Spectral characteristics of the photoreceptor pigment of phototaxis in Dictyostelium discoideum

Abstract —A photoresponsive pigment, described previously as the photoreceptor pigment for phototaxis in Dictyostelium discoideum, was examined by low-temperature spectroscopy. The absorption spectrum of the purified pigment frozen in darkness indicates the reduced form of a high-spin heme protein, and the spectrum of the pigment frozen during irradiation (to freeze in the photoproduct) indicates the oxidized form of the heme protein. The light-induced absorbance changes measured at room temperature also indicate photooxidation of a heme pigment. The action spectrum for the light-induced absorbance change shows a primary peak at about 430 nm and a broad, less active maximum in the 550–580 nm region. The absorption spectrum of the reduced pigment and the action spectrum for its photooxidation are both similar to the action spectrum for phototaxis of the pseudoplasmodia of D. discoideum.     

LIGHT-INDUCED ACCUMULATIONS OF DICTYOSTELIUM DISCOIDEUM AMOEBAE

Abstract— Light-induced accumulations of amoebae of Dictyostelium discoideum in 'light traps' have been observed. The greatest accumulations are obtained with cell densities of about 8 × 10⁴ cells/mm². Accumulations are observed at incident fluence rates over about one decade both for white and for monochromatic light; higher fluence rates cause dispersal from the 'light trap'. An action spectrum for the photoaccumulation, calculated from fluence-response curves using the zero thresholds, shows a major peak between 405 and 410 nm and extends through most of the visible spectrum. This action spectrum does not coincide with any of the pigments known to be present in D. discoideum . The cellular basis for the photoaccumulation has been studied. No light effects on cell divisions or cell aggregation are observed during the 2 h duration of an accumulation experiment. Microvideographic analysis of single amoebae is consistent with the hypothesis that the amoebae are positively phototactic and move toward the light scattered from cells in the 'light trap', thus accumulating in the trap.     

Étude des isotopes du plomb de masse 200, 198, 196 et 194 formés par réaction (ions lourds, xn)

The low-lying excited states of ^{194, 196, 198, 200}Pb have been studied through the de-excitation γ-rays following the reactions Os(¹²C, 4n), W(¹⁶O, 4n) and Re(¹⁴N, 5n). Prompt and delayed spectra, γ-γ coincidences, γ-ray angular distributions and γ-ray time distributions have been obtained. Half-lives were measured for the 12⁺ level in the four isotopes, for the 9^? level in ^{194, 196, 200}Pb, for the ^7? level in ²⁰⁰Pb and for the ^5? level in ^{196, 198}Pb. The results of microscopic calculations, performed with either a Gaussian interaction or a surface delta interaction, are compared to the experimental level schemes and transition probabilities, and discussed.     

New results on the superdeformed band in192Hg

New results on the¹⁹²Hg superdeformed band have been obtained with EUROGAM. The experiment has been performed with the¹⁶⁰Gd(³⁶S,4n) reaction at 159 MeV. Above 800 keV the γ-ray energies differ from the previously published ones. Thus the rise of the dynamical moment of inertia $\mathfrak{F}^{(2)} $ above ?ω=0.4 MeV is no longer observed. This is in better agreement with recent cranked Hartree-Fock-Bogoliubov calculations.