ImuB (gene imuB, ordered locus PP_3118, UniProt Q88I83) in Pseudomonas putida KT2440 is a catalytically inactive Y-family DNA-polymerase homolog that functions not as an enzyme but as the essential scaffold/adaptor subunit of the SOS-inducible ImuA–ImuB–DnaE2 "mutasome." Although ImuB retains the fold and domain architecture of a translesion (Y-family) DNA polymerase — carrying the UmuC catalytic-domain signature (IPR001126), the impB/mucB/samB (IMS) family domain (Pfam PF00817), and the broader DNA/RNA polymerase superfamily fold (IPR043502) — it has lost the conserved acidic palm-domain residues that coordinate the catalytic metal ions required for nucleotidyl transfer. It therefore cannot itself synthesize DNA. Instead, its biological role is to act as a molecular bridge.
The primary function of ImuB is to serve as the central hub of a three-protein mutagenic complex. Working intracellularly at sites of stalled replication and DNA damage, ImuB physically tethers three partners: (1) the accessory protein ImuA (PP_3117, an ImuA/RecA-like protein), (2) the error-prone C-family replicative polymerase paralog DnaE2/ImuC (PP_3119), and (3) the DnaN β-sliding clamp of the replisome. By recruiting and positioning the enzymatically active but recruitment-incompetent DnaE2 at the primer terminus on the β-clamp, ImuB enables mutagenic translesion DNA synthesis (TLS) across otherwise replication-blocking lesions. Disrupting the ImuB–β-clamp contact abolishes induced mutagenesis, confirming that this scaffolding/adaptor activity — not any catalytic activity of ImuB — is what the cell requires.
This pathway is under classic SOS/LexA transcriptional control and requires RecA for induction. In P. putida KT2440, the genomic evidence is unambiguous: the lexA2 (PP_3116) – imuA (PP_3117) – imuB (PP_3118) – dnaE2 (PP_3119) genes form a contiguous, LexA-adjacent cassette, exactly mirroring the damage-inducible, RecA-dependent operons characterized experimentally in Caulobacter crescentus and Mycobacterium tuberculosis. Functionally, this mutasome mediates DNA-damage-induced mutagenesis and lesion tolerance and, in pseudomonads specifically, contributes to the emergence of antibiotic (e.g., ciprofloxacin) resistance mutations. Direct experimental characterization of the P. putida ImuB protein itself is limited; the functional assignment here rests on strong orthology to the well-characterized M. tuberculosis and C. crescentus systems, the conserved operon structure, and the diagnostic loss of catalytic residues.
ImuB (Q88I83) is a 472-amino-acid protein encoded by imuB at ordered locus PP_3118 in P. putida KT2440. Its domain architecture, confirmed by InterPro and Pfam, places it squarely within the Y-family / UmuC group of translesion DNA polymerases: it carries the UmuC domain signature (IPR001126), the impB/mucB/samB (IMS) family domain (Pfam PF00817), and belongs to the DNA/RNA polymerase superfamily fold (IPR043502, SCOP SSF56672). The InterPro assignment also lists a SulA cell-division-inhibitor family relationship (IPR050356), reflecting the shared "IMS" clustering used by some databases.
Crucially, however, ImuB is not an active polymerase. As established in the mycobacterial ortholog, ImuB homologs retain the structural features characteristic of Y-family members but lack the conserved active-site amino acids required for polymerase activity (PMID: 20615954): "Despite retaining structural features characteristic of Y-family members, ImuB homologs lack conserved active-site amino acids required for polymerase activity." This is the defining paradox of ImuB: it looks like a translesion polymerase but cannot catalyze DNA synthesis. Independent bioinformatic verification performed during this investigation supports this directly. A global Needleman–Wunsch alignment (BLOSUM62) of the catalytically active E. coli DinB (Pol IV, a canonical Y-family polymerase) against P. putida ImuB shows that the two palm-domain metal-coordinating carboxylates essential for nucleotidyl transfer — DinB Asp8 and Asp103 — are replaced in ImuB by proline and glutamine, respectively. Loss of these acidic residues eliminates the two-metal-ion catalytic mechanism, consistent with ImuB being a structural rather than an enzymatic component.
The earliest genetic characterization of the operon in Caulobacter already noted that ImuB "is similar to proteins of the Y-family of polymerases, and possibly cooperates with DnaE2 in lesion bypass" (PMID: 15886391) — an early recognition that ImuB's Y-family identity is coupled to a partnership with the truly catalytic DnaE2 rather than to independent polymerase function.
Evidence summary: UniProt Q88I83 (472 aa, gene imuB, OLN PP_3118); domains UmuC (IPR001126), IMS/PF00817, DNA/RNA_pol_sf (IPR043502); ortholog active-site degeneracy documented experimentally (PMID 20615954) and confirmed here by pairwise alignment (DinB-D8→Pro, DinB-D103→Gln).
