The Bacillus subtilis TatAdCd system exhibits an extreme level of substrate selectivity

The Tat system preferentially transports correctly folded proteins across the bacterial membrane although little is known of the proofreading mechanism. Most research has focused on TatABC systems from Gram-negative bacteria, especially Escherichia coli, and much less is known of the TatAC-type systems from Gram-positive organisms. We have previously shown that the Bacillus subtilis TatAdCd system is functional in an E. coli tat null background and able to transport TorA-GFP and native TorA (TMAO reductase); here, we examined its ability to transport other proteins bearing a TorA signal sequence. We show that whereas E. coli TatABC transports a wide range of biotherapeutics including human growth hormone, interferon ?2b, a VH domain protein and 2 different scFvs, TatAdCd transports the scFvs but completely rejects the other proteins. The system also rejects two native E. coli substrates, NrfC and FhuD. Moreover, we have shown that TatABC will transport a wide range of folded scFv variants with the surface altered to incorporate multiple salt bridges, charged residues (5 glutamate, lysine or arginine), or hydrophobic residues (up to 6 leucines). In contrast, TatAdCd completely rejects many of these variants including those with 5 or 6 added Leu residues. The combined data show that the TatABC and TatAdCd systems have very different substrate selectivities, with the TatAdCd system displaying an extreme level of selectivity when compared to the E. coli system. The data also provide a preliminary suggestion that TatAdCd may not tolerate surface domains with a level of hydrophobicity above a certain threshold

The Advanced Composition Explorer

The Advanced Composition Explorer (ACE) was recently selected as one of two new Explorer‐class missions to be developed for launch during the mid‐1990’s ACE will observe particles of solar, interplanetary, interstellar, and galactic origins, spanning the energy range from that of the solar wind (∼1 keV/nucleon) to galactic cosmic ray energies (several hundred MeV/nucleon). Definitive studies will be made of the abundance of nearly all isotopes from H to Zn (1≤Z≤30), with exploratory isotope studies extending to Zr(Z=40). To accomplish this, the ACE payload includes six high‐resolution spectrometers, each designed to provide the optimum charge, mass, or charge‐state resolution in its particular energy range, and each having a geometry factor optimized for the expected flux levels, so as to provide a collecting power a factor of 10 to 1000 times greater than previous or planned experiments. The payload also includes several instruments of standard design that will monitor solar wind and magnetic field conditions and energetic H, He, and electron fluxes. We summarize here the scientific objectives, instrumentation, spacecraft, and mission approach that were defined for ACE during the Phase‐A study period

Comments on Inan’s Notions of Objectual and Propositional Curiosity

In this paper I comment on Inan’s notions of propositional and objectual curiosity. Even though Inan offers an interesting and intuitive distinction between propositional and objectual curiosity, I want to question two aspects of his theory of curiosity. One aspect concerns his thesis that propositional curiosity is interdependent on epistemic attitudes such as belief, certainty and interest. Another aspect of his theory that I discuss is his thesis that objectual curiosity is not reducible to propositional curiosity. In more detail, in the fi rst part, I start off by explaining what propositional curiosity is according to Inan and I bring up two worries that I call: (i) over-complexity as a result of subjectivity and (ii) overcomplexity as a result of dynamics for the above mentioned epistemic attitudes. Both worries stress the problem of over-complexity of Inan’s theory of propositional curiosity. In the second part, I argue that objectual curiosity is, contrary to Inan’s hypothesis, reducible to propositional curiosity. I further argue that the object of wh- questions that, according to Inan, express objectual curiosity can either be about the truth value of general or singular proposition. In addition, I suggest that only the reading where wh- questions express curiosity in a form of de re reading and have a singular proposition as their content is the one that is compatible with Inan’s notion of objectual curiosity

mRNA localization, reaction centre biogenesis and thylakoid membrane targeting in cyanobacteria

The thylakoid membranes of cyanobacteria form a complex intracellular membrane system with a distinctive proteome. The sites of biogenesis of thylakoid proteins remain uncertain, as do the signals that direct thylakoid membrane-integral proteins to the thylakoids rather than to the plasma membrane. Here, we address these questions by using fluorescence in situ hybridization to probe the subcellular location of messenger RNA molecules encoding core subunits of the photosystems in two cyanobacterial species. These mRNAs cluster at thylakoid surfaces mainly adjacent to the central cytoplasm and the nucleoid, in contrast to mRNAs encoding proteins with other locations. Ribosome association influences the distribution of the photosynthetic mRNAs on the thylakoid surface, but thylakoid affinity is retained in the absence of ribosome association. However, thylakoid association is disrupted in a mutant lacking two mRNA-binding proteins, which probably play roles in targeting photosynthetic proteins to the thylakoid membrane