Isolation and characterization of paramecium mutants defective in their response to magnesium.

Four mutant strains of Paramecium tetraurelia with a reduced ability to respond behaviorally to Mg2+ have been isolated. Voltage-clamp analyses showed that their Mg2+ insensitivity is associated with a reduced Ca(2+)-dependent Mg2+ current. The four mutants, which have been doubled "eccentric," result from recessive mutations in two unlinked loci, xntA and xntB. Further analysis of xntA1 showed it to be unlinked to any of the behavioral mutants of P. tetraurelia described previously, but it is allelic to d4-521, a "K(+)-resistant" strain, and d4-596, a "Ba(2+)-shy" mutant. The varied pleiotropic effects of xntA1, which include increased resistance to Ni2+ and Zn2+ poisoning, suggest that the locus encodes a central regulator of cell function in Paramecium.