OSCILLATING RESPONSE TO A PURINE NUCLEOTIDE DISRUPTED BY MUTATION IN PARAMECIUM-TETRAURELIA

The purine nucleotide GTP, when added extracellularly, induces oscilla tions in the swimming behaviour of the protist Paramecium tetraurelia. For periods as long as 10 min the cell swims backwards and forwards r epetitively. The oscillations in swimming behaviour are driven by chan ges in membrane potential of the cell, which in turn are caused by per iodic activation of inward Mg2+- and Na+-specific currents. We screene d for and isolated mutants that are defective in this response, exploi ting the fact that the net result of GTP on a population of cells is r epulsion. One mutant, GTP-insensitive (gin A), is not repelled by GTP. In addition, GTP fails to induce repetitive backwards swimming in gin A mutants, although they swim backwards normally in response to other stimuli. GTP fails to evoke oscillations in membrane potential or Mg2 + and Na+ currents in the mutant, although the Mg2+ and Na+ conductanc es are not themselves measurably affected. A small, oscillating Ca2+ c urrent induced by GTP in the wild type, which might be part of the mec hanism that generates oscillations, is also missing from gin A cells. To our knowledge, gin A is the first example of a mutant defective in a purinergic response. We discuss the possibility that the gin A lesio n affects the oscillator itself.