly affected its operator binding activity. Of these ions, Mg(2+) unfolded CI most severely at room temperature and, compared to Mg(2+), Na(+) provided improved thermal stability to CI. Furthermore, the intrinsic tryptophan fluorescence of CI was changed notably upon replacing Na(+) with Mg(2+) and these opposing effects of Mg(2+) 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 Mg(2+) for maintaining the biologically active conformation of CI needed for its optimal binding to operator DNA.