DNA polymerase III, clamp loader chi/psi subcomplex

GO:0043847 · Other · Multiprotein complex · Proposed

A dimer composed of the chi and psi subunits which is a subassembly of the DNA polymerase III clamp loader complex and serves as a bridge between the DnaX complex and the single-stranded DNA-binding protein (SSB). [GO:0043847]

Synonyms: DNA polymerase III, DnaX complex, chi/psi subcomplex

Part of: DNA polymerase III, clamp loader complex (GO:0043846)

UniProt location names: UniProt Consortium, CC BY 4.0. Reference label and route sources.

Components

Component	Type	Part of / acts on	Grounding	Genes	Stoichiometry	Essential	Role
DNA polymerase III chi subunit	Protein	part of	Reviewed label only (reviewed name; no accession asserted)	holC	1	UNKNOWN	HolC chi constituent of the 1:1 chi/psi accessory dimer in the DNA polymerase III clamp-loader complex. `GO:0043847` GO:0043847 defines the chi/psi subcomplex as a dimer composed of the chi and psi subunits. `PMID:8389364` Xiao et al. 1993 identified holC as the E. coli gene encoding the DNA polymerase III chi accessory protein. `PMID:8505305` Xiao et al. 1993 characterized E. coli chi and psi as monomeric proteins that associate to form a 1:1 complex. `UniProtKB:P28905` UniProtKB P28905 is the reviewed E. coli K-12 holC entry for DNA polymerase III subunit chi and is annotated to GO:0043846.
DNA polymerase III psi subunit	Protein	part of	Reviewed label only (reviewed name; no accession asserted)	holD	1	UNKNOWN	HolD psi constituent that binds chi and links it into the DNA polymerase III gamma clamp-loader complex. `GO:0043847` GO:0043847 defines the chi/psi subcomplex as a dimer composed of the chi and psi subunits. `PMID:8389364` Xiao et al. 1993 identified holD as the E. coli gene encoding the DNA polymerase III psi accessory protein. `PMID:8505305` Xiao et al. 1993 characterized E. coli psi as the gamma-binding partner that bridges chi into the gamma clamp-loader complex. `UniProtKB:P28632` UniProtKB P28632 is the reviewed E. coli K-12 holD entry for DNA polymerase III subunit psi and is annotated to GO:0043846.

Organism-specific protein examples

Named proteins in which each component's role was established, with the organism the evidence comes from. The component above is the taxon-agnostic family; these are the specific entries behind it.

DNA polymerase III chi subunit

Accession	Protein	Gene	Organism	Entry
`UniProtKB:P28905`	DNA polymerase III subunit chi	holC	Escherichia coli K-12 (`NCBITaxon:83333`)	REVIEWED
Role and evidence (1 reference) Reviewed E. coli K-12 HolC accession for the clamp-loader chi subunit. `UniProtKB:P28905` UniProtKB P28905 is the reviewed Escherichia coli K-12 holC entry for DNA polymerase III subunit chi.

DNA polymerase III psi subunit

Accession	Protein	Gene	Organism	Entry
`UniProtKB:P28632`	DNA polymerase III subunit psi	holD	Escherichia coli K-12 (`NCBITaxon:83333`)	REVIEWED
Role and evidence (1 reference) Reviewed E. coli K-12 HolD accession for the clamp-loader psi subunit. `UniProtKB:P28632` UniProtKB P28632 is the reviewed Escherichia coli K-12 holD entry for DNA polymerase III subunit psi.

Canonical examples

NCBITaxon:562 Escherichia coli — Source species for biochemical characterization of chi and psi as a 1:1 gamma-complex accessory dimer. [PMID:8505305]

Functions

DNA replication GO:0006260 — The chi/psi dimer contributes to Pol III replication as an accessory clamp-loader subcomplex that brings chi into the gamma complex through psi.
- PMID:8505305 Xiao et al. 1993 characterized E. coli chi and psi as a 1:1 gamma-complex accessory dimer and found that psi links chi to gamma.
- GO:0006260 GO:0006260 is the Gene Ontology biological-process term for DNA replication.

Mechanism graphs

Chi/psi joins the Pol III clamp loader (FUNCTION)

The DNA polymerase III chi/psi heterodimer combines HolC and HolD into an accessory subcomplex that is incorporated into the clamp loader through psi.

