Protein p17 is a 166-residue early protein of the Bacillus subtilis phage phi29, encoded by the first gene of the right-side early operon that is driven by the C2 promoter and that enters the cell during the first step of genome injection. It is a homodimer with a putative coiled-coil region, is not a membrane protein, and has two demonstrated roles in the early stages of infection. First, it is required for efficient internalisation of the phage genome: phi29 injects its linear DNA right-to-left in two steps, and after capsid pressure has pushed in about 65% of the genome, p17 is a component of the energy-dependent machinery that actively pulls in the remaining left-hand portion. Second, it stimulates protein-primed replication of the phi29 genome. Purified p17 stimulates phi29 DNA amplification in vitro when the input DNA is scarce, and in vivo it is required at low multiplicity of infection but dispensable at high multiplicity, consistent with a role in recruiting limiting initiation factors to the replication origins early in infection. It is not needed for the terminal-protein-dAMP initiation reaction itself; it acts instead by optimising binding of the viral architectural protein p6 along the genome and at the DNA ends, which favours initiation, and the same p6-mediated effect represses p17's own promoter. The protein is conserved throughout the phi29-like phages, where it is treated as an accessory component of the phage DNA-replicating machinery.
Definition: The active, energy-requiring internalisation of the part of a symbiont genome that is not delivered by the pressure stored in the virion, carried out by symbiont proteins that are translated inside the host cell from the portion of the genome already delivered. The process is distinct from the ejection apparatus of the virion, acts from the host cytoplasm rather than across the envelope from outside, and requires an electrochemical gradient across the host membrane.
Justification: GO represents genome entry by the apparatus that performs it: GO:0039678 symbiont genome ejection through host cell envelope and its children are defined in terms of contractile, long flexible or short tails, that is, structures of the virion. phi29 shows that a second, mechanistically separate phase exists, performed by early proteins made inside the host from the first-delivered part of the genome: about 65% of the genome enters passively by capsid pressure, and the remainder is pulled in by a motor that includes p17 and p16.7 and that depends on a proton gradient and on negatively supercoiled DNA. There is currently no term that distinguishes this host-side pull phase, so p17 and p16.7 can only be annotated to the virion-apparatus parent. The phenomenon is unlikely to be confined to phi29, since a push-pull internalisation has been proposed for other phages of Gram-positive hosts.
Parent term: symbiont genome ejection through host cell envelope
Supporting Evidence:
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0006260 DNA replication | IEA GO_REF:0000043 | MODIFY | Summary: Keyword-mapped row from KW-0235 DNA replication. The claim is true - p17 stimulates replication of the phi29 genome - but the gene also carries the specific term GO:0039693 viral DNA genome replication three times, and that is what the evidence actually shows. Reason: The right term for p17 is the viral one. Note that this is a lateral re-pointing rather than a refinement: GO:0006260 is not an ancestor of GO:0039693 (the is_a / part_of ancestors of GO:0039693 run through GO:0006259 DNA metabolic process, GO:0071897 DNA biosynthetic process and GO:0019079 viral genome replication, not through GO:0006260), so the keyword row is not strictly redundant - it is a less informative statement on a neighbouring branch. Every replication experiment on p17, from the 1976 nonsense-mutant screen to the 1998 in vitro amplification assay, concerns phi29 genome replication specifically, so GO:0039693 should carry the claim. No GO term is being lost: the keyword KW-1194 Viral DNA replication already maps the same protein to GO:0039693. Proposed replacements: viral DNA genome replication Supporting Evidence: PMID:9858710 A stimulatory effect of protein p17 was demonstrated under in vitro conditions required to amplify phi29 DNA |
