The Role of Silicate in the Parasitism of Pleurosigma sp. Diatoms by the Thraustochytrid Phycophthorum isakeiti

The recently described thraustochytrid diatom parasite Phycophthorum isakeiti has been isolated from the marine system of northern Norway (Hassett, 2020). Protist P. isakeiti’s ecological role, biodiscovery potential, and interactions with a possible diatom host, Pleurosigma sp. remain largely unexplored. Here I present culturing experiments to test whether P. isakeiti is an obligate parasite. To supplement this analysis, incidence of infection and division was explored in a P. isakeiti-Pleurosigma sp. model system and assessed as a function of time in the presence of standard and reduced silicate conditions. Nutrient analysis and cell counting experiments spanning 15 to 31 days were conducted to investigate rates of free silicate uptake, free nitrate uptake, and the effect of P. isakeiti on rates of Pleurosigma sp. division and infection. Resin and ethyl acetate extractions were performed to characterize and screen for allelopathic chemicals involved in diatom defense or thraustochytrid parasitism. Lastly, bioassays were undertaken to detect whether any primary or secondary metabolites produced during host parasitism had biotechnologically relevant activities. Contrary to my hypothesis that P. isakeiti would be able to grow in the absence of its diatom host due to its presence in highly seasonal, light limited climates like northern Norway, I found no evidence to support a facultative strategy of P. isakeiti in a variety of tested medium. In coculturing experiments, the proportion of host Pleurosigma sp. cells dividing increased over time in the presence of the parasite, P. isakeiti. The silicate was depleted in media to a greater extent in parasitized cultures compared to non-parasitized cultures suggesting greater uptake of silicate in parasitized diatom populations. Two compounds, C17H27NO2 (4.2422 min; 278.20923 m/z) and C23H16O2 (9.2357min, 325.12084 m/z), were detected through liquid chromatography mass spectrophotometry exclusively in Pleurosigma sp. cultures parasitized by P. isakeiti. Limited bioactivity was detected in anti-bacterial assays against gram-positive Staphylococcus aureus and in the inhibition of TNF-a production during the anti-inflammation screening. No bioactivity was observed in the anti-cancer or biofilm assays. Experiments and observations in this thesis characterize the role of silicate in the parasitism of Pleurosigma sp. by P. isakeiti. The present research is multidisciplinary, spanning the fields of ecology and biodiscovery to yield novel, fundamental knowledge on a newly described species, P. isakeiti and to describe the interaction with its host, Pleurosigma sp., an ecologically important diatom species

N-Terminal Ubiquitination of Amyloidogenic Proteins Triggers Removal of Their Oligomers by the Proteasome Holoenzyme.

Aggregation of amyloidogenic proteins is an abnormal biological process implicated in neurodegenerative disorders. Whereas the aggregation process of amyloid-forming proteins has been studied extensively, the mechanism of aggregate removal is poorly understood. We recently demonstrated that proteasomes could fragment filamentous aggregates into smaller entities, restricting aggregate size [1]. Here, we show in vitro that UBE2W can modify the N-terminus of both α-synuclein and a tau tetra-repeat domain with a single ubiquitin. We demonstrate that an engineered N-terminal ubiquitin modification changes the aggregation process of both proteins, resulting in the formation of structurally distinct aggregates. Single-molecule approaches further reveal that the proteasome can target soluble oligomers assembled from ubiquitin-modified proteins independently of its peptidase activity, consistent with our recently reported fibril-fragmenting activity. Based on these results, we propose that proteasomes are able to target oligomers assembled from N-terminally ubiquitinated proteins. Our data suggest a possible disassembly mechanism by which N-terminal ubiquitination and the proteasome may together impede aggregate formation.Wellcome Trus

Mobile Technology as a Leverage Point for the Spread of Permaculture in the Food System

This thesis argues that the current food system is untenable in the long term due to its significant negative impacts on the global ecosystem and society

Components of the ubiquitin-proteasome pathway compete for surfaces on Rad23 family proteins

Background: The delivery of ubiquitinated proteins to the proteasome for degradation is a key step in the regulation of the ubiquitin-proteasome pathway, yet the mechanisms underlying this step are not understood in detail. The Rad23 family of proteins is known to bind ubiquitinated proteins through its two ubiquitin-associated (UBA) domains, and may participate in the delivery of ubiquitinated proteins to the proteasome through docking via the Rad23 ubiquitin-like (UBL) domain. Results: In this study, we investigate how the interaction between the UBL and UBA domains may modulate ubiquitin recognition and the delivery of ubiquitinated proteins to the proteasome by autoinhibition. We have explored a competitive binding model using specific mutations in the UBL domain. Disrupting the intramolecular UBL-UBA domain interactions in HHR23A indeed potentiates ubiquitin-binding. Additionally, the analogous surface on the Rad23 UBL domain overlaps with that required for interaction with both proteasomes and the ubiquitin ligase Ufd2. We have found that mutation of residues on this surface affects the ability of Rad23 to deliver ubiquitinated proteins to the proteasome. Conclusions: We conclude that the competition of ubiquitin-proteasome pathway components for surfaces on Rad23 is important for the role of the Rad23 family proteins in proteasomal targeting

Theory of Significance Testing

Mathematics and Statistic

Observation of extremely slow hole spin relaxation in self-assembled quantum dots

We report the measurement of extremely slow hole spin relaxation dynamics in small ensembles of self-assembled InGaAs quantum dots. Individual spin orientated holes are optically created in the lowest orbital state of each dot and read out after a defined storage time using spin memory devices. The resulting luminescence signal exhibits a pronounced polarization memory effect that vanishes for long storage times. The hole spin relaxation dynamics are measured as a function of external magnetic field and lattice temperature. We show that hole spin relaxation can occur over remarkably long timescales in strongly confined quantum dots (up to ~270 us), as predicted by recent theory. Our findings are supported by calculations that reproduce both the observed magnetic field and temperature dependencies. The results suggest that hole spin relaxation in strongly confined quantum dots is due to spin orbit mediated phonon scattering between Zeeman levels, in marked contrast to higher dimensional nanostructures where it is limited by valence band mixing.Comment: Published by Physical Review