Evidence for the transport of manganous ion against an activity gradient by mitochondria

Mn²⁺ taken up by mitochondria through the often discussed energy dependent process has been previously shown to be separable by EPR spectroscopy into two spectral fractions (T. E. Gunter and J. S. Puskin, Biophys. J., 12 (1972) 625). One of these spectral fractions shows the characteristics of spin exchange. The other fraction, comprising roughly 10 % of the spectrally observed Mn²⁺, shows a hyperfine sextet.

Evidence is presented supporting the view that under conditions of maximum uptake of Mn²⁺ by the mitochondria in the absence of exogenous phosphate, the bulk of the hyperfine sextet fraction is in the Mn²⁺(H₂O)₆ form.

Spectral data is then used to show that if this view is correct, this fraction is found within mitochondria under conditions where it can be shown to have been accumulated against an activity gradient.

Spectral line width data for this fraction is interpreted in such a way as to provide an upper limit to intramitochondrial local viscosity where this hexahydrate form of manganese is present. This upper limit is approximately 1.5 cP at 37 °C and 0.25 M external osmolarity for example.     

Structural and functional insights into the fly microRNA biogenesis factor Loquacious

In the microRNA (miRNA) pathway, Dicer processes precursors to mature miRNAs. For efficient processing, double-stranded RNA-binding proteins support Dicer proteins. In flies, Loquacious (Loqs) interacts with Dicer1 (dmDcr1) to facilitate miRNA processing. Here, we have solved the structure of the third double-stranded RNA-binding domain (dsRBD) of Loqs and define specific structural elements that interact with dmDcr1. In addition, we show that the linker preceding dsRBD3 contributes significantly to dmDcr1 binding. Furthermore, our structural work demonstrates that the third dsRBD of Loqs forms homodimers. Mutations in the dimerization interface abrogate dmDcr1 interaction. Loqs, however, binds to dmDcr1 as a monomer using the identified dimerization surface, which suggests that Loqs might form dimers under conditions where dmDcr1 is absent or not accessible. Since critical sequence elements are conserved, we suggest that dimerization might be a general feature of dsRBD proteins in gene silencing.     

Effect of u.v.-B irradiance on the response of 15N-nitrate uptake of Lauderia annulate and Synedra planctonica

The marine diatoms Lauderia annulata and Synedra planctonicaharvested during exponential growth were exposed to differentdoses of u.v.-B (286, 439 and 710 J m^–2 d^–1) for2 days. Uptake of ¹⁵N-nitrate was estimated before, during andafter u.v.-B radiation over 2 days. Exposure to high levelsof u.v.-B (710 J m^–2 d^–1) caused irreversible damageat the second daily irradiance. Lauderia cells were less affectedby u.v.-B stress than Synedra cells. ¹⁵N-nitrate uptake wasreduced under u.v.-B irradiance but could be reactivated within1 day following exposure to a low dose (286 J m^–2 d^–1).Higher levels of u.v.-B radiation (710 J m^–2 d^–1)led to irreversible damage. The pattern of ¹⁵N-incorporationinto several amino acids of Lauderia varied after 2 days ofu.v.-B radiation. ¹⁵N enrichment of glutamine increased markedlyafter u.v.-B stress (717 J m^–2 d^–1) whereas ^I5Nexcess of aspartic acid was significantly reduced. Results arediscussed with reference to the u.v.-B damage of the nitratetransport system.