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Example sentences for: rdrps
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The only proteins with evolutionarily conserved DxDGD motifs that were detected in these searches were the RDRPs, the β' subunit of DDRPs, the integrin calcium-binding module, and the EF-hand domain (data not shown).
The RDRPs of RNA viruses define one major lineage of nucleic acid polymerases, which additionally includes reverse transcriptases, archaeo-eukaryotic DNA polymerases, and nucleotide cyclases [ 8 9 10 11 12 13 ] . The DNA-dependent RNA polymerase of certain bacteriophages, such as T7, and the archaeo-eukaryotic primase (also detected in some bacteria) are divergent derivatives of the same fold [ 11 14 ] . The core catalytic domain of all these enzymes, the so-called "palm" domain, has an RNA-recognition motif (RRM)-like fold with strategically placed metal-coordinating residues, which form the active site [ 11 15 16 ] . In contrast, bacterial DnaG-type primases (also present in archaea and some eukaryotes) contain a polymerase domain of the Rossmann-like TOPRIM fold, which is shared with topoisomerases and OLD-family nucleases [ 17 18 19 ] . The recently solved structures of the DDRPs from yeast and the thermophilic bacterium Thermus thermophilus indicate that the β' subunit (according to the subunit nomenclature of Escherichia coli DDRP, which we hereinafter employ to designate all orthologs of the respective E. coli subunits) of these enzymes defines another distinct catalytic scaffold, which is unrelated to any of the above template-dependent RNA polymerases [ 20 21 22 23 24 ] . Additionally, the structural and evolutionary affinities of two other template-dependent RNA polymerases, namely RDRPs involved in PTGS [ 25 26 27 ] and primases of herpesviruses [ 28 ] , remain obscure.
Closely spaced acidic residues that coordinate divalent cations are characteristic of the active sites of most nucleic acid polymerases, in spite of the fact that they belong to several unrelated structural folds [ 9 13 14 17 28 29 59 ] . Given that the DbDGD motif is the only set of closely spaced acidic residues shared by the RDRPs and the YRHs, it is likely to form part of the nucleotidyltransferase active site of these enzymes.
The high-scoring segment pairs (HSPs) detected in these searches aligned the highly conserved region of the RDRPs between the predicted strands 18 and 20, including the DbDGD motif, with the portion of the DDRP β' subunit sequence, which contains the metal-chelating active site, with a similar conserved motif, DxDGD.
In contrast to the ubiquitous DDRP, cellular RDRPs that are involved in PTGS so far have been detected only in eukaryotes [ 1 26 ] . The PTGS phenomenon covers a variety of complementarity-dependent silencing pathways, such as RNA interference (RNAi) in animals and slime mold, co-suppression (silencing of transgene and the corresponding endogenous genes) and virus gene resistance in plants, and quelling in fungi, all of which share a common mechanism of RNA turnover [ 28 37 38 39 40 ] . Essentially, double-stranded (ds) RNA, which is formed in these processes, triggers the activation of a sequence-specific RNA degradation system, which targets homologous RNAs [ 41 42 ] . The dsRNA is broken down by the Dicer enzyme into 21-25 nucleotide (nt) fragments called small interfering RNA or siRNAs [ 43 44 45 ] . The siRNAs subsequently associate with a complex of proteins called RISC and target homologous RNA by serving as guides for multiple rounds of RNA cleavage [ 45 46 ] .
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