ИСТИНА

Tryptone induces chemotaxis and inhibits photoreceptor currents in gametes, hut not in vegetative cells of Chlamydomonas reinhardtii. A close correlation between these two phenomena shows convergence of initial steps in photo- and chemosensory transduction pathways. Therefore, measurement of the tryptone influence on photoreceptor currents allows probing activation of the chemosensory system. The presence of acetate in the medium during gametogenesis increases the sensitivity of the photoreceptor currents to tryptone, as compared to acetate-free conditions. This increase corresponds to an increase in the external pH due to acetate consumption. Elevation of the external pH in the absence of acetate leads to an immediate increase in the sensitivity. The action of tryptone depends on the presence of monovalent cations in the medium with a selectivity sequence K+>Rb+>>Cs+>Na+>Li+ (Eisenman sequence IV) and is inhibited by extracellular BaH and Ca2+ in concentrations known to block K+ channels. These data indicate the involvement of an inward K+ current across the plasma membrane in cbemosensory transduction in C. reinhardtii. Preincubation of gametes with 10 mM dibutyrylcAMP or the addition of 1 mM IBMX, an inhibitor of cyclic nucleotide hosphodiesterases, decreases the tryptone effect. A possible explanation is that K+ channels involved in chemotaxis toward tryptone are modulated by intracellular concentration of cyclic nucleotides. Unlike tryptone, NH4+ suppresses the photoreceptor currents in both gametes and nitrogen-deprived vegetative cells of C. reinhardtii, although mature gametes show no chemotaxis toward this ion. It can therefore be concluded that chemotactic responses toward tryptone and NH4+ are mediated by different transduction mechanisms. This work was supponed by RFBR grant 02-0449179.