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Evidence for a dual functional role of a conserved histidine in RNA · DNA heteroduplex cleavage by human RN ase H1
Author(s) -
Alla Nageswara R.,
Nicholson Allen W.
Publication year - 2012
Publication title -
the febs journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.981
H-Index - 204
eISSN - 1742-4658
pISSN - 1742-464X
DOI - 10.1111/febs.12035
Subject(s) - rna , microbiology and biotechnology , oligonucleotide , rnase h , dna , cleavage (geology) , biochemistry , enzyme , chemistry , ribonuclease , mutant , rnase p , biology , gene , paleontology , fracture (geology)
Ribonuclease H 1 is a conserved enzyme that cleaves the RNA strand of RNA · DNA heteroduplexes and has important functions in the nuclear and mitochondrial compartments. The therapeutic action of antisense oligodeoxynucleotides involves the recruitment of RN ase H 1 to cleave disease‐relevant RNA targets. Recombinant human ( H s) RN ase H 1 was purified from a bacterial expression host, and conditions were identified that provided optimal oligonucleotide‐directed RNA cleavage in vitro . H s‐ RN ase H 1 exhibits optimal catalytic activity in pH 7.5 HEPES buffer and a salt ( KC l) concentration of ~ 100–150 m m . M g 2+ best supports H s‐ RN ase H 1 with an optimal concentration of 10 m m , but at higher concentrations inhibits enzyme activity. M n 2+ and C o 2+ also support catalytic activity, while N i 2+ and Z n 2+ exhibit only modest activities as cofactors. The optimized assay was used to show that an antisense oligonucleotide, added in substoichiometric amounts to initiate RNA cleavage, supports up to 30 rounds of reaction in 30 min. Mutation to alanine of the conserved histidine at position 264 causes an ~ 100‐fold decrease in k cat under multiple‐turnover conditions, but does not alter K m . Under single‐turnover conditions, the H 264 A mutant exhibits a 12‐fold higher exponential time constant for substrate cleavage. The defective activity of the H 264 A mutant is not rescued in either assay condition by higher M g 2+ concentrations. These data implicate the H 264 side chain in phosphodiester hydrolysis as well as in product release, and are consistent with a proposed model in which the H 264 side chain interacts with a divalent metal ion to support catalysis.