Zeus at HERA II

The ZEUS detector has been upgraded in a number of areas to prepare for the physics opportunities of HERA II. These upgrades, and their physics rationale and promise, are briefly outlined. The measurement of polarisation at HERA II, and its importance for the HERA II physics programme, is also discussed.Comment: 9 pages, 7 figure

Foster, B. B.

https://digitalmaine.com/cw_med_exam_name/1208/thumbnail.jp

An in vitro biochemical investigation into the conformation, binding and E3-ubiquitin ligase activity of mammalian UHRF1 with reconstituted chromatin

In the eukaryotic genome, DNA and histone modifications regulate chromatin function and mediate basic processes such as gene transcription, DNA repair and DNA replication. Maintaining chromatin modifications after DNA replication is essential for chromatin homeostasis, especially for regions of the genome that need to be kept silenced such as repetitive elements. The maintenance DNA methyltransferase, DNMT1, is responsible for ensuring that cytosine methylation at CpG dinucleotides, and thus proper transcriptional programmes, are propagated to the daughter cells. DNMT1 is specifically recruited to newly replicated, hemi-methylated DNA and the E3-ubiquitin ligase UHRF1 (Ubiquitin-like containing PHD- and RING-finger domains protein 1) plays a critical role for this. The mechanisms of the recruitment of DNMT1 to chromatin via UHRF1 are currently an area of active investigation. Several studies using modified nucleosomes, histone peptides and DNA oligonucleotides have identified UHRF1 to bind to hemi-methylated CpG dinucleotides and to histone H3 di- or tri-methylated at Lys-9. Since UHRF1 was also found to interact with DNMT1, it was postulated that UHRF1 acts as an adapter that directly recruits DNMT1 to newly replicated DNA. Additionally, it has recently been reported that the E3-ubiquitin ligase activity of its C-terminal RING-finger is required for the recruitment of DNMT1 to replication forks. Ubiquitylation of either K18 or K23 on histone H3 that is recognised by a ubiquitin-interacting motif within DNMT1 appears to be critical for DNMT1 targeting but the recruitment mechanism has so far not been completely elucidated. This study has investigated the binding and E3-ubiquitin ligase activity of UHRF1 in the context of physiologically relevant chromatin substrates. Using a fully reconstituted system, the chromatin binding and enzymatic activity of UHRF1 and how this is linked to its intra-molecular arrangement have been elucidated. In the context of modified nucleosome substrates, we observe an increase in binding of recombinant UHRF1 in the presence of hemi-methylated DNA whilst with histone H3K9me2/3, only a small increase in binding is detected. We also provide evidence that binding to nucleosome core particles is enhanced by a basic region between the SRA-domain and the RING-finger. This so called polybasic region or PBR has previously been implicated in the regulation of UHRF1 binding to H3K9me2/3 marks. Our findings therefore suggest that binding of UHRF1 to physiological chromatin substrates is more complex than previously thought. In-solution crosslinking/mass spectrometry experiments using the full-length protein confirm that UHRF1 exhibits complex intra-molecular contacts that can potentially regulate its interaction with chromatin or other factors. In addition to reported contacts between the PBR with the Tandem-Tudor domain and between the PHD-finger and the SRA-domain, the UBL-domain also makes extensive contacts to other regions within UHRF1. These appear to be weak and dynamic. Crucially, removal of the UBL-domain does not affect nucleosome binding but does result in a strong reduction in UHRF1 E3-ubiquitin ligase activity. Further experiments suggest that the UBL-domain is involved in establishing the enzyme/substrate complex between the E2-conjugating enzyme and the chromatin substrate and in stimulating the transfer of ubiquitin from the E2~Ub complex to histone H3. In summary, by combining a crosslinking/mass spectrometry approach to interrogate the intra-molecular arrangement of UHRF1 with fully reconstituted enzyme and chromatin-binding assays using physiologically relevant substrates, we have identified a function for the UBL-domain of UHRF1. Our results suggest that the UBL is highly flexible in solution and that it forms transient contacts with other parts of UHRF1 and the E2-conjugating enzyme that are required for the formation of the E2/E3/substrate complex in allosterically activating ubiquitin transfer from the E2~Ub to the histone target substrate. These findings assign, for the first time, a function for the UBL-domain and pave the way for further investigation of the involvement of this domain in the physiological role of UHRF1.Open Acces

