Silicon is beneficial to plant growth and helps plants to overcome abiotic and biotic stresses by preventing lodging (falling over) and increasing resistance to pests and diseases, as well as other stresses. Silicon is essential for high and sustainable production of rice, but the molecular mechanism responsible for the uptake of silicon is unknown. Here we describe the Low silicon rice 1 (Lsi1) gene, which controls silicon accumulation in rice, a typical silicon-accumulating plant. This gene belongs to the aquaporin family and is constitutively expressed in the roots. Lsi1 is localized on the plasma membrane of the distal side of both exodermis and endodermis cells, where casparian strips are located. Suppression of Lsi1 expression resulted in reduced silicon uptake. Furthermore, expression of Lsi1 in Xenopus oocytes showed transport activity for silicon only. The identification of a silicon transporter provides both an insight into the silicon uptake system in plants, and a new strategy for producing crops with high resistance to multiple stresses by genetic modification of the root's silicon uptake capacity.
Structure-activity correlation experiments demonstrated that the fine structure of 3-oxo-C 12 -HSL, the HSL backbone, and side chain length are required for maximal activity. These data suggest that Pseudomonas 3-oxo-C 12 -HSL specifically promotes induction of apoptosis, which may be associated with 3-oxo-C 12 -HSL-induced cytotoxicity in macrophages and neutrophils. Our data suggest that the quorum-sensing molecule 3-oxo-C 12 -HSL has critical roles in the pathogenesis of P. aeruginosa infection, not only in the induction of bacterial virulence factors but also in the modulation of host responses.
Escherichia coli TUH12191, which is resistant to piperacillin, cefazolin, cefotiam, ceftizoxime, cefuzonam, and aztreonam but is susceptible to cefoxitin, latamoxef, flomoxef, and imipenem, was isolated from the urine of a patient treated with -lactam antibiotics. The -lactamase (Toho-1) purified from the bacteria had a pI of 7.8, had a molecular weight of about 29,000, and hydrolyzed -lactam antibiotics such as penicillin G, ampicillin, oxacillin, carbenicillin, piperacillin, cephalothin, cephaloridine, cefoxitin, cefotaxime, ceftazidime, and aztreonam. Toho-1 was markedly inhibited by -lactamase inhibitors such as clavulanic acid and tazobactam. Resistance to -lactams, streptomycin, spectinomycin, sulfamethoxazole, and trimethoprim was transferred by conjugational transfer from E. coli TUH12191 to E. coli ML4903, and the transferred plasmid was about 58 kbp, belonging to incompatibility group M. The cefotaxime resistance gene for Toho-1 was subcloned from the 58-kbp plasmid by transformation of E. coli MV1184. KTG). Toho-1 was about 83% homologous to the -lactamase mediated by the chromosome of K. oxytoca D488 and the -lactamase mediated by the plasmid of E. coli MEN-1. Therefore, the newly isolated -lactamase Toho-1 produced by E. coli TUH12191 is similar to -lactamases produced by K. oxytoca D488, K. oxytoca E23004, and E. coli MEN-1 rather than to mutants of TEM or SHV enzymes. Toho-1 has shown the highest degree of similarity to K. oxytoca class A -lactamase. Detailed comparison of Toho-1 with other -lactamases implied that replacement of Asn-276 by Arg with the concomitant substitution of Thr for Arg-244 is an important mutation in the extension of the substrate specificity.Expanded-spectrum cephalosporins have chemical structures which confer stability to many -lactamases from gram-negative bacteria. However, many members of the family Enterobacteriaceae other than Escherichia coli developed resistance to the expanded-spectrum cephems (40). The primary mechanism of this resistance was demonstrated to be excessive production of a chromosomal -lactamase (AmpC) (23). However, bacteria that show resistance mediated by other -lactamases appeared in 1984 (8). Species of the Enterobacteriaceae such as Klebsiella pneumoniae, Klebsiella oxytoca, and E. coli acquired resistance against expanded-spectrum cephem antibiotics by producing extended-spectrum -lactamase. The extended spectrum of the -lactamase was often acquired by the variation of -lactamase genes on transmissible plasmids (43,44). Under the influence of antimicrobial agents, bacteria producing primarily TEM-type or SHV-type -lactamases developed point mutations in structural genes which served to extend the substrate specificity of the enzymes (44). These TEM-type and SHV-type -lactamases show about 65% amino acid sequence homology, with isoelectric points of 5.5 to 6.3 and 7.0 to 8.2, respectively (8, 9).
One of the most distinctive features of human sweet taste perception is its broad tuning to chemically diverse compounds ranging from low-molecular-weight sweeteners to sweet-tasting proteins. Many reports suggest that the human sweet taste receptor (hT1R2–hT1R3), a heteromeric complex composed of T1R2 and T1R3 subunits belonging to the class C G protein–coupled receptor family, has multiple binding sites for these sweeteners. However, it remains unclear how the same receptor recognizes such diverse structures. Here we aim to characterize the modes of binding between hT1R2–hT1R3 and low-molecular-weight sweet compounds by functional analysis of a series of site-directed mutants and by molecular modeling–based docking simulation at the binding pocket formed on the large extracellular amino-terminal domain (ATD) of hT1R2. We successfully determined the amino acid residues responsible for binding to sweeteners in the cleft of hT1R2 ATD. Our results suggest that individual ligands have sets of specific residues for binding in correspondence with the chemical structures and other residues responsible for interacting with multiple ligands.
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