BIMODAL ACTION OF ALKALINE-EARTH CATIONS ON DICTYOSTELILUM-DISCOIDEUMRIBONUCLEASE-P ACTIVITY

The ribonucleoprotein ribonuclease P (RNase P) cleaves all tRNA precur sors endonucleolitically to produce the mature 5'-end. Dictyostelium d iscoideum RNase P displays an absolute requirement for Mg2+. Only the alkaline earth cations Ca2+, Sr2+, and Ba2+, under appropriate conditi ons can substitute to some extent fur Mg2+. The transition metals Mn2, Co2+, Ni2+, and Cd2+ are efficient inhibitors of the enzyme activity . Ca2+, Sr2+ and Ba2+, in the presence of Mg2+, exhibit a bimodal acti on at the kinetic phase of the reaction. Kinetic analysis of the activ ation phase revealed that Ca2+, Sr2+, or Ba2+ attached on a specific s ite of RNase P act as nonessential-noncompetitive activators. Further additions of Ca2+, Sr2+, or Ba2+ cause noncompetitive inhibition on th e RNase P reaction, indicating that RNase P possesses a second binding site responsible for the inhibitory effect of Ca2+, Sr2+, and Ba2+. B oth activator and inhibitory sites can be occupied by Ca2+, Sr2+, or B a2+ at the same time.