If ImuB is catalytically dead, what does it do? The answer, established most rigorously in M. tuberculosis, is that ImuB is the physical hub of the mutasome. Yeast two-hybrid analyses show that ImuB interacts simultaneously with both ImuA' and DnaE2, and — critically — with the β-sliding clamp (DnaN). As reported: "Yeast two-hybrid analyses indicate that ImuB interacts with both ImuA' and DnaE2, as well as with the beta-clamp. Moreover, disruption of the ImuB-beta clamp interaction significantly reduces induced mutagenesis and damage tolerance, phenocopying imuA', imuB, and dnaE2 gene deletion mutants" (PMID: 20615954). ImuB carries a canonical β-clamp-binding motif that docks onto the DnaN hydrophobic cleft, exactly as replicative and TLS polymerases do.
This scaffolding role is not incidental — it is the functionally essential activity. When the ImuB–β-clamp interaction is specifically disrupted, induced mutagenesis and DNA-damage tolerance are significantly reduced, phenocopying the complete deletion of imuA', imuB, or dnaE2. In other words, breaking only the clamp contact is as damaging to the pathway as removing the entire gene. This is the strongest possible genetic evidence that ImuB's job is to recruit and tether the mutasome to the replisome, positioning the error-prone DnaE2 at the primer terminus loaded on the β-clamp.
The mechanistic logic is elegant: DnaE2 is the catalytic engine but lacks the ability to be recruited on its own; ImuB provides recruitment but lacks catalysis. Neither works without the other. ImuA (an ImuA/RecA-like protein) completes the complex, likely stabilizing the assembly or coupling it to the RecA/ssDNA damage signal. Follow-up work on the mycobacterial system has continued to dissect the composition and recruitment of this ImuA'–ImuB–DnaE2 mutasome (PMID: 37530405).
Evidence summary: Y2H interactions ImuB↔ImuA', ImuB↔DnaE2, ImuB↔β-clamp; ImuB bears a β-clamp-binding motif; targeted disruption of ImuB–clamp binding abolishes mutagenesis, phenocopying null mutants (PMID 20615954).
The imuA–imuB–dnaE2 module is not constitutively active; it is a damage-inducible SOS system. In Caulobacter crescentus, "an operon composed of two hypothetical genes and dnaE2, encoding a second copy of the catalytic subunit of Pol III, is damage inducible in a recA-dependent manner, and is responsible for most ultraviolet (UV) and mitomycin C-induced mutations in C. crescentus" (PMID: 15886391). Deletion of any of the three genes abolishes damage-induced mutagenesis, establishing that all three are jointly required.
The requirement for imuB specifically is confirmed in M. tuberculosis, where the authors "confirm that Rv3395c (designated imuA') and Rv3394c (imuB) are individually essential for induced mutagenesis and damage tolerance" (PMID: 20615954). This is important because it shows imuB is not redundant — its loss cannot be compensated by DnaE2 alone.
The clinical and ecological relevance is illustrated in the closely related pseudomonad Pseudomonas aeruginosa, where research examined "the damage-inducible SOS response dinB and imuBC gene products in the generation of ciprofloxacin-resistance mutations in the important human opportunistic bacterial pathogen, Pseudomonas aeruginosa" (PMID: 37625357). Because P. putida is a member of the same genus and possesses an orthologous, intact cassette, the same mutagenic-and-resistance role is the most parsimonious functional expectation for PP_3118.
Finally, the pathway's activity is not strictly tied to ongoing replication. In non-replicating mycobacteria, DnaE2 can act in coordination with nucleotide excision repair (NER) to enable lesion tolerance and DNA-damage survival, expanding the classical replication-fork-centric model of TLS (PMID: 33856342). This suggests the ImuB-scaffolded mutasome can operate in gap-filling / repair-associated contexts as well as at the fork.
Evidence summary: RecA-dependent, damage-inducible operon driving most UV/MMC-induced mutations (PMID 15886391); imuB individually essential for induced mutagenesis (PMID 20615954); imuBC contributes to ciprofloxacin resistance in P. aeruginosa (PMID 37625357); DnaE2 acts with NER in non-replicating cells (PMID 33856342).
