Ultrastructure of Weddellomyces epicallopisma (Dothideales, Dacampiaceae, Ascomycota), especially of its ascospores]

An ultrastructural study of Weddellomyces epicallopisma (ascomata wall, asci, ascospores and vegetative hyphae), the first done on the family Dacampiaceae, confirms most of the observations made in light microscopy. Moreover it shows that ascospores are provided with an endospore (not visible in light microscope) and that the structure of the ascospore septum is more complex. The similarity of the wall structure between the ascospore and the hyphoid appendages, developed on the upper part of the ascoma, is emphasized.     

PHOTOACCLIMATION IN THE TROPICAL CORALLINE ALGA HYDROLITHON ONKODES (RHODOPHYTA, CORALLINACEAE) FROM A FRENCH POLYNESIAN REEF

Pigment concentration, in vivo absorption, and photosynthetic parameters of the coralline alga Hydrolithon onkodes (Heydrich) Penrose and Woelkerling were compared among samples from a lagoon and from a reef crest of Tahiti Island. Four groups of specimens were considered, differing in their natural exposure to PAR. For specimens collected from the lagoon, the tissues from low-light samples had significantly higher pigment concentration, particularly chl a and phycobilins, compared with the high-light exposed plants that contained more total carotenoids. The in vivo absorption spectra normalized to chl a (called a* values) also revealed differences. The low-light samples had a reduced absorption capacity and a well-marked phycobilin absorption signature, whereas sunlit samples showed a greater absorption at wavelengths absorbed mainly by chl a and carotenoids. The decrease of a* when pigment concentration increased is interpreted as a consequence of the pigment packaging. Significantly lower α (chlorophyll basis) and higher E_k values were found in the shaded plants. The values of P _max for the four groups of specimens were not significantly different. The samples showed various degrees of photoinhibition depending on the light exposure during growth, and this effect was more pronounced in the shaded plants. The specimens from the reef crest deviate from the general model presented for the lagoon samples and show a mix of sun- and shade-exposed characteristics. We have shown that the coralline alga H. onkodes responds to its light environment, probably by acclimation rather than ecotypic genetic variation, by adjusting its physiology, but some morphological differences are also involved. Photoacclimation can explain partly the wide distribution of this species over the reef ecosystem and its major contribution to the building of the reef.     

Target size analysis of neurotensin receptors

The neurotensin receptors from rat brain synaptosomal membranes differed in subunit structure from those in rat gastric fundus smooth muscle plasma membranes when studied following photoaffinity labelling or after exposure to cross-linking reagents. However, these two receptors were similar in their recognition properties. In this study we compared the target size of the two receptors by radiation inactivation and observed that the receptors in both tissues had similar target sizes (mean values 103,000 and 108,000 daltons). This suggests that the differences in size observed in biochemical studies may reflect changes occurring during the isolation procedures, or, on the other hand, there might be inherent difference in the subunit structure of these receptors.     

Carboxylmethylation of Calmodulin in Cultured Pituitary Cells

We have used fast protein liquid chromatography (FPLC) and reverse-phase HPLC to rapidly resolve carboxylmethylated proteins in cultured pituitary GH3 cells. This procedure preserves labile carboxylmethyl esters, which are lost under the usual procedures employed for protein fractionation. GH3 cells were incubated with [methyl-3H]-methionine in culture and incorporation of label into the soluble fraction, total cell protein, and protein carboxylmethyl esters was determined; protein carboxylmethyl ester formation was shown to be resistant to cycloheximide. Fractionation of protein carboxylmethyl esters from GH3 cells by gel permeation FPLC, anion-exchange FPLC, and reverse-phase HPLC in the presence of calcium and in the presence of EGTA identified two proteins that are major substrates for protein carboxylmethyltransferase and indicated that one of these proteins is calmodulin. Similar results were obtained when a cytosolic fraction from GH3 cells was incubated with S-adenosyl-L-[methyl-3H]methionine. These results indicate that rapid chromatography at low temperature and low pH is useful for the analysis of eucaryotic carboxylmethylated proteins and that contrary to reports obtained in other systems, calmodulin is carboxylmethylated in intact pituitary cells.     

Cortical activity in vertebrate eggs. I: The activation waves

We present a physical model for the propagation of chemical and mechanical waves on the surface of vertebrate eggs. As a first step we analyzed the propagation of the calcium wave observed to sweep over the surface of the Medaka egg (Gilkey et al., 1978). It has been assumed that this wave is driven by a mechanism of calcium-stimulated-calcium-release. By formulating this hypothesis mathematically we can use the observed wavefront data to obtain a map of cortical reactivity. This map indicates a gradient of reactivity along the egg: highest in the animal hemisphere and tapering off towards the vegetal hemisphere. The cortex of Xenopus eggs is also capable of propagating a calcium wave (Busa & Nuccitelli, 1985). At about the same time a wave of expansion followed by a wave of contraction sweeps across the egg surface (Takeichi et al., 1984). We have proposed a mechanism for this wave pair based on the physical chemistry of actomyosin gels. The calcium wave activates solation factors which sever some of the actin chains which leads to an osmotic swelling of the gel. Calcium also activates the contractile machinery of the actomyosin system which causes the gel to contract. The contraction lags the swelling because of the nature of the kinetics: solation and swelling is a more rapid process than contraction. By writing the equations for gel expansion and contraction we can mimic the mechanical and chemical wave propagation by a computer simulation. If the model is correct this provides a method for using the waves as a diagnostic of the mechanochemical properties of the egg cortex.