Structure and mechanism of E. coli RNA 2′,3′-cyclic phosphodiesterase

2H (two-histidine) phosphoesterase enzymes are distributed widely in all domains of life and are implicated in diverse RNA and nucleotide transactions, including the transesterification and hydrolysis of cyclic phosphates. Here we report a biochemical and structural characterization of the Escherichia coli 2H protein YadP, which was identified originally as a reversible transesterifying "nuclease/ligase" at RNA 2',5'-phosphodiesters. We find that YadP is an "end healing" cyclic phosphodiesterase (CPDase) enzyme that hydrolyzes an (HO)RNA>p substrate with a 2',3'-cyclic phosphodiester to a (HO)RNAp product with a 2'-phosphomonoester terminus, without concomitant end joining. Thus we rename this enzyme ThpR (two-histidine 2',3'-cyclic phosphodiesterase acting on RNA). The 2.0 angstrom crystal structure of ThpR in a product complex with 2'-AMP highlights the roles of extended histidine-containing motifs (43)HxTxxF(48) and (125)HxTxxR(130) in the CPDase reaction. His43-N epsilon makes a hydrogen bond with the ribose O3' leaving group, thereby implicating His43 as a general acid catalyst. His125-N epsilon coordinates the O1P oxygen of the AMP 2'-phosphate (inferred from geometry to derive from the attacking water nucleophile), pointing to His125 as a general base catalyst. Arg130 makes bidentate contact with the AMP 2'-phosphate, suggesting a role in transition-state stabilization. Consistent with these inferences, changing His43, His125, or Arg130 to alanine effaced the CPDase activity of ThpR. Phe48 makes a pi-pi stack on the adenine nucleobase. Mutating Phe28 to alanine slowed the CPDase by an order of magnitude. The tertiary structure and extended active site motifs of ThpR are conserved in a subfamily of bacterial and archaeal 2H enzymes.