MazG (UniProt Q88MB7, locus PP_1657) of Pseudomonas putida KT2440 is a cytoplasmic, divalent-metal-dependent nucleoside triphosphate pyrophosphohydrolase (NTP-PPase; EC 3.6.1.8) that catalyzes the hydrolysis of nucleoside triphosphates to their corresponding nucleoside monophosphates plus inorganic pyrophosphate (NTP + H₂O → NMP + PPᵢ). It belongs to the MazG-like all-α NTP-PPase superfamily (Pfam PF03819; InterPro IPR011551), a group of "house-cleaning" enzymes whose principal physiological role is to sanitize the cellular nucleotide pool by removing non-canonical and oxidatively damaged (deoxy)nucleoside triphosphates before they can be misincorporated into DNA or RNA. Characterized MazG orthologs hydrolyze all eight canonical ribo- and deoxyribonucleoside triphosphates, with a preference for deoxynucleotides, and additionally destroy mutagenic species such as 8-oxo-dGTP, dUTP, and 2-hydroxy-(iso-)ATP.
No primary experimental study has been published on the P. putida KT2440 protein itself. However, the functional assignment for Q88MB7 rests on an unusually strong chain of homology-based evidence. Global sequence alignment shows Q88MB7 is 56.7 % identical (74.7 % similar) to the biochemically and structurally characterized Escherichia coli MazG (P0AEY3), whose crystal structure and NTPase mechanism were solved by Lee et al. (2008). Q88MB7 is a 277-residue, two-domain (tandem) enzyme with duplicated MazG cores (residues 28–101 and 180–236), and it retains the conserved catalytic E-x-GDLLF metal-coordinating motifs in both lobes. An AlphaFold model confirms this tandem architecture at high confidence (overall mean pLDDT 88.5), with both α-helical catalytic cores modeled at very high confidence and a lower-confidence flexible inter-domain linker.
At the genomic level, PP_1657 lies immediately downstream of relA (PP_1656, the (p)ppGpp synthetase), preserving a relA–mazG synteny seen across many bacteria and pointing to a role in tuning the stringent-response alarmone ppGpp and nucleotide homeostasis. Notably, and unlike E. coli, the P. putida locus lacks an adjacent mazEF toxin–antitoxin module, so the E. coli-specific link between MazG and mazEF-mediated programmed cell death should not be assumed for this organism. In short, MazG in P. putida is best described as a soluble cytoplasmic nucleotide-pool-sanitizing enzyme coupled, through genomic and biochemical context, to the stringent stress response.
Before presenting findings, the identity of the target was verified against the UniProt record, satisfying the mandatory checks in the research brief:
| Attribute | Expected (UniProt Q88MB7) | Verified in this investigation |
|---|---|---|
| Gene symbol | mazG | ✅ Consistent with MazG-family assignment |
| Organism | Pseudomonas putida KT2440 | ✅ Locus PP_1657 confirmed in KT2440 genome |
| EC number | 3.6.1.8 | ✅ Reaction NTP → NMP + PPᵢ confirmed for family |
| Protein family | MazG-like NTP pyrophosphohydrolase | ✅ PF03819 / IPR011551, duplicated cores |
| Key domains | MazG-like N + C (IPR048015, IPR048011) | ✅ Two tandem MazG domains (res 28–101, 180–236) |
Conclusion of verification: The gene symbol mazG is not ambiguous in this case. The literature for MazG-family NTP pyrophosphohydrolases aligns precisely with the UniProt description, domain architecture, and EC number for Q88MB7. The one caveat is that mazG is genomically associated with the mazEF toxin–antitoxin system in E. coli, and some literature conflates MazG with programmed cell death; this investigation explicitly checked and found that the mazEF module is absent adjacent to PP_1657, so that association does not transfer to P. putida. Research therefore proceeded on the correct gene.
The primary, defining function of MazG is the hydrolysis of a nucleoside triphosphate into a nucleoside monophosphate plus inorganic pyrophosphate. This reaction (EC 3.6.1.8) is established by direct biochemical characterization of multiple MazG orthologs. E. coli MazG was shown to hydrolyze all eight of the canonical ribo- and deoxyribonucleoside triphosphates to their respective monophosphates and PPᵢ, with a notable preference for deoxynucleotides (PMID: 12218018). The Thermotoga maritima ortholog (Tm-MazG) similarly "catalyzes the hydrolysis of all eight canonical ribo- and deoxyribonucleoside triphosphates to their corresponding nucleoside monophosphates and PPᵢ," and in a second step hydrolyzes the resultant PPᵢ to Pᵢ, giving it an additional pyrophosphatase activity (PMID: 12657645).
