HeTR Portable Space Heater 1500 Watt Forced Air Heater With Ceramic Heater Element And Overheat Protection For Office Home Garage Workshop, Etl Listed
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HeTR Portable Space Heater 1500 Watt Forced Air Heater With Ceramic Heater Element And Overheat Protection For Office Home Garage Workshop, Etl Listed
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In the bistable region the system behaves as a switch that can be either OFF in a vegetative state (lower branch, with a small production of HetR and NtcA) or ON in a heterocyst state (upper branch, with a high production of HetR and NtcA). A sufficient large perturbation may result in the system crossing the manifold of the saddle and falling into the other stable branch of solutions. The distance between the saddle and the nodes determines the size of the perturbation needed to activate or inactivate the system. Du Y, Cai Y, Hou S, Xu X: Identification of the HetR recognition sequence upstream of hetZ in Anabaena sp. strain PCC 7120. J Bacteriol. 2012, 194 (9): 2297-2306. 10.1128/JB.00119-12. Similar to the structures of Fischerella HetR 23, 24, each Anabaena HetR subunit contains three distinct domains: the N-terminal DBD (residues 1–98), the middle flap domain (residues 99–216) and a slightly smaller C-terminal hood domain (residues 217–299) ( Fig. 2a). The DBD has a HTH motif (α4 and α5) that inserts into the major groove of DNA. Notably, the partially palindromic DNA sequence in our structure is exactly the same as the HetR recognition sequence in the hetP promoter 12, whereas the DNA sequence in the previous Fischerella HetR structures is derived from the hetP promoter region but modified to be perfectly palindromic 24. Due to a sequence identity of 90% between Anabaena and Fischerella HetR proteins, the two DNA-complexed structures (PDB 4YRV and 4IZZ) are quite similar to each other with a root-mean-square deviation (RMSD) of 1.0 Å over 462 Cα atoms. In both structures, HetR adopts an active conformation, with the two 33.0 Å-apart HTH motifs perfectly accommodated in a successive DNA major groove. Moreover, the two flap domains in both complex structures adopt the same conformation and orientation ( Fig. 2b), which are stabilized by the duplex DNA via interactions with the two α10 helices. Let us briefly review the previous studies on the mathematical modeling of heterocyst pattern formation. In references [ 13, 14] Rutenberg and coworkers analyzed a model to explain heterocyst patterns by means of the study of cN diffusion along a cyanobacterial filament. On the other hand, Gerdtzen et al. [ 15] modeled cyanobacterial filaments based on a time-discrete dynamical system incorporating the main interactions between the most important proteins that take part in heterocyst formation. Buikema WJ, Haselkorn R: Characterization of a gene controlling heterocyst differentiation in the cyanobacterium Anabaena 7120. Genes Dev. 1991, 5 (2): 321-330. 10.1101/gad.5.2.321.
