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1.  Rational engineering of sequence specificity in R.MwoI restriction endonuclease 
Nucleic Acids Research  2012;40(17):8579-8592.
R.MwoI is a Type II restriction endonucleases enzyme (REase), which specifically recognizes a palindromic interrupted DNA sequence 5′-GCNNNNNNNGC-3′ (where N indicates any nucleotide), and hydrolyzes the phosphodiester bond in the DNA between the 7th and 8th base in both strands. R.MwoI exhibits remote sequence similarity to R.BglI, a REase with known structure, which recognizes an interrupted palindromic target 5′-GCCNNNNNGGC-3′. A homology model of R.MwoI in complex with DNA was constructed and used to predict functionally important amino acid residues that were subsequently targeted by mutagenesis. The model, together with the supporting experimental data, revealed regions important for recognition of the common bases in DNA sequences recognized by R.BglI and R.MwoI. Based on the bioinformatics analysis, we designed substitutions of the S310 residue in R.MwoI to arginine or glutamic acid, which led to enzyme variants with altered sequence selectivity compared with the wild-type enzyme. The S310R variant of R.MwoI preferred the 5′-GCCNNNNNGGC-3′ sequence as a target, similarly to R.BglI, whereas the S310E variant preferentially cleaved a subset of the MwoI sites, depending on the identity of the 3rd and 9th nucleotide residues. Our results represent a case study of a REase sequence specificity alteration by a single amino acid substitution, based on a theoretical model in the absence of a crystal structure.
PMCID: PMC3458533  PMID: 22735699
2.  MetalionRNA: computational predictor of metal-binding sites in RNA structures 
Bioinformatics  2011;28(2):198-205.
Motivation: Metal ions are essential for the folding of RNA molecules into stable tertiary structures and are often involved in the catalytic activity of ribozymes. However, the positions of metal ions in RNA 3D structures are difficult to determine experimentally. This motivated us to develop a computational predictor of metal ion sites for RNA structures.
Results: We developed a statistical potential for predicting positions of metal ions (magnesium, sodium and potassium), based on the analysis of binding sites in experimentally solved RNA structures. The MetalionRNA program is available as a web server that predicts metal ions for RNA structures submitted by the user.
Availability: The MetalionRNA web server is accessible at
Supplementary information: Supplementary data are available at Bioinformatics online.
PMCID: PMC3259437  PMID: 22110243
3.  The balance between pre- and post-transfer editing in tRNA synthetases 
FEBS letters  2010;584(2):455-459.
The fidelity of tRNA aminoacylation is dependent in part on amino acid editing mechanisms. A hydrolytic activity that clears mischarged tRNAs typically resides in an active site on the tRNA synthetase that is distinct from its synthetic aminoacylation active site. A second pre-transfer editing pathway that hydrolyzes the tRNA synthetase aminoacyl adenylate intermediate can also be activated. Pre- and post-transfer editing activities can co-exist within a single tRNA synthetase resulting in a redundancy of fidelity mechanisms. However, in most cases one pathway appears to dominate, but when compromised, the secondary pathway can be activated to suppress tRNA synthetase infidelities.
PMCID: PMC2859721  PMID: 19941860
Fidelity; Amino acid editing; Protein synthesis; Aminoacylation
4.  In vitro assays for the determination of aminoacyl-tRNA synthetase editing activity 
Methods (San Diego, Calif.)  2008;44(2):119-128.
Aminoacyl-tRNA synthetases are essential enzymes that help to ensure the fidelity of protein translation by accurately aminoacylating (or “charging”) specific tRNA substrates with cognate amino acids. Many synthetases have an additional catalytic activity to confer amino acid editing or proofreading. This activity relieves ambiguities during translation of the genetic code that result from one synthetase activating multiple amino acid substrates. In this review, we describe methods that have been developed for assaying both pre- and post-transfer editing activities. Pre-transfer editing is defined as hydrolysis of a misactivated aminoacyl-adenylate prior to transfer to the tRNA. This reaction has been reported to occur either in the aminoacylation active site or in a separate editing domain. Post-transfer editing refers to the hydrolysis reaction that cleaves the aminoacyl-ester linkage formed between the carbonyl carbon of the amino acid and the 2′ or 3′ hydroxyl group of the ribose on the terminal adenosine. Post-transfer editing takes place in a hydrolytic active site that is distinct from the site of amino acid activation. Here, we focus on methods for determination of steady-state reaction rates using editing assays developed for both classes of synthetases.
PMCID: PMC2270698  PMID: 18241793
5.  Type II restriction endonuclease R.Hpy188I belongs to the GIY-YIG nuclease superfamily, but exhibits an unusual active site 
Catalytic domains of Type II restriction endonucleases (REases) belong to a few unrelated three-dimensional folds. While the PD-(D/E)XK fold is most common among these enzymes, crystal structures have been also determined for single representatives of two other folds: PLD (R.BfiI) and half-pipe (R.PabI). Bioinformatics analyses supported by mutagenesis experiments suggested that some REases belong to the HNH fold (e.g. R.KpnI), and that a small group represented by R.Eco29kI belongs to the GIY-YIG fold. However, for a large fraction of REases with known sequences, the three-dimensional fold and the architecture of the active site remain unknown, mostly due to extreme sequence divergence that hampers detection of homology to enzymes with known folds.
R.Hpy188I is a Type II REase with unknown structure. PSI-BLAST searches of the non-redundant protein sequence database reveal only 1 homolog (R.HpyF17I, with nearly identical amino acid sequence and the same DNA sequence specificity). Standard application of state-of-the-art protein fold-recognition methods failed to predict the relationship of R.Hpy188I to proteins with known structure or to other protein families. In order to increase the amount of evolutionary information in the multiple sequence alignment, we have expanded our sequence database searches to include sequences from metagenomics projects. This search resulted in identification of 23 further members of R.Hpy188I family, both from metagenomics and the non-redundant database. Moreover, fold-recognition analysis of the extended R.Hpy188I family revealed its relationship to the GIY-YIG domain and allowed for computational modeling of the R.Hpy188I structure. Analysis of the R.Hpy188I model in the light of sequence conservation among its homologs revealed an unusual variant of the active site, in which the typical Tyr residue of the YIG half-motif had been substituted by a Lys residue. Moreover, some of its homologs have the otherwise invariant Arg residue in a non-homologous position in sequence that nonetheless allows for spatial conservation of the guanidino group potentially involved in phosphate binding.
The present study eliminates a significant "white spot" on the structural map of REases. It also provides important insight into sequence-structure-function relationships in the GIY-YIG nuclease superfamily. Our results reveal that in the case of proteins with no or few detectable homologs in the standard "non-redundant" database, it is useful to expand this database by adding the metagenomic sequences, which may provide evolutionary linkage to detect more remote homologs.
PMCID: PMC2630997  PMID: 19014591

Results 1-5 (5)