Q88MB7 is assigned to this family and reaction by UniProt (EC 3.6.1.8; MazG-like NTP-PPase superfamily; PF03819 / IPR011551). Because the reaction chemistry and broad substrate range are conserved features of the enzyme family and are supported by the very high sequence identity to characterized enzymes (see Finding 7), the P. putida protein is confidently assigned the same catalytic activity.
Reaction:
NTP + H2O ──MazG (Mg2+/Mn2+)──► NMP + PPi
(dNTP) (dNMP)
Beyond turning over canonical nucleotides, the biologically important role of MazG is to act as a "house-cleaning" enzyme — one that eliminates aberrant nucleotides from the pool so they cannot be incorporated into nucleic acids, where they would be mutagenic or cytotoxic. A structure-guided classification placed dimeric dUTPases, HisE, and MazG in a new superfamily of all-α NTP pyrophosphohydrolases and predicted that "uncharacterized members of this superfamily perform 'house-cleaning' functions by hydrolyzing abnormal NTPs and are functionally analogous to the structurally unrelated hydrolases of the Nudix superfamily" (PMID: 15740738). The same analysis pointed to 2-hydroxy-adenosine (isoguanosine) triphosphate — a product of oxidative damage of ATP — as the most likely substrate for the Sulfolobus MazG active site.
This prediction was confirmed experimentally in mycobacteria: mycobacterial MazG "is a potent NTP-PPase capable of hydrolyzing all canonical (d)NTPs, as well as the mutagenic dUTP and 8-oxo-7,8-dihydro-2'-dGTP" (PMID: 20529853). Because 8-oxo-dGTP and 2-hydroxy-ATP arise from reactive oxygen species and are strongly mutagenic if incorporated into DNA, MazG's ability to hydrolyze them ties the enzyme directly to genome-protection and oxidative-stress defense. This house-cleaning role is the most likely primary physiological function of P. putida MazG.
Crystal structures of MazG orthologs show a conserved all-α architecture built from a characteristic four-helix module. The Bacillus-conserved MazG "assembles into a tetrameric architecture. Each monomer adopts a four-α-helix bundle that accommodates a metal ion using four acidic residues, and presents one putative substrate-binding site" (PMID: 26920050). This confirms two hallmarks of the family: (i) an all-α fold and (ii) a metal-coordinating cluster of acidic residues (glutamates/aspartates) at the active site that positions the catalytic divalent cation (Mg²⁺ or Mn²⁺).
Crucially for Q88MB7, the E. coli enzyme — the closest characterized homolog — is a two-domain monomer: "composed of two similarly folded globular domains in tandem. Among the two putative catalytic domains, only the C-terminal domain has well ordered active sites and exhibits an NTPase activity" (PMID: 18353782). This tandem two-domain organization matches the domain annotation for Q88MB7 exactly (see Finding 5) and indicates that in the P. putida enzyme, catalysis is likely concentrated in the ordered C-terminal MazG core, with the N-terminal core playing a structural/regulatory or accessory role.
MazG's enzymatic activity has a documented downstream consequence for the stringent response. In E. coli, "enzymatic activity of MazG in vivo affects the cellular level of guanosine 3',5'-bispyrophosphate (ppGpp), synthesized by RelA under amino acid starvation. The reduction of ppGpp, caused by MazG, may extend the period of cell survival under nutritional stress" (PMID: 18353782). MazG thereby acts as a counterweight to RelA — trimming the ppGpp alarmone and modulating how long cells persist during starvation.
MazG also physically engages the translation/ribosome-associated GTPase machinery: "we discovered that Era interacts with MazG" — a direct protein–protein interaction with the essential GTPase Era (PMID: 12218018). In E. coli, mazG is "being transcribed in the same polycistronic mRNA with mazEF" (PMID: 16390452), placing it in the operon context of the mazEF toxin–antitoxin/programmed-cell-death module in that organism. Finally, the stress relevance of MazG is functionally demonstrated in mycobacteria, where "deletion of mazG in M. smegmatis rendered the mycobacteria defective in response to oxidative stress" (PMID: 20529853). As a soluble metabolic enzyme acting on cytosolic nucleotide pools, MazG carries out its function in the cytoplasm.
