C-terminal domain of Escherichia coli uncharacterized protein yjhR and similar proteins
C-terminal domain of Escherichia coli uncharacterized protein yjhR, encoded by the o338 gene, and similar proteins. Although the biological function of yjhR remains unknown, it shows sequence similarity to the C-terminal portions of superfamily I DNA and RNA helicases, which are ubiquitous enzymes mediating ATP-dependent unwinding of DNA and RNA duplexes, and play essential roles in gene replication and expression. Moreover, The C-termini of yjhR and similar proteins contain one HKD motif (H-x-K-x(4)-D, where x represents any amino acid residue) that characterizes the phospholipase D (PLD) superfamily. The PLDc-like domain of yjhR is similar to bacterial endonucleases, Nuc and BfiI, both of which have only one copy of the HKD motif per chain. They function as homodimers, with a single active site at the dimer interface containing the HKD motifs from both subunits. They utilize a two-step mechanism to cleave phosphodiester bonds. Upon substrate binding, the bond is first attacked by a histidine residue from one HKD motif to form a covalent phosphohistidine intermediate, which is then hydrolyzed by water with the aid of a second histidine residue from the other HKD motif in the opposite subunit.
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