Aeromonas proteolyrica bacteria in aerospace environments

Preflight studies on Aeromonas proteolytica are reported to investigate the possibility of genetic alterations resulting in increased proteolysis in spacecraft environments. This organism may be present on human tissue and could pose medical problems if its endopeptidase and a hemolysin were to be produced in ususually high quantities or altered in such a way as to be more effective in their activities. Considered are: (1) Development of a nutrative holding medium for suspension of organisms; (2) the establishment of baseline information for the standardization of the assay for endopeptidase levels and hemolytic titers; (3) formulation of a method by which intracutaneous hemorrhage could be quantitated in guinea pig tissue; and (4) the responses of these organisms to parameters of spaceflight and experimentation

Lymphoid cell kinetics under continuous low dose-rate gamma irradiation: A comparison study

The mechanism of cell proliferation is studied in the lymphoid tissue of the mouse spleen under the stress of continuous irradiation at a dose-rate of 10 roentgens per day for 105 days. Autoradiography and specific labeling with tritiated thymidine were utilized. It was found that at least four compensatory mechanisms maintained a near-steady state of cellular growth: (1) an increase in the proportion of PAS-positive cells which stimulate mitotic activity, (2) maturation arrest of proliferating and differentiating cells which tend to replenish the cells damaged or destroyed by irradiation, (3) an increase in the proportion of cells proliferating, and (4) an increase in the proportion of precursor cells. The results are compared to previous findings observed in the thymus

The effect of continuous low dose-rate gamma irradiation on cell population kinetics of lymphoid tissue

Cellular response and cell population kinetics were studied during lymphopoiesis in the thymus of the mouse under continuous gamma irradiation using autoradiographic techniques and specific labeling with tritiated thymidine. On the basis of tissue weights, it is concluded that the response of both the thymus and spleen to continuous low dose-rate irradiation is multiphasic. That is, alternating periods of steady state growth, followed by collapse, which in turn is followed by another period of homeostasis. Since there are two populations of lymphocytes - short lived and long-lived, it may be that different phases of steady state growth are mediated by different lymphocytes. The spleen is affected to a greater extent with shorter periods of steady-state growth than exhibited by the thymus

Lymphoid cell kinetics under continuous low dose-rate gamma irradiation: A comparison study

A comparison study was conducted of the effects of continuous low dose-rate gamma irradiation on cell population kinetics of lymphoid tissue (white pulp) of the mouse spleen with findings as they relate to the mouse thymus. Experimental techniques employed included autoradiography and specific labeling with tritiated thymidine (TdR-(h-3)). The problem studied involved the mechanism of cell proliferation of lymphoid tissue of the mouse spleen and thymus under the stress of continuous irradiation at a dose rate of 10 roentgens (R) per day for 105 days (15 weeks). The aim was to determine whether or not a steady state or near-steady state of cell population could be established for this period of time, and what compensatory mechanisms of cell population were involved

The effects of spacecraft environments on some hydrolytic enzyme patterns in bacteria

The effects of space flight on the production and characteristics of proteolytic enzymes are studied for a number of bacterial species isolated from crew members and spacecraft. Enzymatic make-up and cultural characteristics of bacteria isolated from spacecraft crew members are determined. The organism Aeromonas proteolytica and the proteolytic enzymes which it produces are used as models for future spacecraft experiments

Annotated SAS Output (ASO)

92 pages, 1 article*Annotated SAS Output (ASO)* (Meredith, Michael P.; Lansky, David M.; Cady, Foster B.) 92 page