Domain analysis of UniProt Q88MB7 shows a 277-residue protein containing two MazG-like NTP-pyrophosphohydrolase domains (residues 28–101 and 180–236). Pfam PF03819, Gene3D 1.10.287.1080, and SUPFAM SSF101386 are each annotated in duplicate ("2"), and the protein maps to eggNOG COG3956 (MazG family). Sequence analysis identifies the conserved MazG metal-coordinating acidic signature E-x-GD-x-x-F in both domains — E59-LGDLLF in the N-terminal lobe and E203-VGDLLF in the C-terminal lobe — each flanked by additional acidic residues (…ERGDF…, …DGDADALED…) that form the metal-binding cluster.
This tandem two-domain organization mirrors E. coli MazG (a two-domain monomer in which catalytic activity resides in the C-terminal domain; PMID: 18353782) rather than the single-domain minimal MazG proteins such as T. maritima TM0360 (PMID: 25758186) or D. radiodurans DR2231 (PMID: 21733847). This is an important classification: it means Q88MB7's closest functional model is the well-studied E. coli enzyme, whose broad canonical-NTP substrate range and ppGpp-modulating physiology are the most directly transferable.
Genomic-neighborhood analysis (KEGG) shows PP_1657 (mazG) is directly adjacent to PP_1656, annotated as ATP:GTP 3'-pyrophosphotransferase, KEGG orthology K00951 = GTP pyrophosphokinase / (p)ppGpp synthetase RelA (EC 2.7.6.5; UniProt Q88MB8), within the purine-metabolism pathway. The other immediate neighbors — PP_1654 (cysteine synthase), PP_1655 (RlmD rRNA methyltransferase), and PP_1658–1660 (conserved proteins of unknown function) — do not include mazE or mazF toxin–antitoxin genes.
This is a significant organism-specific distinction. In E. coli the chromosomal arrangement is relA–mazEF–mazG, embedding mazG in the programmed-cell-death operon. In P. putida the relA–mazG adjacency is retained but the intervening mazEF module is absent. The conserved relA–mazG linkage strengthens the interpretation that MazG participates in tuning the ppGpp stringent response (consistent with Finding 4), while the missing mazEF module means the E. coli-specific programmed-cell-death connection should not be extrapolated to P. putida.
E. coli: relA ── mazE ── mazF ── mazG (mazG in mazEF operon)
P. putida: relA ── mazG (relA-mazG synteny; no mazEF)
(PP_1656) (PP_1657)
A global Needleman–Wunsch alignment (BLOSUM62) of Q88MB7 (277 aa) against E. coli MazG P0AEY3 (263 aa) yields 148/261 identical positions (56.7 % identity) and 74.7 % similarity across essentially the full length. P0AEY3 is the enzyme whose crystal structure and NTPase mechanism were solved by Lee et al. (2008) — the "MazG-ATP complex structure and subsequent mutagenesis studies explain the peculiar active site environment accommodating all eight canonical NTPs as substrates" (PMID: 18353782).
Both tandem MazG domains are conserved between the two proteins. The N-terminal metal-binding motif GDLLFQVV is identical, and the C-terminal catalytic-domain signature aligns residue-for-residue:
| Region | E. coli MazG (P0AEY3) | P. putida MazG (Q88MB7) |
|---|---|---|
| N-terminal metal motif | GDLLFQVV | GDLLFQVV (identical) |
| C-terminal catalytic signature | LEEE-MGDLLFATVNLARHL | LEDE-VGDLLFAAVNLARHL |
| Downstream MazG signature | ANxKFERRFR | ANxKFERRFR (conserved) |
At 56.7 % identity, well above the ~30–40 % threshold generally accepted for confident transfer of enzyme function and identical reaction chemistry, and with all catalytically essential acidic residues conserved, Q88MB7 can be assigned the same substrate range and mechanism as E. coli MazG with high confidence.
The AlphaFold DB model for Q88MB7 (AF-Q88MB7-F1, v6, 277 residues) has a high overall mean pLDDT of 88.5 (median 91.9); 93.1 % of residues are modeled at confident level (pLDDT > 70) and 61.7 % at very high confidence (> 90). The two MazG-like domains are each predicted with very high confidence — N-terminal domain (res 28–101) mean pLDDT 93.8 and C-terminal domain (res 180–236) mean pLDDT 93.9. In contrast, the inter-domain segment (res 102–179) is lower (mean 79.6, min 50), consistent with a flexible linker, and the extreme C-terminal tail (237–277) is partly disordered (min pLDDT 33).
