| Category | Summary | Quantitative / specific data | Citations |
|---|---|---|---|
| Target identity | Correct target is **AmtB** from **Pseudomonas putida KT2440**; gene **amtB**, ordered locus **PP_5233**, UniProt **Q88CE8**. In P. putida literature, **amtB (PP5233)** is explicitly described as the **high-affinity ammonium transporter** and is linked to upstream **glnK**. | qRT-PCR/microarray study explicitly maps **amtB = PP5233** in KT2440. | (pqac-00000000, pqac-00000019) |
| Primary function / substrate | AmtB is a **cytoplasmic membrane ammonium transporter/channel** of the **Amt/Mep/Rh family**. Current consensus is that the physiological substrate enters as **NH4+**, then is **deprotonated** and transferred largely as **NH3** through a hydrophobic pore; bacterial AmtB is therefore best viewed as an ammonium uptake system for nitrogen scavenging under limitation. | Family-level high-affinity behavior is reported around **Km ~10 µM** in classic AmtB discussions; direct bacterial AmtB electrophysiology suggests net ammonium translocation rather than simple passive NH3 equilibration. | (pqac-00000003, pqac-00000005, pqac-00000006, pqac-00000010, pqac-00000011, pqac-00000022) |
| Operon context | In **P. putida KT2440**, **amtB** is genetically linked with **glnK** in a **glnK-amtB operon**; evidence also suggests an **internal promoter** upstream of amtB. | glnK and amtB are co-oriented; amtB can also show stronger induction than glnK, consistent with additional promoter control. | (pqac-00000000, pqac-00000001, pqac-00000019, pqac-00000024) |
| Transcriptional regulation | **NtrC** directly activates the nitrogen-responsive promoter(s) controlling **glnK-amtB** in **P. putida**; these promoters are **σ54/RpoN-dependent**, and **IHF** is required for open-complex formation at the glnK promoter. | Two contiguous **NtrC binding sites** were identified upstream of the N-dependent glnK promoter. | (pqac-00000000, pqac-00000001, pqac-00000019, pqac-00000024) |
| Nitrogen-responsive expression | **amtB** is strongly induced during nitrogen limitation in **P. putida KT2440**, consistent with a role in scavenging low external ammonium. | **~62-fold induction** in wild type under serine vs ammonium+serine; **~75.8- to 76-fold NtrC-dependent difference** versus the ntrC mutant / corresponding comparison. | (pqac-00000000, pqac-00000018, pqac-00000019) |
| Post-translational control | Beyond transcriptional control, AmtB activity is regulated by the **PII protein GlnK**. Non-uridylylated **GlnK** binds AmtB and **blocks the channel**; **GlnK-UMP** does not bind, allowing transport. This links transport to intracellular **glutamine, 2-oxoglutarate, ATP/ADP, and Mg2+** status. | GlnK is **trimeric** and inserts T-loops into AmtB vestibules/pore entrances; metabolite-dependent complex formation couples N/C status to transport. | (pqac-00000004, pqac-00000006, pqac-00000020, pqac-00000021, pqac-00000022, pqac-00000023) |
| Structural motifs | AmtB family proteins are **trimers** with one pore per protomer. Conserved motifs include the **periplasmic NH4+ recruitment site**, **Phe-gate** (**F107/F215** in E. coli AmtB), and central **twin-His motif** (**H168/H318** in E. coli AmtB). | Recent structural work also describes 11 TM helices per monomer and conserved vestibule features (Trp/Ser, Phe pair, coplanar His pair). | (pqac-00000011, pqac-00000012, pqac-00000013, pqac-00000016) |
| Mechanism model | The leading 2024 model favors **electrogenic ammonium transport** with **NH4+ deprotonation** and a **“two-lane” pathway** in which **H+** and **NH3** traverse separately, rather than simple passive NH3 diffusion alone. However, mechanism/energetics remain debated and assay-dependent. | Solid-supported membrane electrophysiology estimated **~30–300 NH4+ s−1 per trimer** for microbial Amts/AmtB-like transport. | (pqac-00000010, pqac-00000015, pqac-00000017) |
| Technical limitations / evidence quality | Functional interpretation is complicated by assay limitations: methylammonium is an imperfect surrogate, proteoliposome orientation/copy number are hard to control, high ammonium concentrations can be nonphysiological, and lipid environment strongly affects activity. | Early studies reported apparent intracellular ammonium accumulation of **~60–3000-fold**; transport assays often used **5–200 mM** ammonium, which may distort mechanism inference. | (pqac-00000013, pqac-00000014, pqac-00000015) |


*Table: This table summarizes verified identity, function, regulation, quantitative data, and mechanistic interpretation for P. putida KT2440 AmtB/PP_5233. It is useful as a compact evidence map linking organism-specific annotation to broader 2024 mechanistic understanding of bacterial AmtB transporters.*