Spectral Dependence of Photoregulation of Inorganic Nitrogen Metabolism in Chlamydomonas reinhardii

The utilization of NO₃⁻ by green algae growing photoautotrophically under air, which are growth conditions close to their more habitual situations in nature, is associated with the excretion of NO₂⁻ and NH₄⁺ to the culture medium. The entire process is promoted by blue light and depends on photosynthetically active radiation for the required reducing equivalents. The stimulation of NO₃⁻ utilization and of its associated NO₂⁻ and NH₄⁺ excretions saturated at very low quantum fluxes of blue light (15 microequivalents per square meter per second) in Chlamydomonas reinhardii cells sparged with CO₂-free air and irradiated with 50 microequivalents per square meter per second background red light. The wavelength dependence data of this stimulation correlated closely with the in situ photoactivation of nitrate reductase and also with the light induced increase in its biosynthesis and/or assembly.

These results indicate that the photoregulation of inorganic N metabolism in C. reinhardii is mainly due to the blue light modulation of nitrate reductase. Although flavins are the most suitable candidates to act as physiological photoreceptors, the wavelength dependence data only show a major peak in the blue region between 400 and 500 nanometers.

     

MARCKS phosphorylation by PKC strongly impairs cell polarity in the chick neural plate

Neurulation involves a complex coordination of cellular movements that are in great part based on the modulation of the actin cytoskeleton. MARCKS, an F‐actin‐binding protein and the major substrate for PKC, is necessary for gastrulation and neurulation morphogenetic movements in mice, frogs, and fish. We previously showed that this protein accumulates at the apical region of the closing neural plate in chick embryos, and here further explore its role in this process and how it is regulated by PKC phosphorylation. PKC activation by PMA caused extensive neural tube closure defects in cultured chick embryos, together with MARCKS phosphorylation and redistribution to the cytoplasm. This was concomitant with an evident disruption of neural plate cell polarity and extensive apical cell extrusion. This effect was not due to actomyosin hypercontractility, but it was reproduced upon MARCKS knockdown. Interestingly, the overexpression of a nonphosphorylatable form of MARCKS was able to revert the cellular defects observed in the neural plate after PKC activation. Altogether, these results suggest that MARCKS function during neurulation would be to maintain neuroepithelial polarity through the stabilization of subapical F‐actin, a function that appears to be counteracted by PKC activation.     

Characterization of the blue light-induced extracellular alkalinization associated with the monovalent anion uptake by Monorapidium braunii. Competition between NO3 and Cl−

The blue light-elicited monovalent anion-dependent alkalinization of the medium of Monoraphidium braunü (Legnerová, 202–7d) was characterized for the NO-³ and Cl- uptake. The maximal H⁺ uptake rates for these two anions have a similar optimum pH around 8.5, and quite similar K^s values for high (38 üM for Cl- and 35üM for NO-³) and low (320 üM for Cl- and 335 üM for NO-³) affinities. The steady H⁺ uptake associated with the uptake and reduction of NO-³ showed a K^s of 125 üM. which in this alga corresponds to the NO-³ reductase (EC 1.6.6.2) K^m for NO-³. The only and striking difference found in the uptake properties of these anions was the delay time between the switching on of the blue light and the start of the alkalinization, which increased from 10 to 90 s as the initial pH decreased from 8.5 to 6.5 in the presence of NO-³, whereas for Cl- uptake this delay time (10s) did not vary in relation to the initial pH. When the NO-³ concentration in the medium was low (100 üM), the presence of relatively high concentrations of Cl- (3 üM), on the one hand, greatly stimulated the maximal alkalinization rates but, on the other, Cl- severely reduced the steady NO-³-dependent rate of alkalinization. The data indicate that Cl- inhibits competitively NO-³ uptake with a Kⁱ of 750 üM. Moreover, high concentrations of NO-³ (above 5 üM) reduced its own maximal, but not the steady, uptake rates. The above results allow us to propose that most of the components of the individual NO-³ and Cl- transport systems are under identical light control and, as the competition data suggest, that these two anions may be taken up by the same transport system.     

The effects of cyclophosphamide on the gonadotrophic cells of the normal rat

The effects of the administration of cyclophosphamide to male rats at doses of 400 mg/m2 for a period of 5 days and of 200 mg/m2 over two five-day cycles interrupted by a 21-day break reveal differences in the ultrastructural morphology of gonadotrophic cells between treated animals and normal controls. The ultrastructural data obtained coincide with the results obtained from the measurement of serum FSH and LH levels, indicating hypofunction. A significant fall was also recorded in blood testosterone levels. The two types of treatment did not give rise to morphological or functional differences.     

Spore coat is altered in modB glycosylation mutants of Dictyostelium discoideum.

The modB mutation eliminates specific carbohydrate epitopes from glycoproteins which are expressed primarily in prespore and spore cells of differentiating Dictyostelium discoideum. Spores formed by the mutant show several phenotypes. Whereas mutant spores germinate efficiently after heat activation, they germinate poorly after urea activation. Following germination, at least one glycosylation-defective glycoprotein is cleaved, and the larger fragment is released in soluble form from the spore coat. However, an earlier difference in the spore coat can be traced back to the nongerminated spore coat, as detected by the elutability of protein from intact spores by chemical extraction. An altered character of the pregermination spore coat is also suggested by increased labeling by a fluorescent lectin which binds to its interior. The findings are consistent with a change in the character of certain molecular contacts leading to altered characteristics of the mutant spore coat, which are specific because they are distinctive from changes observed in another glycosylation mutant which affects a different epitope.