| GO:0039693 viral DNA genome replication | IDA PMID:9858710 Bacteriophage phi29 early protein p17 is conditionally requi... | ACCEPT | Summary: UniProt IDA row from the paper that cloned gene 17, purified p17 and showed that the purified protein stimulates phi29 DNA amplification in vitro under conditions starting from a low amount of input DNA. This is the direct-evidence row for p17's replication role and the strongest of the four. Reason: Direct in vitro evidence with purified protein in a reconstituted phi29 DNA amplification reaction, and the term is at the right granularity. The in vivo counterpart in the same paper is conditional on multiplicity of infection - p17 is required at low moi, the condition the authors argue is natural, and dispensable at high moi - which refines rather than weakens the claim. The role is downstream of the initiation reaction itself, since the terminal protein-dAMP initiation complex forms in extracts lacking p17 (PMID:6402761), and the mechanism proposed later is that p17 optimises p6 binding at the viral DNA ends (PMID:12480935). The replication defect is not merely a knock-on of the genome-internalisation defect described below: it persists once the whole sus17 genome is inside the cell. Supporting Evidence: PMID:9858710 A stimulatory effect of protein p17 was demonstrated under in vitro conditions required to amplify phi29 DNA PMID:9858710 protein p17 is required when phage infection occurs at a low multiplicity of infection (moi), which is probably the natural condition for infection, but is PMID:12480935 p17 function is to optimize binding of protein p6 at the viral DNA ends, thus PMID:17526715 the absence of p17 was also shown to drastically affect the level of Ο29 DNA replication once the entire sus17 genome was internalized |
| GO:0039693 viral DNA genome replication | IEA GO_REF:0000120 | ACCEPT | Summary: Automated row combining the InterPro family signature IPR035184 Phage_Gp17 (Pfam PF17549) with the UniProt keyword KW-1194 Viral DNA replication. It restates, by family membership, the claim the IDA and IMP rows establish experimentally. Reason: The signature is the p17 family itself rather than a generic domain, so membership is not in question, and the term it projects is the one the experiments support. The family is conserved across all phi29-like phages and was used as a phylogenetic marker precisely as a component of the phage DNA-replicating machinery, so the family-level inference is sound as well as correct for this member. The row reinforces the experimental rows rather than duplicating them in any problematic sense. Supporting Evidence: PMID:9929388 terminal proteins and of the gene 17 protein product, an accessory component of file:BPPH2/17/17-uniprot.txt Pfam; PF17549; Phage_Gp17; 1. |
| GO:0039693 viral DNA genome replication | IMP PMID:820555 Bacillus subtilis phage phi29. Characterization of gene prod... | ACCEPT | Summary: CACAO IMP row from the 1976 Carrascosa et al. survey of phi29 gene products and functions, a bulk nonsense-mutant screen whose abstract reports that mutants in six genes fail to synthesise phage DNA. Gene 17 is one of those six. Reason: What the paper establishes for this gene specifically: that a sus17 mutant is one of the DNA-synthesis-negative mutants of the screen. The abstract does not name the six cistrons and the cached record is abstract-only, with GONUTS now permanently retired (gowiki.tamu.edu returns HTTP 410 "GONUTS, the GO wiki, has been retired", and the Internet Archive holds no snapshot of the relevant pages), so the student's reading cannot be inspected. It can, however, be checked independently: the same laboratory named the set seven years later while describing the mutant extracts it worked with - genes 2, 3, 5, 6 and 17 - which with gene 1 completes the six, and three later papers restate gene 17's replication role as established. The attribution is therefore correct, not a guess, and the row is accepted under curator deference. The qualifier acts_upstream_of_or_within_positive_effect is also the right choice here, not a defect: the evidence is a bulk mutant screen that demonstrates necessity without demonstrating participation, and the modern literature shows a large part of the sus17 DNA-synthesis defect is indirect, arising from failed genome internalisation, altered viral DNA topology and reduced p6 binding. Direct participation is established by the separate IDA row, not by this one. Supporting Evidence: PMID:820555 Mutants in six genes do not synthesize phage DNA. PMID:6402761 extracts from B. subtilis infected with phi29 mutants in genes 2, 3, 5, 6 and 17, involved in DNA synthesis, have been used PMID:10650217 Gene 17 of Bacillus subtilis bacteriophage Phi29 is an early gene playing a role |
