Crystal Structure Analysis Reveals Functional Flexibility in the Selenocysteine-Specific tRNA from Mouse

Selenocysteine tRNAs (tRNA(Sec)) exhibit a number of unique identity elements that are recognized specifically by proteins of the selenocysteine biosynthetic pathways and decoding machineries. Presently, these identity elements and the mechanisms by which they are interpreted by tRNA(Sec)-interacting factors are incompletely understood.We applied rational mutagenesis to obtain well diffracting crystals of murine tRNA(Sec). tRNA(Sec) lacking the single-stranded 3'-acceptor end ((ΔGCCA)RNA(Sec)) yielded a crystal structure at 2.0 Å resolution. The global structure of (ΔGCCA)RNA(Sec) resembles the structure of human tRNA(Sec) determined at 3.1 Å resolution. Structural comparisons revealed flexible regions in tRNA(Sec) used for induced fit binding to selenophosphate synthetase. Water molecules located in the present structure were involved in the stabilization of two alternative conformations of the anticodon stem-loop. Modeling of a 2'-O-methylated ribose at position U34 of the anticodon loop as found in a sub-population of tRNA(Sec)in vivo showed how this modification favors an anticodon loop conformation that is functional during decoding on the ribosome. Soaking of crystals in Mn(2+)-containing buffer revealed eight potential divalent metal ion binding sites but the located metal ions did not significantly stabilize specific structural features of tRNA(Sec).We provide the most highly resolved structure of a tRNA(Sec) molecule to date and assessed the influence of water molecules and metal ions on the molecule's conformation and dynamics. Our results suggest how conformational changes of tRNA(Sec) support its interaction with proteins

Chemomechanical removal of dental caries in deciduous teeth: Further studies in vitro

Objective: To further investigate the efficacy of N-monochloro-DL-2-aminobutyrate (NMAB) and NMAB containing 2M urea (NMAB-urea) as chemomechanical caries removal reagents in deciduous teeth using standardised lesions and limited applicator pressure. Design: In vitro. Method: Carious dentine was removed from standardised lesions in deciduous teeth using NMAB, NMAB-urea or isotonic saline (control); 50 lesions were studied with each reagent. The surface of the dentine remaining in cavities where complete caries removal was achieved was examined by light and scanning electron microscopy. Main results: NMAB-urea (but not NMAB) gave significantly improved caries removal compared with saline. The dentine surfaces remaining after complete caries removal were irregular and approximately one third were bacterially contaminated. Conclusions: The improved efficacy of NMAB by the addition of urea has been confirmed. Toxicity studies are still necessary prior to clinical use of this reagent. © British Dental Journal 1999.link_to_subscribed_fulltex