Changes in the Functional Activity of Phi11 Cro Protein is Mediated by Various Ions

Phi11, a temperate bacteriophage of Staphylococcus aureus, has been found to harbor a cro repressor gene which facilitates Phi11 to adopt the lytic mode of development. The Cro protein has been found to bind very specifically to a 15-bp operator DNA, located in the Phi11 cI-cro intergenic region [1]. To investigate the effects exerted by different ions upon the interaction between Cro and its cognate operator DNA, we have employed gel shift assays as well as circular dichroism spectral analysis. In this communication, we have shown that NH₄⁺ and acetate^- ions better facilitated the binding of Cro with its cognate operator as compared to Na⁺, K⁺ and Li⁺. Interestingly, Mg²⁺, carbonate^2- and Citrate^3- have an inhibitory effect upon the binding. The effect of the said ions upon the structure of Cro was also investigated by circular dichroism and it was found that other than Citrate^3- ions, none of the other ions destabilised the protein. On the other hand, Mg²⁺ and carbonate^2- ions maintained the structure of the protein but severely hampered its functional activity. Citrate^3- ions severely unfolded Cro and also inhibited its function. Considering all the data, NH₄⁺ and acetate^- ions appeared to be more suitable in maintaining the biological activity of Cro.