Lh. Nguyen et al., The human homolog of Escherichia coli orn degrades small single-stranded RNA and DNA oligomers, J BIOL CHEM, 275(34), 2000, pp. 25900-25906
We report here the identification of human homologues to the essential Esch
erichia coli Orn protein and the related yeast mitochondrial DNA-escape pat
hway regulatory factor Ynt20, The human proteins appear to arise from alter
natively spliced transcripts, and are thus identical, except the human Ynt2
0 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 less than or equal to 5 nucleotides in length) single-stranded RN
A and DNA oligomers, We have named this human protein Sfn for Small fragmen
t nuclease to reflect its broad substrate range, and have termed the longer
protein hSfn alpha. Sfn prefers Mn2+ as a metal cofactor and displays a te
mperature-resistant (to 50 degrees C) nuclease activity. Kinetic analysis i
ndicates that Sfn exhibits a similar affinity for small RNAs and DNAs (K-m
of similar to 1.5 mu M), but degrades small RNAs similar to 4-fold more eff
iciently than DNA, Mutation of a conserved aspartate (Asp(136)) to alanine
abolishes both nuclease activities of Sfn, Northern blot analysis revealed
that a l-kilobase transcript corresponding to SFN and/or SFN alpha (these m
RNAs differ by only two nucleotides) is expressed at varying levels in all
fetal and adult human tissues examined. Expressed tag sequence clone analys
is found that the two splice variants, SFN to SFN alpha, are present at a r
atio of roughly 4 to 1, respectively. The results presented within suggest
a role for human Sfn in cellular nucleotide recycling.