Biosynthesis of NAD(P) cofactors is of special importance for cyanobacteria due to their role in photosynthesis and respiration. Despite significant progress in understanding NAD(P) biosynthetic machinery in some model organisms, relatively little is known about its implementation in cyanobacteria. Cyanobacteria are among the oldest life forms on earth (64). Their unique role in biogeochemical cycles such as photosynthesis, nitrogen fixation (6, 20) and generation, and homeostasis of oxygenic atmosphere (14) makes this diverse group of bacteria essential for all terrestrial life. Not surprisingly, cyanobacteria attract growing attention in various areas of basic and applied research (11). Recent breakthroughs in genome sequencing opened new opportunities for a fundamental understanding of cyanobacterial physiology and evolution. For example, a comparative genome analysis provided new insights into the photosynthetic machinery of cyanobacteria and related species (12,54,59).Biosynthesis of NAD(P) cofactors is of special importance for cyanobacteria due to their role in photosynthesis and respiration. In addition to its role in innumerable redox reactions, NAD participates as a cosubstrate in a number of metabolic and regulatory processes. Some of these processes, such as NAD-dependent protein deacetylation catalyzed by the CobB/ SIR2 enzyme family (68), are leading to the depletion of the NAD pool. Among other enzymes consuming NAD are a NAD-dependent DNA ligase characteristic of bacteria (73) Although most of the biochemical transformations involved in NAD biosynthesis, salvage, and recycling were analyzed in great detail (for reviews, see references 3, 33, 35, and 48), those studies were historically focused on a few model species. NADmediated links between metabolic and signaling networks recently revealed in eukaryotic cells (16,69,78) triggered a new wave of research in yeast and mammals (for recent reviews, see references 31 and 60). At the same time, relatively little is known about NAD biosynthesis in cyanobacteria. Although many NAD biosynthetic genes are annotated in public archives (e.g., Cyanobase [http://www.kazusa.or.jp/cyanobase/] and KEGG [http://www.genome.ad.jp/kegg/]), some of these annotations are imprecise and have not been consistently projected across all the sequenced cyanobacteria. Moreover, we are not aware of any attempt at metabolic reconstruction of respective pathways in any of the divergent cyanobacterial species. When we began this study, only one of the NAD biosynthetic enzymes, archaea-like nicotinamide mononucleotide (NMN) adenylyltransferase from Synechocystis sp. strain PCC 6803 (encoded by