Involved in the regulation of the intracellular balance of NAD and NADP, and is a key enzyme in the biosynthesis of NADP. Catalyzes specifically the phosphorylation on 2'-hydroxyl of the adenosine moiety of NAD to yield NADP (PubMed:22056937, PubMed:30693583). Functions as a growth repressor under light-activated heterotrophic growth conditions and light and dark cycle conditions in the presence of glucose (PubMed:33560438). NADP(H)/NAD(H) maintenance by slr0400 probably plays a significant role in modulating glycolysis and the TCA cycle to repress the growth rate and maintain the photosynthetic capacity (PubMed:33560438). ATP + NAD(+) = ADP + H(+) + NADP(+) Expression decreases after transfer to photoheterotrophic conditions. Insertion mutant shows a slight decrease in NADK activity compared to the wild-type (PubMed:22056937, PubMed:27657983). Under photoheterotrophic conditions, mutation does not affect growth rate (PubMed:22056937, PubMed:30693583). Under photoautotrophic conditions, the growth curves of the wild-type parent and the mutant do not show any differences, but mutant shows large differences in NAD(P)(H) levels along with massive changes in metabolite profile (PubMed:27657983). Mutant exhibits a fast-growth phenotype under light-activated heterotrophic growth conditions and light and dark cycle conditions in the presence of glucose (PubMed:33560438). NADK activity derived from sll1415 makes probably a larger contribution to total cellular NADK activity than activity derived from slr0400 (PubMed:22056937, PubMed:27657983). Slr0400 cannot functionally replace sll1415 under photoheterotrophic conditions (PubMed:30693583). Belongs to the NAD kinase family.