Table 2 Generation time (minutes) of Escherichia coli strains in different culture media* No. Strain Pathway Deficiency Medium M9 M9 + NA M9 + NAD+ M9 + NAM Expected Observed Expected Observed Aurora Kinase inhibitor Expected Observed Expected Observed 1 BW25113 None + 65.8 + 49.8 + 50.5 + 49.4 2 ΔnadC dn – – + 49.4 + 49.4 + 53.4 3 ΔnadCΔpncA dn, I – – + 50.3 + 49.2 – 380.8 4 ΔnadCΔpncAΔxapA dn, I – – + 49.2 + 50.0 – 620.4 5 ΔnadCΔpncAΔxapA/pBAD-xapA dn, I – NT + NT + + – 376.4 6 ΔnadCΔpncAΔxapA/pBAD-EGFP dn, I – NT + NT + + – 626.8 7 ΔnadCΔpncAΔnadR
dn, I, III – – + 51.1 + NT – – 8 ΔnadCΔpncAΔxapAΔnadR dn, I, III – – + 49.7 + NT – – *Notes: NA, nicotinic acid; NAM, nicotinamide; NAD+, nicotinamide Protein Tyrosine Kinase inhibitor adenine dinucleotide; NT, not tested; –, No proliferation; +, proliferation; dn, de novo NAD+ synthesis; I, NAD+ salvage pathway I; III, NAD+ salvage pathway III. We then generated double-deletion mutant BW25113ΔnadCΔpncA to also interrupt the conversion from NAM to NA in NAD+ salvage pathway I. This mutant was expected to only survive in the absence of NA, but not NAM due to the lack of NAD+ salvage pathway II in E. coli (Figure 1). The growth of BW25113ΔnadCΔpncA YH25448 chemical structure mutant in the absence of NA was confirmed as expected, but we
also unexpectedly observed its survival in M9/NAM medium, albeit with a much slower growth rate (i.e., 380.8 min generation time vs. 53.4 min for BW25113ΔnadC mutant) (Table 2 and Figure 2). This result suggested the presence of another unknown salvage pathway can participate in the conversion of NAM from medium into NAD+. Genetic evidence on the Non-specific serine/threonine protein kinase involvement of xapA in NAD+ salvage pathway The ability for BW25113ΔnadCΔpncA to grow in M9/NAM medium implied a previously undefined enzyme(s) might be involved in feeding NAM into the NAD+ synthesis. The poor efficiency in utilizing NAM was indicative of the presence of an enzyme that might use NAM as an atypical substrate, but the activity was sufficient for
bacterial growth when other NAD+ intermediates were unavailable. Based on the substrate preference of xapA towards purine nucleosides and the fact that its sister enzyme deoD (PNP-I) is able to use NR as a non-typical substrate to form NAM in vitro, we hypothesized that xapA might be a candidate enzyme responsible for converting NAM to NR. To test this hypothesis, we developed three multiple gene deletion mutants, namely, BW25113ΔnadCΔpncAΔxapA, BW25113ΔnadCΔpncAΔnadR, and BW25113ΔnadCΔpncAΔxapAΔnadR (Table 1). Among them, the growth of BW25113ΔnadCΔpncAΔxapA was worse than that of BW25113ΔnadCΔpncA in the M9/NAM medium (i.e., 620.4 min generation time in BW25113ΔnadCΔpncAΔxapA vs. 380.8 min in BW25113ΔnadCΔpncA) (Figure 2 and Table 2). When a complementary plasmid pBAD-xapA (but not the control vector pBAD-EGFP) was reintroduced into this triple-deletion mutant, its growth rate was restored to a similar level of that of BW25113ΔnadCΔpncA (Table 2).