Dual Activities of ACC Synthase: Novel Clues Regarding the Molecular Evolution of Acs Genes

Ethylene plays profound roles in plant development. The rate-limiting enzyme of ethylene biosynthesis is 1-aminocyclopropane-1-carboxylate (ACC) synthase (ACS), which is generally believed to be a single-activity enzyme evolving from aspartate aminotransferases. Here, we demonstrate that, in addition to catalyzing the conversion of S-adenosyl-methionine to the ethylene precursor ACC, genuine ACSs widely have Cβ-S lyase activity. Two N-terminal motifs, including a glutamine residue, are essential for conferring ACS activity to ACS-like proteins. Motif and activity analyses of ACS-like proteins from plants at different evolutionary stages suggest that the ACC-dependent pathway is uniquely developed in seed plants. A putative catalytic mechanism for the dual activities of ACSs is proposed on the basis of the crystal structure and biochemical data. These findings not only expand our current understanding of ACS functions but also provide novel insights into the evolutionary origin of ACS genes

Building a Low-Cost Soyuz Simulator to Teach Orbital Navigation

Space flight presents many dynamics unseen in any other domain. As such, they can be often hard to teach, and to have students develop an intuition for, even at a university level. A great benefit of cost efficient low-fidelity simulators is that they can enable users to directly interact with complex situations such as those encountered during space missions, whilst keeping a low learning curve for the user thanks to abstraction or simplification of the situation in question. Simulators can enable their users to start developing an intuitive understanding of complex topics such as orbital rendezvous, spacecraft docking, re-entry and navigation by instruments after only a few hours. This paper presents the design and build process, as well as the use cases of a low-fidelity simulator for the Soyuz TMA spacecraft built at the International Space University for educational purposes. The simulator is designed to provide a simplified and cost-efficient rendition of the on-board instruments and controls in a Soyuz TMA spacecraft descent capsule using commercially available components andallow students to manually perform in-orbit maneuvers that astronauts might perform over the course of a real mission. The paper focuses on design challenges associated with developing a user-friendly simulator without losing important aspects of how the Soyuz is controlled and behaves, discussing design decisions and trade-offs performed. The exact hardware and software architecture used in the final version of the simulator is also detailed.</div

N-Terminus-Mediated Degradation of ACS7 Is Negatively Regulated by Senescence Signaling to Allow Optimal Ethylene Production during Leaf Development in Arabidopsis

Senescence is the final phase of leaf development, characterized by key processes by which resources trapped in deteriorating leaves are degraded and recycled to sustain the growth of newly formed organs. As the gaseous hormone ethylene exerts a profound effect on the progression of leaf senescence, both the optimal timing and amount of its biosynthesis are essential for controlled leaf development. The rate-limiting enzyme that controls ethylene synthesis in higher plants is ACC synthase (ACS). In this study, we evaluated the production of ethylene and revealed an up-regulation of ACS7 during leaf senescence in Arabidopsis. We further showed that the promoter activity of ACS7 was maintained at a relatively high level throughout the whole rosette development process. However, the accumulation level of ACS7 protein was extremely low in the light-grown young seedlings, and it was gradually restored as plants aging. We previously demonstrated that degradation of ACS7 is regulated by its first 14 N-terminal residues, here we compared the phenotypes of transgenic Arabidopsis overexpressing a truncated ACS7 lacking the 14 residues with transgenic plants overexpressing the full-length protein. Results showed that seedlings overexpressing the truncated ACS7 exhibited a senescence phenotype much earlier than their counterparts overexpressing the full-length gene. Fusion of the 14 residues to SSPP, a PP2C-type senescence-suppressed protein phosphatase, effectively rescued the SSPP-induced suppression of rosette growth and development but had no effect on the delayed senescence. This observation further supported that N-terminus-mediated degradation of ACS7 is negatively regulated by leaf senescence signaling. All results of this study therefore suggest that ACS7 is one of the major contributors to the synthesis of ‘senescence ethylene’. And more importantly, the N-terminal 14 residue-mediated degradation of this protein is highly regulated by senescence signaling to enable plants to produce the appropriate levels of ethylene required