| GO:0039678 symbiont genome ejection through host cell envelope | IMP PMID:15066038 The push-pull mechanism of bacteriophage Γ29 DNA injection. | NEW | Summary: Proposed row for p17's best-documented in vivo role, which is absent from GOA altogether. phi29 injects its linear genome right-to-left in two steps; after capsid pressure has pushed in about 65% of it, p17 is a component of the energy-dependent machinery that actively pulls the remaining left-hand portion into the cytoplasm. Reason: p17 does the work of this process rather than merely being required for it. Three papers from two groups converge: X-ChIP measurement of left-end internalisation in sus17-infected cells shows the pull step is strongly impaired without p17 and that p17 affects it more severely than the membrane protein p16.7 does; dissection with energy poisons shows the pull step needs an electrochemical proton gradient and that azide has no further effect in p17's absence, placing the energy-transduction step on p17 itself. GO:0039678 is the mechanism-agnostic parent term for entry of a symbiont genome into the host cell through the host cell envelope. The short-tail child GO:0099002 is deliberately not used: its definition is about the tail tube, connector and released core proteins, and p17 is not part of that apparatus - it is an early protein translated inside the host from the already-injected right-hand operon. Whether GO wants a distinct term for this host-side, energy-dependent pull phase is raised in suggested_questions rather than asserted here. Supporting Evidence: PMID:15066038 second step at least one of the viral early proteins, p17, participates in the PMID:16517096 p17 suggests that this protein is essential for energy transduction PMID:17526715 These results corroborate earlier findings that protein p17 is required for efficient ejection of the left half of the Ο29 genome PMID:17526715 at least two proteins encoded by the right-side operon, p17 and p16.7, are required for an efficient pull step of DNA ejection, although the absence of p17 affects this step more drastically than the absence of p16.7 |
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Download this section (compressed HTML)Q: Should p17's role in the pull step of phi29 genome internalisation be annotated to GO:0039678, whose definition is written around the virion's ejection apparatus, or does GO want a separate term for the host-side, energy-dependent pull phase performed by early proteins translated from the already-injected part of the genome? The same question applies to phi29 p16.7, which has no ejection annotation either.
Q: Is the reduction of p6 binding along phi29 DNA in sus17-infected cells a direct consequence of the p17-p6 interaction, or an indirect consequence of impaired genome internalisation and altered DNA topology? The answer decides whether p17's contribution to replication should be described as an activity on p6 or as an upstream requirement.
Q: Does the CACAO practice of using acts_upstream_of_or_within_positive_effect for IMP annotations drawn from bulk nonsense-mutant screens reflect an explicit instruction? Every 2025-dated phi29 CACAO row carries that qualifier while the same organism's 2012-2017 rows use involved_in, and on this gene the conservative qualifier is the better fit for the evidence.
Experiment: Reconstitute the minimal phi29 amplification reaction (terminal protein, DNA polymerase, p5, p6, DNA-protein p3 template) at limiting template concentration and titrate purified p17 and the coiled-coil-defective, dimerisation-deficient p17 derivative. Measure initiation and amplification yield, and in parallel measure p6 occupancy on the template ends by footprinting or by ChIP from the reaction, so that a stimulation of amplification can be attributed to, or dissociated from, a change in p6 binding.
Hypothesis: p17 stimulates phi29 replication by increasing p6 occupancy at the DNA ends, and not by acting on the terminal protein/DNA polymerase heterodimer.
Type: in vitro reconstitution with footprinting
Experiment: Compare p6 occupancy by X-ChIP along the phi29 genome in cells infected with sus17 phage and in cells where the sus17 genome has been fully internalised - for example by infecting a suppressor host, by complementing p17 in trans from a plasmid after internalisation is complete, or by selecting the time window in which the X-ChIP left-end/right-end ratio shows the left end has arrived. A persisting deficit would establish a replication-specific role for p17; its disappearance would place the effect entirely on internalisation.
Hypothesis: The p6-binding deficit of sus17-infected cells is downstream of incomplete genome internalisation, so it disappears when the full genome is delivered independently of p17.
Type: chromatin immunoprecipitation with genetic complementation
Experiment: Fractionate infected B. subtilis and ask whether p17 is recovered with the membrane only when p16.7 is present, using sus16.7 phage as the comparator, and map the interaction by crosslinking or proximity labelling from p17 in infected cells. Combine with measurement of the pull step in cells carrying the p16.7 membrane-anchor deletion to test whether anchoring, rather than p16.7 itself, is what p17 requires.
Hypothesis: p17 is the energy-transducing component of the pull motor and contacts the host membrane machinery through p16.7 rather than directly.
Type: subcellular fractionation and proximity labelling
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