The gene for human nuclear NAD+ ADP-ribosyltransferase [NAD+:poly(adenosine diphosphate D-ribose) ADP-D-ribosetransferase, EC 2.4.2.30; pADPRT] was localized to chromosome 1 at q41-q42 by in situ hybridization with a pADPRT-specific cDNA probe. Expression of a pADPRT cDNA under control of the lac promoter in Escherichia coli induces the synthesis of a group of related proteins that were immunoreactive with pADPRT antibody and that had catalytic properties very similar to those of the human enzyme. Purification of this enzymatic activity was performed essentially as described for the human enzyme. The Km, pH optimum, optimal reaction temperature, and inhibition by 3-aminobenzamide and 3-methoxybenzamide were found to be similar for the recombinant and the human enzymes. The purified recombinant enzyme consists of two major proteins of Mr 99,000 and Mr 89,000. Both proteins show pADPRT activity in activity gel analysis with [32P]NAD+ as substrate. Microsequencing of these two proteins isolated by denaturing gel electrophoresis and deletion mutagenesis of the pADPRT expression plasmid shows that the Mr 99,000 and Mr 89,000 proteins derive from initiation of translation at internal translational start signals located within the pADPRT cDNA.