MOUSE NICOTINAMIDE N-METHYLTRANSFERASE GENE - MOLECULAR-CLONING, STRUCTURAL CHARACTERIZATION, AND CHROMOSOMAL LOCALIZATION

Nicotinamide N-methyltransferase (NNMT) catalyzes the N-methylation of nicotinamide and structurally related compounds. There are large stra in-dependent variations in the expression of NNMT activity in mouse li ver during growth and development, raising the possibility of developm ental regulation of the gene. Therefore, we set out to clone and struc turally characterize the mouse NNMT gene, Nnmt, The gene spanned appro ximately 16 kb and consisted of three exons, 348 bp, 208 bp, and 487 b p in length, with an initial 1228-bp intron and a second intron that w as approximately 14 kb in length. The locations of the splice junction s within the gene were highly conserved compared with those in genes f or structurally related methyltransferase enzymes. The Nnmt gene conta ined no canonical TATA box sequences, but an ''initiator'' (Inr) seque nce was located at the site of transcription initiation as determined by 5' rapid amplification of cDNAs ends. A promoter was located within the initial 750 bp of the 5' flanking region of the gene according to studies of the expression of a reporter gene in HepG2 cells. 5'-Flank ing region sequences for mouse strains with high and low hepatic NNMT activity differed with regard to a series of nucleotide substitutions, insertions, and deletions, with the most striking difference being a 12-bp insertion/deletion. The Nnmt gene mapped to mouse chromosome 9 i n an area of conserved synteny to human chromosome 11q, consistent wit h the localization of the human NNMT gene to 11q23, Cloning and struct ural characterization of the mouse Nnmt gene will make it possible to study molecular genetic mechanisms involved in the expression of this important methyltransferase.