FIG. 4.

Inferred pyridine nucleotide salvage cycle encoded by KVP40. The components of the NAD salvage pathway (solid lines) are most related to enzymes of bacteria, none of which have been previously identified in phages. PnuC (CDS215) and NadR (CDS211) are shown as a complex for active transport of nicotinamide mononucleotide (NMN) across the bacterial membrane (hatched bar). Nicotinamide (NAm) is assimilated, but an active transport system has not been described. NadV (CDS264) is a nicotinamide phosphoribosyltransferase (NAmPRT) resembling the enzyme from Haemophilus ducreyi that catalyzes the formation of nicotinamide mononucleotide from nicotinamide. NadR is bifunctional, having an apparent nicotinamide mononucleotide adenylyltransferase domain similar to that of bacterial enzymes. The two-domain Nudix hydrolase (here designated NatV; CDS162) resembles one found in the marine cyanobacterial species Synechocystis, which also has a nicotinamide mononucleotide adenylyltransferase domain. Hydrolysis of NADH (dashed lines) can occur by the NatV Nudix hydrolase and by a KVP40 enzyme (CDS043) that is a Sir2/CobB-like enzyme prevalent in eukaryotes and bacteria (also not previously seen in a phage). See references 37, 47, 49, 50, and 61 for details on the pathway and related enzymes.