| Aspect | Experimentally supported finding | Example quantitative/phenotypic detail | Key citations |
|---|---|---|---|
| Peptide size / sequence | T4 **Stp** is a minuscule phage-encoded peptide of ~26–27 aa; Penner et al. reported the deduced sequence **MSNFHNEHVMQFYRNNLKTKGVFGRQ** (stop codon omitted). | Table 4 in Penner et al. lists the wt peptide and mutant/variant sequences; Snyder describes an ORF of about **26 aa**. | (pqac-00000014, pqac-00000009, pqac-00000031) |
| Primary function 1: anti-restriction | Stp inhibits **EcoprrI** type IC restriction activity while sparing cognate DNA modification, consistent with action on the restriction-specific Hsd machinery. | Lower Stp levels suffice for anti-restriction than for ACNase activation; anti-restriction does **not** require PrrC. | (pqac-00000003, pqac-00000005, pqac-00000008) |
| Primary function 2: PrrC activation | Stp is **necessary and sufficient** to activate the latent host **PrrC anticodon nuclease (ACNase)**, leading to cleavage of **tRNA^Lys**. | Induction of cloned **stp** in uninfected **prr+** E. coli elicits ACNase activity; stp mutations abolish activation. | (pqac-00000001, pqac-00000006, pqac-00000012, pqac-00000030) |
| Mechanistic model | Stp likely binds an **Hsd/EcoPrrI** component and relays a conformational change to the **Hsd–PrrC** interface, unmasking latent PrrC. | Proposed “single-hit” inactivation of EcoPrrI versus a higher/continued Stp requirement for ACNase activation. | (pqac-00000001, pqac-00000007, pqac-00000012) |
| Biochemical requirements | In vitro Stp-dependent activation is favored when **EcoPrrI is DNA-bound**, is **DNase-sensitive**, and requires nucleotide turnover with **GTP hydrolysis** likely through the PrrC NTPase domain. | Activation reactions lose activity with DNase; GTP hydrolysis is required for activation of the holoenzyme. | (pqac-00000002, pqac-00000011, pqac-00000028) |
| Alternative activator | A normal host metabolite can substitute for Stp: **dTTP** is the most potent reported surrogate activator in vitro, apparently acting through a mechanistically distinct holoenzyme/DNA state. | Effective level reported at approximately **5 × 10^-7 M dTTP** under assay conditions. | (pqac-00000011, pqac-00000013) |
| Target reaction / substrate context | The biologically relevant downstream nuclease reaction is cleavage of **tRNA^Lys** at/near the wobble position, generating **5′-OH** and **2′,3′-cyclic phosphate** termini; this reaction is catalyzed by host **PrrC**, not by Stp itself. | Cleaved tRNA lesions can block late T4 protein synthesis unless repaired by phage **Pnk/Rnl1** functions. | (pqac-00000001, pqac-00000004, pqac-00000010) |
| Infection timing / localization notes | Stp is reported as a **delayed-early** T4 gene product and is **unlikely to be virion packaged**; it therefore most likely acts **inside the infected host cytoplasm** after infection rather than being injected as a preformed virion protein. | Timing inference comes from delayed-early expression; localization is inferred from diffusible intracellular action on EcoPrrI/PrrC. | (pqac-00000003, pqac-00000010) |
| Phenotypic consequences during infection | Because Stp activates PrrC, **stp** mutations suppress the requirement for T4 tRNA-repair enzymes on **prr+** hosts; without Stp-mediated activation, pnk/rli become nonessential in that context. | Stp is thus a “double-edged” effector: anti-restriction for phage benefit, but it can also trigger host tRNA restriction. | (pqac-00000001, pqac-00000004, pqac-00000021) |
| Mutational readouts | The conserved **N-proximal ~18 aa** are most important; residues such as **F4, E7, H8, R14** are critical for activity, while much of the C-terminus is more dispensable. | Example Table 5 values: **prr:stp = EOP 0.4, ACNase +**; **stpF4S = 0.2, −**; **stpE7V = 10^-3, −**; **stp(Baker) = 0.8, +**. | (pqac-00000003, pqac-00000014, pqac-00000023, pqac-00000031, pqac-00000032) |
| Recent framing (2023–2024) | Recent reviews still cite the **Stp–EcoprrI–PrrC** axis as a classic example of phage anti-restriction provoking abortive infection/tRNA cleavage; in 2023 Stp was also included as a known accessory-gene control in a large phage AG screen. | Silas et al. screened **10,888 putative AGs** from **1,217 phages** and included **Stp** among 11 known controls. | (pqac-00000015, pqac-00000018, pqac-00000024) |


*Table: This table summarizes experimentally supported facts about bacteriophage T4 Stp, including its size, anti-restriction and PrrC-activating roles, mechanistic requirements, infection context, and quantitative mutant phenotypes. It is useful as a compact evidence map for functional annotation of UniProt P62765.*