In-Space Propulsion: Connectivity to In-Space Fabrication and Repair

The connectivity between new in-space propulsion technologies and the ultimate development of an in-space fabrication and repair infrastructure are described in this Technical Memorandum. A number of advanced in-space propulsion technologies are being developed by NASA, many of which are directly relevant to the establishment of such an in-space infrastructure. These include aerocapture, advanced solar-electric propulsion, solar-thermal propulsion, advanced chemical propulsion, tethers, and solar photon sails. Other, further-term technologies have also been studied to assess their utility to the development of such an infrastructure

Rewriting Baranī? The description of the Delhi Sultanate in the Riḥla of Ibn Baṭṭūṭa/Ibn Djuzayy and the Tārīkh-i Fīrūz Shāhī of Ḍiyā’ al-Dīn Baranī

The Riḥla of the famous Moroccan Ibn Baṭṭūṭa describes his travels, which led him through the whole Islamic world and beyond that to the South Seas and China, in an elaborate and rousing way: different people and their mannerisms, famous metropolises, the flora and fauna of distant territories and, last but not least, his adventures on the road. All this made the Riḥla, besides its being an entertaining and enthralling text, one of the main sources on the relatively poorly documented Islamic World of the 14th century. It retained this status until today. The fact that over the years more and more forgeries and plagiarisms could be proven to Ibn Baṭṭūṭa has not principally altered the approach to regard this text as an authentic travelogue as long as no opposite is definitively proven. This article deals with the issue of what one needs to write a travelogue; this is, besides writing skill and imaginativeness, information. It focuses on the question where this information comes from or, to get to the heart of it, whether one needs to have travelled. The subject of this study is one of the most significant parts of the Riḥla, Ibn Baṭṭūṭa’s description of the Delhi Sultanate. If he probably never was in India, how could he have gained his vast amount of information about this distant region? The answer to this question is, as I think, the Tārīkh-i Fīrūz Shāhī of the Indian court scribe Ḍiyā’ al­-Dīn Baranī

Impaired nucleotide excision repair upon macrophage differentiation is corrected by E1 ubiquitin-activating enzyme

Global nucleotide excision repair is greatly attenuated in terminally differentiated mammalian cells. We observed this phenomenon in human neurons and in macrophages, noting that the transcription-coupled repair pathway remains functional and that there is no significant reduction in levels of excision repair enzymes. We have discovered that ubiquitin-activating enzyme E1 complements the repair deficiency in macrophage extracts, and although there is no reduction in the concentration of E1 upon differentiation, our results indicate a reduction in phosphorylation of E1. In preliminary studies, we have identified the basal transcription factor TFIIH as the potential target for ubiquitination. We suggest that this unusual type of regulation at the level of the E1 enzyme is likely to affect numerous cellular processes and may represent a strategy to coordinate multiple phenotypic changes upon differentiation by using E1 as a “master switch.

Nucleotides Adjacent to the Ligand-Binding Pocket are Linked to Activity Tuning in the Purine Riboswitch

AbstractDirect sensing of intracellular metabolite concentrations by riboswitch RNAs provides an economical and rapid means to maintain metabolic homeostasis. Since many organisms employ the same class of riboswitch to control different genes or transcription units, it is likely that functional variation exists in riboswitches such that activity is tuned to meet cellular needs. Using a bioinformatic approach, we have identified a region of the purine riboswitch aptamer domain that displays conservation patterns linked to riboswitch activity. Aptamer domain compositions within this region can be divided into nine classes that display a spectrum of activities. Naturally occurring compositions in this region favor rapid association rate constants and slow dissociation rate constants for ligand binding. Using X-ray crystallography and chemical probing, we demonstrate that both the free and bound states are influenced by the composition of this region and that modest sequence alterations have a dramatic impact on activity. The introduction of non-natural compositions result in the inability to regulate gene expression in vivo, suggesting that aptamer domain activity is highly plastic and thus readily tunable to meet cellular needs

Structural basis for diversity in the SAM clan of riboswitches

In bacteria, sulfur metabolism is regulated in part by seven known families of riboswitches that bind S-adenosyl-l-methionine (SAM). Direct binding of SAM to these mRNA regulatory elements governs a downstream secondary structural switch that communicates with the transcriptional and/or translational expression machinery. The most widely distributed SAM-binding riboswitches belong to the SAM clan, comprising three families that share a common SAM-binding core but differ radically in their peripheral architecture. Although the structure of the SAM-I member of this clan has been extensively studied, how the alternative peripheral architecture of the other families supports the common SAM-binding core remains unknown. We have therefore solved the X-ray structure of a member of the SAM-I/IV family containing the alternative "PK-2" subdomain shared with the SAM-IV family. This structure reveals that this subdomain forms extensive interactions with the helix housing the SAM-binding pocket, including a highly unusual mode of helix packing in which two helices pack in a perpendicular fashion. Biochemical and genetic analysis of this RNA reveals that SAM binding induces many of these interactions, including stabilization of a pseudoknot that is part of the regulatory switch. Despite strong structural similarity between the cores of SAM-I and SAM-I/IV members, a phylogenetic analysis of sequences does not indicate that they derive from a common ancestor