Characterization of the Nicotinamide Adenine Dinucleotide Biosynthesis Pathway and Regulatory Mechanisms in Streptococcus mutans.

NAD and its derivatives, which act as redox coenzymes, are crucial for cellular metabolism and energy production. Nevertheless, the processes by which Streptococcus mutans, a bacterium known for causing dental caries, synthesizes NAD are not well elucidated. Through a genome-wide screen, we identified the nicotinic acid salvage pathway and the evolutionarily incomplete PnuC-NadR pathway involved in NAD biosynthesis in S. mutans UA159. The nicotinic acid pathway is regulated by SmNiaR, a nicotinic acid-responsive transcription regulator featuring an N-terminal DNA-binding winged helix-turn-helix-like domain and a C-terminal 3-histidine domain. Notably, a single-site amino acid substitution at site K97 in SmNiaR can reverse its DNA-binding ability, an effect mediated by acetylation at this site, which impacts the intracellular production of NAD and NADH. Additionally, the deletion of niaR in S. mutans UA159 impaired bacterial proliferation, reduced acid production, and altered biofilm formation, resulting in attenuated virulence in the rat caries model. Conclusively, the regulation of NAD homeostasis via SmNiaR contributes significantly to the cariogenic virulence of S. mutans.