FGF-2 antisense RNA encodes a nuclear protein with MutT-like antimutator activity

Bidirectional transcription of the basic fibroblast growth factor (FGF-2) gene gives rise to multiple polyadenylated sense mRNAs and a unique 1.5 kb antisense transcript (FGF-AS) which is complementary to the 3'-untranslated region of the FGF-2 mRNA. The rat FGF-AS cDNA encodes a novel 35 kDa nuclear protein (GFG) with homology to the MutT family of antimutator NTPases. Antibodies against the deduced amino acid sequence of GFG detected intense immunoreactivity in the nuclei of adult rat hepatocytes. Subcellular fractionation and Western blotting confirmed the presence of a 35 kDa immunoreactive protein in the nuclear fraction and, to a lesser extent, in the mitochondrial fractions of rat liver homogenates. Recombinant GFG suppressed the spontaneous mutation rate of MutT-deficient E. coli in a complementation assay. In-frame deletion of the 53 amino acids encompassing the MutT domain eliminated this activity, confirming the catalytic function of this region in the FGF antisense gene product. These findings demonstrate for the first time that the FGF-AS transcript encodes a functional nuclear protein with MutT-related enzymatic activity.