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J. Biol. Chem., Vol. 275, Issue 34, 25900-25906, August 25, 2000

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The Human Homolog of Escherichia coli Orn Degrades Small Single-stranded RNA and DNA Oligomers^*

Lam H. Nguyen, Jan P. Erzberger, Jeffrey Root, and David M. Wilson III^§

From the Molecular and Structural Biology Division, Lawrence Livermore National Laboratory, Livermore, California 94551

We report here the identification of human homologues to the essential Escherichia coli Orn protein and the related yeast mitochondrial DNA-escape pathway regulatory factor Ynt20. The human proteins appear to arise from alternatively spliced transcripts, and are thus identical, except the human Ynt20 equivalent contains an NH₂-terminal extension that possesses a predicted mitochondrial protease cleavage signal. In vitro analysis revealed that the smaller human protein exhibits a 3' to 5' exonuclease activity for small (primarily 5 nucleotides in length) single-stranded RNA and DNA oligomers. We have named this human protein Sfn for small fragment nuclease to reflect its broad substrate range, and have termed the longer protein hSfn. Sfn prefers Mn²⁺ as a metal cofactor and displays a temperature-resistant (to 50 °C) nuclease activity. Kinetic analysis indicates that Sfn exhibits a similar affinity for small RNAs and DNAs (K_m of ~1.5 µM), but degrades small RNAs ~4-fold more efficiently than DNA. Mutation of a conserved aspartate (Asp¹³⁶) to alanine abolishes both nuclease activities of Sfn. Northern blot analysis revealed that a 1-kilobase transcript corresponding to SFN and/or SFN (these mRNAs differ by only two nucleotides) is expressed at varying levels in all fetal and adult human tissues examined. Expressed tag sequence clone analysis found that the two splice variants, SFN to SFN, are present at a ratio of roughly 4 to 1, respectively. The results presented within suggest a role for human Sfn in cellular nucleotide recycling.

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