This structural prediction independently corroborates the two-domain, all-α architecture inferred from sequence, and mirrors the experimentally observed E. coli two-domain fold. The lower-confidence linker is expected for a flexible connector between two rigid catalytic lobes.
Integrating the eight findings yields a coherent model for the role of MazG in P. putida KT2440:
Molecular function. MazG is a Mg²⁺/Mn²⁺-dependent NTP pyrophosphohydrolase that cleaves the α–β phosphoanhydride bond of nucleoside triphosphates, releasing PPᵢ and the nucleoside monophosphate. It is broad-specificity toward all eight canonical (d)NTPs (deoxynucleotide-preferring) but, physiologically, its most important substrates are aberrant nucleotides.
Cellular role — nucleotide-pool sanitation. The enzyme's central job is "house-cleaning": destroying non-canonical / oxidatively damaged nucleotides (8-oxo-dGTP, dUTP, 2-hydroxy-ATP/isoguanosine triphosphate) before DNA/RNA polymerases can incorporate them. By converting these to monophosphates, MazG removes their triphosphate "currency," preventing mutagenic misincorporation and preserving genome integrity — functionally parallel to the Nudix hydrolases (e.g., MutT) but structurally unrelated.
Regulatory coupling — the stringent response. MazG's genomic position immediately downstream of relA and the demonstrated ability of MazG activity to lower ppGpp levels connect nucleotide catabolism to the stringent response. RelA synthesizes the alarmone ppGpp during nutritional/oxidative stress; MazG activity trims ppGpp, modulating the depth and duration of stringent shutdown and, in E. coli, extending survival under starvation. The retention of relA–mazG synteny in P. putida suggests this regulatory partnership is conserved.
Oxidative / nutritional stress
│
▼
ROS ──► damaged (d)NTPs (8-oxo-dGTP, 2-OH-ATP, dUTP)
│ ┌────────────────┐
▼ │ RelA (PP_1656) │──► ppGpp ↑
┌─────────┐ └────────────────┘ │
│ MazG │ hydrolyzes damaged (d)NTPs (genomically │
│(PP_1657)│──► NMP + PPi adjacent) ▼
└─────────┘ │ MazG NTPase ──► ppGpp ↓
│ ▼ (tunes stringent response,
▼ clean nucleotide pool extends survival)
genome protection │
(prevents mutagenic ▼
misincorporation) faithful DNA/RNA synthesis
Localization. As a soluble enzyme acting on cytosolic nucleotide pools, MazG operates in the cytoplasm. There is no signal peptide or transmembrane region implied by its all-α globular fold.
What differs in P. putida vs. E. coli. The E. coli literature ties MazG to mazEF-mediated programmed cell death because of operon co-transcription. In P. putida, the mazEF module is absent adjacent to PP_1657. Therefore, the P. putida enzyme is best interpreted as a nucleotide-sanitizing / stringent-response-modulating enzyme, and claims about programmed cell death should be treated as E. coli-specific and not automatically transferred.
| PMID | Study (organism / system) | How it supports the findings |
|---|---|---|
| 12218018 | E. coli MazG; Era interaction | Defines EC 3.6.1.8 reaction; all 8 canonical (d)NTPs, deoxynucleotide preference; MazG–Era interaction (F1, F4) |
| 12657645 | T. maritima Tm-MazG | Confirms NTP→NMP+PPᵢ for all 8 NTPs; extra pyrophosphatase activity (F1) |
| 15740738 | Superfamily classification | Predicts house-cleaning role; 2-OH-ATP as substrate; all-α superfamily (F2) |
| 20529853 | Mycobacterial MazG | Experimental hydrolysis of dUTP & 8-oxo-dGTP; oxidative-stress defect on deletion (F2, F4) |
| 26920050 | Bacillus MazG crystal structure | Tetrameric all-α fold; metal-coordinating acidic active site (F3) |
| 18353782 | E. coli MazG crystal structure | Two-domain tandem architecture; catalytic C-terminal domain; ppGpp modulation; reference for homology transfer (F3, F4, F7) |
| 16390452 | E. coli mazEF/mazG regulation | mazG co-transcribed with mazEF in E. coli (F4; contrast with F6) |
| 25758186 | T. maritima TM0360 structure | Single-domain minimal MazG; contrast to two-domain Q88MB7 (F5) |
| 21733847 | D. radiodurans DR2231 | dUTP-specific MazG-like house-cleaning enzyme; oxidative-stress framework (F2, F5) |
| 36937295 | Housecleaning MazG structural analysis | Reinforces house-cleaning nomenclature/role for the family (F2) |
| 26075750 | Human DCTPP1 (MazG-like) | Illustrates substrate-specialization within superfamily; tetrameric form (context) |
| 21972224 | E. coli cisplatin stress proteomics | MazG among stress-responsive proteins (context) |
| 33265965 | Oxidative-stress transcriptomics | Broader oxidative-stress-response context (context) |
Concordance and tension. The core reaction and house-cleaning role are supported by concordant biochemistry across at least three organisms (E. coli, T. maritima, mycobacteria) plus structure-based prediction. The main point of tension is the mazEF/programmed-cell-death association, which is documented in E. coli (PMID: 16390452) but does not apply to P. putida because the mazEF module is genomically absent there — an important negative finding that prevents over-interpretation.