Subject	Predicate	Object	Evidence
DNA polymerase III chi subunit	is part of	DNA polymerase III, clamp loader chi/psi subcomplex	`GO:0043847` GO:0043847 defines the clamp-loader chi/psi subcomplex as a dimer composed of chi and psi.
DNA polymerase III psi subunit	is part of	DNA polymerase III, clamp loader chi/psi subcomplex	`GO:0043847` GO:0043847 defines the clamp-loader chi/psi subcomplex as a dimer composed of chi and psi.
DNA polymerase III, clamp loader chi/psi subcomplex	is part of	DNA polymerase III, clamp loader complex	`GO:0043847` GO:0043847 defines chi/psi as a subassembly of the DNA polymerase III clamp-loader complex.
DNA polymerase III psi subunit	binds	DnaX tau/gamma products	`PMID:8505305` Xiao et al. 1993 found that psi binds gamma tightly and bridges the interaction of chi with gamma.
DNA polymerase III, clamp loader chi/psi subcomplex	binds	DnaX tau/gamma products	`PMID:7494000` Olson et al. 1995 purified E. coli chi/psi and showed that it binds either gamma or tau DnaX protein.
DNA polymerase III, clamp loader chi/psi subcomplex	bridges to	single-stranded DNA-binding protein	`GO:0043847` GO:0043847 states that the chi/psi subcomplex serves as a bridge between the DnaX complex and SSB.
DNA polymerase III, clamp loader chi/psi subcomplex	supports	DNA replication	`PMID:8505305` Xiao et al. 1993 showed that psi stimulates the ATPase and replication activities of the gamma clamp-loader complex.

Discussions

CURATION_TODO (OPEN): Resolve exact source-neutral HolC and HolD family grounding for DNA polymerase III clamp-loader chi/psi components. This first GO:0043847 split captures reviewed E. coli K-12 UniProtKB examples for the chi and psi subunits while leaving source-neutral protein-family grounding to a later InterPro/Pfam/NCBIfam review.

Evidence

GO:0043847 Gene Ontology cellular-component term for the DNA polymerase III clamp-loader chi/psi subcomplex.
GO:0043846 Gene Ontology cellular-component term for the parent DNA polymerase III clamp-loader complex.
GO:0032991 Gene Ontology cellular-component parent term for protein-containing complexes.
GO:0006260 Gene Ontology biological-process term for DNA replication.
PMID:12940977 PubMed identifier for McHenry 2003, Chromosomal replicases as asymmetric dimers: studies of subunit arrangement and functional consequences.
DOI:10.1046/j.1365-2958.2003.03645.x DOI for McHenry 2003 as reported by PubMed.
PMID:8389364 PubMed identifier for Xiao et al. 1993, DNA polymerase III accessory proteins. III. holC and holD encoding chi and psi.
PMID:8505305 PubMed identifier for Xiao et al. 1993, DNA polymerase III accessory proteins. IV. Characterization of chi and psi.
PMID:7494000 PubMed identifier for Olson, Dallmann and McHenry 1995, DnaX complex of Escherichia coli DNA polymerase III holoenzyme: The chi psi complex functions by increasing the affinity of tau and gamma for delta.delta'.
UniProtKB:P28905 Reviewed UniProtKB accession for E. coli K-12 HolC.
UniProtKB:P28632 Reviewed UniProtKB accession for E. coli K-12 HolD.

Curation history

2026-09-28T11:43:34Z codex · CREATED_RECORD — Scaffolded by scripts/new_record.py
2026-09-28T11:44:53Z codex · EXPANDED_RECORD — Added the GO exact synonym, protein-containing-complex parentage, clamp-loader parthood, chi and psi components with E. coli K-12 protein examples, an E. coli canonical example, DNA-replication function evidence, GO/PubMed/UniProtKB evidence, a nonmechanistic adapter graph, and a grounding TODO.
2026-09-28T11:57:05Z codex · REFINED_GRAPH — Clarified the psi graph edge so PMID:8505305 supports psi binding the gamma DnaX protein instead of a one-ended psi-to-chi bridge edge.
2026-09-28T12:08:15Z codex · FIX_REVIEW_ISSUE — Fixed #1722 by adding PMID:8505305 to the chi and psi component evidence lists so the local 1:1 stoichiometry and psi linkage claims are directly sourced.

Source: data/structures/other/dna_polymerase_iii_clamp_loader_chi_psi_subcomplex.yaml