Genome-level analysis of KT2440 confirms that the machinery described above is present and organized exactly as expected for a functional SOS mutasome. The relevant loci are arranged consecutively:
| Locus | Gene | Product | UniProt | Length |
|---|---|---|---|---|
| PP_3116 | lexA2 | LexA repressor 2 (SOS transcriptional repressor) | P59479 | 202 aa |
| PP_3117 | imuA | Translesion DNA synthesis-associated protein ImuA (ImuA/RecA-like) | Q88I84 | 206 aa |
| PP_3118 | imuB | Catalytically inactive Y-family polymerase scaffold (TARGET) | Q88I83 | 472 aa |
| PP_3119 | dnaE2 | Error-prone DNA polymerase (Pol III α paralog / ImuC) | Q88I82 | 1033 aa |
| PP_3120 | yeaE | Methylglyoxal reductase (unrelated; operon boundary) | — | — |
This arrangement — a LexA repressor gene immediately adjacent to a contiguous imuA–imuB–dnaE2 cassette — is the hallmark of the SOS-regulated mutagenesis modules experimentally validated in Caulobacter and Mycobacterium. The presence of lexA2 directly upstream strongly implies LexA-box regulation of the cassette, placing imuB under RecA/LexA (SOS) control in P. putida, consistent with the recA-dependent induction demonstrated in orthologs.
The same analysis independently reconfirmed the catalytic-residue degeneracy of the P. putida ImuB protein (DinB-D8 and DinB-D103 replaced), tying the sequence-level loss of enzymatic capacity directly to this specific KT2440 protein rather than relying solely on ortholog inference.
Evidence summary: Contiguous KT2440 loci PP_3116 lexA2 → PP_3117 imuA → PP_3118 imuB → PP_3119 dnaE2; operon adjacent to a LexA repressor; ImuB catalytic carboxylates absent by pairwise alignment.
The core insight is a separation of catalysis from recruitment. The bacterial ImuA–ImuB–DnaE2 mutasome distributes the functions normally combined in a single Y-family TLS polymerase across two paralogs plus an accessory factor:
SOS induction (DNA damage)
│
RecA* + ssDNA ─► LexA (PP_3116) autocleaves
│ derepression of LexA box
▼
┌──────────── imuA (PP_3117) ── imuB (PP_3118) ── dnaE2 (PP_3119) ────────────┐
│ │
▼ ▼
ImuA (RecA-like) DnaE2 / ImuC (C-family Pol)
accessory / assembly CATALYTIC engine, error-prone
│ │
└──────────────► ImuB (Y-family FOLD, NO catalysis) ◄────────────────────────┘
• central SCAFFOLD / ADAPTOR
• β-clamp-binding motif
│
▼
DnaN β-sliding clamp (replisome)
│
▼
Mutagenic translesion synthesis across the lesion
│
▼
Damage tolerance + point mutations (e.g., antibiotic resistance)
Step-by-step:
The genetic phenocopy result is the linchpin. If ImuB merely assisted an active site, disrupting one contact might only partially impair function. Instead, breaking the single ImuB–β-clamp interaction is as deleterious as deleting the whole cassette (PMID: 20615954). This tells us the pathway's rate-limiting, non-redundant requirement is getting DnaE2 to the right place on the clamp — precisely ImuB's job. ImuB is best described, in the language of the research question, as a structural adapter/scaffold protein whose "substrate" is the protein–protein and protein–clamp interaction network of the replisome, not a nucleic-acid or small-molecule substrate.
ImuB carries out its function intracellularly, in the cytoplasm, at the DNA replisome / sites of DNA damage. It is a soluble DNA-associated protein with no signal peptide or transmembrane region; its "location" is operationally defined by its docking onto the DnaN β-clamp at the primer terminus of stalled replication forks and, per the NER-coordination evidence, at post-replicative repair gaps in non-replicating cells (PMID: 33856342).
| PMID | Title (organism) | How it supports the annotation |
|---|---|---|
| 20615954 | Essential roles for imuA'- and imuB-encoded accessory factors in DnaE2-dependent mutagenesis in M. tuberculosis | Cornerstone. Establishes ImuB lacks catalytic residues; demonstrates ImuB↔ImuA', ImuB↔DnaE2 and ImuB↔β-clamp interactions; shows clamp-binding disruption phenocopies null mutants; imuB individually essential for induced mutagenesis and damage tolerance. |
| 15886391 | An SOS-regulated operon involved in damage-inducible mutagenesis in C. crescentus | Defines the imuA–imuB–dnaE2 operon as RecA-dependent and damage-inducible, responsible for most UV/mitomycin-C-induced mutations; identifies ImuB as Y-family-like, cooperating with DnaE2 in lesion bypass. |
| 37625357 | DinB, ImuBC and RpoS contribute to ciprofloxacin-resistance mutations in P. aeruginosa | Directly links imuBC in a Pseudomonas species to SOS-dependent generation of antibiotic-resistance mutations — the closest-relative functional evidence for the P. putida ortholog. |
| 37530405 | Investigating the composition and recruitment of the mycobacterial ImuA'–ImuB–DnaE2 mutasome | Reinforces the tripartite mutasome model and the recruitment logic; ImuB as assembly hub. |
| 33856342 | Coordination between NER and DnaE2 enables DNA damage survival in non-replicating bacteria | Extends the model beyond the replication fork: DnaE2 (and by extension the ImuB scaffold) can act with NER in gap-filling / non-replicating contexts. |
| 28533123 | Molecular characterization of C. crescentus mutator strains | Context on imuC/dnaE2 contribution to spontaneous vs. induced mutagenesis; notes an imuC mutant had spontaneous rates comparable to parental — underscoring that this pathway is chiefly damage-induced rather than a driver of baseline mutation. |
Convergence of evidence. Four independent lines converge on the same assignment: (1) sequence/domain analysis (Y-family fold with degenerate active site), (2) protein-interaction data (ImuB as tripartite hub bound to the clamp), (3) genetics (RecA/LexA-dependent, individually essential, clamp-disruption phenocopy), and (4) comparative genomics (intact lexA2–imuA–imuB–dnaE2 cassette in KT2440). No line of evidence contradicts the scaffold model.