Supported (homology + strong cross-species evidence):
- H1 — Q88MB7 is an NTP pyrophosphohydrolase (EC 3.6.1.8) catalyzing NTP → NMP + PPᵢ. Supported.
- H2 — It acts broadly on canonical (d)NTPs with deoxynucleotide preference. Supported.
- H3 — It performs house-cleaning removal of damaged nucleotides (8-oxo-dGTP, dUTP, 2-OH-ATP). Supported.
- H4 — It is a metal-dependent, all-α-helical enzyme with a two-lobed MazG core. Supported.
- H5 — It is cytoplasmic. Supported (inference from soluble metabolic-enzyme role).
Refined / revised for P. putida specifically:
- H6a — PP_1657 is embedded in a mazEF toxin–antitoxin operon (as in E. coli). Refuted by genomic context — no mazE/mazF genes neighbor PP_1657.
- H6b — PP_1657 is functionally linked to ppGpp/stringent-response metabolism. Strengthened — PP_1657 is directly downstream of relA (PP_1656, (p)ppGpp synthetase), conserving E. coli relA–mazG synteny.
- H6c — PP_1657 participates in mazEF-mediated programmed cell death. Unsupported for this strain — the linked toxin–antitoxin module is absent.
| Claim | Evidence type | Confidence |
|---|---|---|
| NTP pyrophosphohydrolase, EC 3.6.1.8 (NTP→NMP+PPᵢ) | Family/EC + ortholog biochemistry + 56.7% id to E. coli MazG | High |
| Broad canonical (d)NTP substrates, dNTP preference | Ortholog biochemistry (E. coli, T. maritima) | High (moderate for exact ranking) |
| House-cleaning of damaged NTPs (8-oxo-dGTP, dUTP, 2-OH-ATP) | Ortholog biochemistry + superfamily prediction | Moderate–High |
| Two-domain, metal-dependent all-α fold | Q88MB7 domains + motifs + AlphaFold (pLDDT 88.5) | High |
| Cytoplasmic localization | Inference (soluble metabolic enzyme) | High |
| Functional link to ppGpp/stringent response | E. coli data + conserved relA–mazG synteny | Moderate |
| NOT in a mazEF operon / not in mazEF-PCD | P. putida genomic context | High (for this strain) |
MazG (Q88MB7 / PP_1657) of Pseudomonas putida KT2440 is, by strong homology and structural inference, a cytoplasmic, divalent-metal-dependent nucleoside triphosphate pyrophosphohydrolase (EC 3.6.1.8) that hydrolyzes NTPs to NMP + PPᵢ, acting on all eight canonical (deoxy)ribonucleoside triphosphates with a deoxynucleotide preference. Its principal biological role is "house-cleaning" sanitation of the nucleotide pool — destroying non-canonical and oxidatively damaged nucleotides such as 8-oxo-dGTP, dUTP, and 2-hydroxy-ATP to prevent their mutagenic incorporation into DNA and RNA — and its conserved genomic linkage downstream of relA implicates it in tuning the ppGpp stringent-response alarmone. Unlike E. coli, it is not embedded in a mazEF toxin–antitoxin operon, so it should not be assumed to drive mazEF-mediated programmed cell death. All conclusions derive from robust protein, genomic, and structural bioinformatics (notably 56.7 % identity to the mechanistically characterized E. coli MazG and a high-confidence AlphaFold two-domain model), as no primary study of the P. putida ortholog itself yet exists.