Nicotinamide adenine dinucleotide (NAD) is an essential cofactor for cellular redox reactions and can act as an important substrate in numerous biological processes. Bacteria maintain its total NADH/NAD+ intracellular pool by synthesizing NAD through the de novo pathway and the pyridine nucleotide salvage pathway. The salvage pathway recycles intracellular NAD breakdown products and preformed pyridine compounds from the environment, such as nicotinic acid. The enzyme nicotinic acid phosphoribosyltransferase, encoded by the pncB gene, catalyzes the first reaction in the NAD salvage I pathway which involves nicotinamide deaminase (pncA), and nicotinate phosphoribosyltransferase (pncB ). Nicotinate phosphoribosyltransferase catalyzes the formation of nicotinate mononucleotide, a direct precursor of NAD, from nicotinic acid. This reaction is believed to be the rate-limiting step in the NAD salvage pathway. The Crystal Structure of Nicotinate Phosphoribosyltransferase from Yersinia pestis was solved at 1.9A. The structure reveals a possible three-domain alpha-beta fold and represents dimer.