No direct biochemistry on the P. putida protein. The functional model is transferred by strong orthology from M. tuberculosis and C. crescentus. The specific PP_3118 protein has not, to our knowledge, been purified and shown biochemically to lack polymerase activity or to bind the KT2440 β-clamp; the catalytic-residue loss is inferred from sequence alignment (robust, but computational).
ImuA/ImuB nomenclature and paralogy. In some organisms the accessory factor is called ImuA' and DnaE2 is called ImuC; database annotations vary. Care is needed to ensure PP_3117/PP_3118/PP_3119 map to imuA/imuB/dnaE2 respectively (the genomic order in KT2440 supports this mapping).
LexA-box confirmation is inferential. The adjacency of lexA2 and the operon strongly implies SOS regulation, but a mapped LexA operator upstream of the P. putida imuA promoter and experimental induction kinetics in KT2440 were not directly verified here.
Mutational spectrum in P. putida is unquantified. While P. aeruginosa imuBC is tied to ciprofloxacin resistance, the specific contribution of PP_3118 to UV/MMC/fluoroquinolone-induced mutagenesis and the mutation signature in KT2440 remain to be measured.
Structural detail is limited. There is no experimental structure of the P. putida ImuB or of the assembled mutasome; the arrangement of the ImuA–ImuB–DnaE2–clamp complex is modeled from interaction data, not solved.
Deletion and complementation phenotyping in KT2440. Construct ΔimuB (PP_3118), ΔimuA, and ΔdnaE2 mutants and measure UV-, mitomycin-C-, and ciprofloxacin-induced mutation rates (rifampicin-resistance / gyrA fluctuation assays). Prediction: each deletion abolishes damage-induced mutagenesis without strongly affecting spontaneous rates.
Clamp-binding motif dissection. Identify the ImuB β-clamp-binding peptide in PP_3118 and generate a point-mutant that abolishes DnaN binding. Prediction (from PMID 20615954): the separation-of-function mutant phenocopies the full deletion for induced mutagenesis.
Biochemical reconstitution. Purify P. putida ImuA, ImuB, DnaE2, and DnaN; confirm the ImuB-centered interaction network by pulldown/SPR and test for (absence of) intrinsic ImuB polymerase activity and DnaE2-dependent TLS in vitro.
SOS regulation mapping. Verify the predicted LexA operator upstream of imuA (PP_3117) by EMSA with purified LexA2 and measure recA-dependent, damage-inducible transcription of imuB by qRT-PCR/reporter fusion in KT2440.
Fluorescence localization. Tag ImuB (functional fluorescent fusion) and confirm damage-induced focus formation colocalizing with replisome/β-clamp markers, testing both replicating and stationary-phase (NER-coordinated) contexts.
Structural work. Pursue a cryo-EM or crystal structure of the ImuA–ImuB–DnaE2–β-clamp assembly, or at minimum an AlphaFold-Multimer model validated against the interaction data, to define the recruitment geometry.
ImuB (PP_3118, Q88I83) in Pseudomonas putida KT2440 is best annotated as the catalytically inactive Y-family-fold scaffold/adaptor subunit of the SOS-inducible ImuA–ImuB–DnaE2 mutasome. Its primary function is protein recruitment, not catalysis: it bridges the accessory factor ImuA, the error-prone polymerase DnaE2, and the DnaN β-sliding clamp, thereby positioning DnaE2 to perform mutagenic translesion DNA synthesis at damaged DNA. It operates intracellularly at the replisome under RecA/LexA (SOS) control, mediating DNA-damage tolerance and damage-induced mutagenesis, and in pseudomonads contributes to the emergence of antibiotic-resistance mutations. This assignment is supported by domain architecture, the diagnostic loss of catalytic carboxylates, the intact lexA2–imuA–imuB–dnaE2 genomic cassette in KT2440, and strong orthology to experimentally characterized systems in Mycobacterium tuberculosis and Caulobacter crescentus.