Effects of Ca2+ and Mg2+ on the switch-on of transcriptase function in reovirus in vitro.

Effects of Ca2+ and Mg2+ on the "switch-on" of transcriptase function in reovirus intermediate subviral particles were examined. Switch-on in this system is triggered by K+ ions, whereas Na+ ions antagonize the effects of K+ ions. Once effected, switch-on is irreversible. In the absence of Ca2+ and Mg2+ ions, switch-on of transcriptase is effected by incubation at 37 degrees C in the presence of a particular ratio of K+/Na+ (total K+ plus Na+ being kept constant). Inclusion of Ca2+ in the activation step lowers the ratio that triggers the switch-on event. This indicates that Ca2+ destabilizes the switched-off state of the transcriptase. Inclusions of Mg2+ ions in the activation step raises the K+/Na+ ratio necessary to trigger switch-on. Thus Mg2+ tends to stabilize the switched-off state of the transcriptase to the triggering action of K+ ions. Inclusion of both Ca2+ and Mg2+ together during the activation step results in mutual inhibition of the effects expected from each divalent cation alone. These results indicate that switch-on of transcriptase function, which is triggered by K+ ions, can be modulated by interactions involving the major physiological cations (Na+, K+, Mg2+, Ca2+).