Nuclear and mitochondrial uracil-DNA glycosylases are generated by alternative splicing and transcription from different positions in the UNG gene

A distinct nuclear form of human uracil-DNA glycosylase [UNG2, open r eading frame (ORF) 313 amino acid residues] from the UNG gene has bee n identified. UNG2 differs from the previously known form (UNG1, ORF 304 amino acid residues) in the 44 amino acids of the N-terminal sequ ence, which is not necessary for catalytic activity. The rest of the sequence and the catalytic domain, altogether 269 amino acids, are id entical. The alternative N-terminal sequence in UNG2 arises by splici ng of a previously unrecognized exon (exon 1A) into a consensus splic e site after codon 35 in exon 1B (previously designated exon 1). The UNG1 sequence starts at codon 1 in exon 1B and thus has 35 amino acid s not present in UNG2. Coupled transcription/translation in rabbit re ticulocyte lysates demonstrated that both proteins are catalytically active. Similar forms of UNG1 and UNG2 are expressed in mouse which h as an identical organization of the homologous gene. Constructs that express fusion products of UNG1 or UNG2 and green fluorescent protein (EGFP) were used to study the significance of the N-terminal sequenc es in UNG1 and UNG2 for subcellular targeting. After transient transf ection of HeLa cells, the pUNG1-EGFP-N1 product colocalizes with mito chondria, whereas the pUNG2-EGFP-N1 product is targeted exclusively t o nuclei.