INHIBITION OF MG2-GENE MUTATION IN PARAMECIUM( CURRENT BY SINGLE)

''Eccentric'' is a newly-isolated mutant of Paramecium tetraurelia tha t fails to swim backwards in response to Mg2+. In the wild type, this backward swimming results from Mg2+ influx via a Mg2+-specific ion con ductance (I-Mg). Voltage-clamp analysis confirmed that, as suspected, step changes in membrane potential over a physiological range fail to elicit I-Mg from eccentric. Further electrophysiological investigation revealed a number of additional ion-current defects in eccentric: (i) The Ca2+ current activated upon depolarization inactivates more slowl y in eccentric than in the wild type, and it requires longer to recove r from this inactivation. (ii) The Ca2+-dependent Na+ current deactiva tes significantly faster in the mutant. (iii) The two K+ currents obse rved upon hyperpolarization are reduced by >60% in eccentric. It is di fficult to envision how these varied pleiotropic effects could result from loss of a single ion current. Rather, they suggest that the eccen tric mutation affects a global regulatory system. Two plausible hypoth eses are discussed.