Our model connects the diffusion of combined nitrogen along the filament with the dynamical properties of the underlying genetic circuit of each single cyanobacterium, capturing both the development of heterocyst patterns and their maintenance. Furthermore, our model shows that noise plays an important role in the onset of differentiation by enabling the development of the characteristic heterocyst patterns for a wide range of model parameters. This reveals that cyanobacteria filaments have developed an efficient response to the noisy conditions that characterize the natural environment. Yoon H (1998) Heterocyst Pattern Formation Controlled by a Diffusible Peptide. Science 282: 935–938. pmid:9794762
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Buikema WJ, Haselkorn R: Expression of the Anabaena hetR gene from a copper-regulated promoter leads to heterocyst differentiation under repressing conditions. Proc Natl Acad Sci U S A. 2001, 98 (5): 2729-2734. 10.1073/pnas.051624898. However, this expression does not behave properly for small concentrations of cN, which are expected under cN deprivation: 2-OG levels would increase without limit. In fact, 2-OG production is controlled by some processes that are not considered in this work and so its value must be limited. We can effectively include such a limiting value by means of a translation on [cN] in Eq. (9) (10) Anabaena (also Nostoc) sp. strain PCC7120, hereafter Anabaena, is a cyanobacterium that fixes atmospheric N 2 in specialized cells called heterocysts. Heterocyst differentiation is regulated by a homodimeric transcription factor, HetR. HetR is expressed at a basal level in all cells but its expression increases in differentiating cells early after nitrogen deprivation. HetR is required for heterocyst development, and therefore nitrogen fixation and diazotrophic growth. Overexpression of HetR leads to multiple contiguous heterocysts (Mch phenotype). HetR binds in vitro to DNA fragments upstream of several genes upregulated in heterocysts, including hetZ, hetP, hepA, patS, pknE, and hetR itself. HetR binds an inverted repeat sequence upstream of a few of these genes; however, HetR binds to promoters that do not contain this sequence, such as the promoter regions for patS and pknE. Results Zhao, M. X. et al. Structural basis for the allosteric control of the global transcription factor NtcA by the nitrogen starvation signal 2-oxoglutarate. Proc. Natl. Acad. Sci. USA 107, 12487–92 (2010). SARS-CoV-2 spike protein plays a key role in mediating viral entry and inducing host immune responses. It can adopt either an open or closed conformation based on the position of its receptor-binding domain (RBD). It is yet unclear what cause these conformational changes or how they influence the spike's functions. Here we show that Lys417 in the RBD plays dual roles in the spike's structure: it stabilizes the closed conformation of the trimeric spike by mediating inter-spike-subunit interactions; it also directly interacts with ACE2 receptor. Hence, a K417V mutation has opposing effects on the spike's function: it opens up the spike for better ACE2 binding while weakening the RBD's direct binding to ACE2. The net outcomes of this mutation are to allow the spike to bind ACE2 with higher probability, mediate viral entry more efficiently, but become more exposed to neutralizing antibodies. Given that residue 417 has been a viral mutational hotspot, SARS-CoV-2 may have been evolving to strike a balance between infection potency and immune evasion, contributing to its pandemic spread.
Citation: Torres-Sánchez A, Gómez-Gardeñes J, Falo F (2015) An Integrative Approach for Modeling and Simulation of Heterocyst Pattern Formation in Cyanobacteria Filaments. PLoS Comput Biol 11(3): Koonin EV, Aravind L: Origin and evolution of eukaryotic apoptosis: the bacterial connection. Cell Death Differ. 2002, 9 (4): 394-404. 10.1038/sj.cdd.4400991. Here, Xu et al. show that HetL cannot diffuse from one cell to the other, and that it binds to HetR at the same place than PatS does. When both PatS and HetL are present, they compete to attach to HetR, which stops PatS from turning off HetR and deactivating the nitrogen-fixing program.Buikema WJ, Haselkorn R: Isolation and complementation of nitrogen fixation mutants of the cyanobacterium Anabaena sp. strain PCC 7120. J Bacteriol. 1991, 173 (6): 1879-1885. Notably, an R223W mutant of HetR was shown to be insensitive in vivo to the overexpressed PatS or HetN 27, indicating that Arg223 might also participate in PatS6 inhibition. Indeed, the R223W mutant has a K d value of 2353 nM towards PatS6 ( Supplementary Fig. 3), indicating a much lower binding affinity compared to the wild-type HetR. In the PatS6−HetR Hood complex structure, the side chain of Arg223 is very close to the electronically complementary C-terminal carboxyl group of PatS6 ( Fig. 3d). Therefore, replacement of the negatively-charged Arg223 with a highly hydrophobic tryptophan should abolish the binding towards PatS6. In addition, ITC assays showed that PatS6 has a much lower binding affinity towards HetR Hood ( K d of 24 μM), compared to that of the full-length HetR with a K d value of 12 nM ( Supplementary Fig. 3). It indicated that the other domains of HetR also influence the binding affinity towards PatS6. The flap domains undergo significant conformational changes upon binding to PatS6 where L n ′ represents the flux of cN from the exterior of the cell. Assuming that the levels of cN relax rapidly we solve Eq. (5) for the steady state. Substituting in Eq. (4) we find:
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