Drosophila melanogaster. FlyBase FBgn0001320. 429 aa. PE1 (evidence at protein level).
Knirps is a zygotic gap gene product and a member of the nuclear receptor superfamily,
subfamily NR0A (AltName in UniProt: "Nuclear receptor subfamily 0 group A member 1"). It is a
sequence-specific, nuclear transcriptional repressor.
Architecture (from UniProt P10734 features):
- N-terminal C4-type (two Zn-finger) nuclear-receptor DNA-binding domain: DNA_BIND 2..78
("Nuclear receptor"), with ZN_FING 5..25 and ZN_FING 42..66 both "NR C4-type".
- The rest of the 429-aa protein is largely intrinsically disordered / low-complexity
(REGION 112..148, 223..250, 338..357, 375..397 "Disordered"; several low-complexity /
polar COMPBIAS regions). There is NO ligand-binding domain (LBD). This is the hallmark of
the NR0 ("knirps-like") subfamily: they retained the NR-type DBD but lost the LBD, so they act
as ligand-independent / orphan factors, not hormone-activated receptors.
- UniProt SIMILARITY: "Belongs to the nuclear hormone receptor family. NR0 subfamily."
- PANTHER family: PTHR48092, cross-referenced from the UniProt record itself
(kni-uniprot.txt:175, DR PANTHER; PTHR48092; KNIRPS-RELATED PROTEIN-RELATED; 1.), and
fetched to interpro/panther/PTHR48092/. Note the naming discrepancy: InterPro/PANTHER's own
entry name for this family is "Nuclear hormone receptor family NR3 subfamily" (integrated into
IPR050200), which does not match the NR0A subfamily assignment UniProt gives kni. The
UniProt DR line is what places kni in this family; the family's NR3 name reflects the
vertebrate steroid receptors that dominate it, and is a further illustration of why the
vertebrate-receptor IBAs (nuclear receptor activity, estrogen response element binding)
over-propagate onto this LBD-less orphan factor.
- UniProt FUNCTION: "Transcriptional repressor. Binds to multiple sites in the eve stripe 3
enhancer element. Plays an essential role in the segmentation process both by refining the
expression patterns of gap genes and by establishing pair-rules stripes of gene expression."
(ECO:0000269|PubMed:8670869)
- UniProt SUBCELLULAR LOCATION: Nucleus.
- UniProt INTERACTION block lists physical partners CtBP (O46036) and gro/Groucho (P16371).
Knirps is the prototypical short-range repressor: it acts locally (~100 bp) to quench
activators or block basal promoters, in contrast to long-range repressors such as Hairy (>1 kb).
Structure-function work identified two separable repression activities in Knirps, one dependent
on dCtBP and one independent of it:
The CtBP-independent activity is executed through Groucho, recruited via an eh1-like motif —
i.e., Groucho is a genuine cofactor of a short-range repressor, overturning the older "CtBP =
short-range / Groucho = long-range" model:
So the core molecular function is best captured as: DNA-binding transcription repressor
activity (RNA Pol II-specific; GO:0001227), enabled by sequence-specific DNA binding through the
C4 Zn-finger DBD (GO:0043565 / GO:0008270 zinc-ion binding), and mechanistically executed by
transcription corepressor binding (GO:0001222) to dCtBP and Groucho.
Genome-wide ChIP (blastoderm) using anti-KNI antibodies shows kni is one of the six maternal/gap
factors binding thousands of genomic regions:
- PMID:18271625
- PMID:18271625
kni is one of the four zygotic gap genes (with hb, Kr, gt) acting downstream of Bicoid/Caudal to
segment the trunk:
- [PMID:18271625 "four targets of BCD and CAD—hunchback (hb), Krüppel (Kr), knirps (kni), and giant
(gt)—the "gap" genes"]
- PMID:18271625
- UniProt: "Plays an essential role in the segmentation process both by refining the expression
patterns of gap genes and by establishing pair-rules stripes of gene expression."
Redundantly with its paralog knrl, kni controls tracheal branching, in part by repressing spalt
downstream of Dpp:
- PMID:9811580
- PMID:9811580
- PMID:9811580
- PMID:9811580
This is a genuine, well-documented developmental deployment but is a secondary biological role
relative to embryonic segmentation; it uses the same repressor MF.
kni/knrl spatially restrict endocycle (endoreduplication) domains in the fore- and hindgut by
repressing S-phase genes:
- PMID:11118880
- PMID:11118880
Note: FlyBase annotates this paper to GO:0007088 "regulation of mitotic nuclear division." The paper
is about endoreduplication (endocycles), not mitosis, so a more accurate term is
GO:0032875 regulation of DNA endoreduplication (candidate MODIFY). Also non-core.
Several bare GO:0005515 protein binding (IPI) annotations exist. "protein binding" is
uninformative, and in every case here the partner is known — the GOA WITH/FROM column names
it, and kni-uniprot.txt resolves the accession. The CC INTERACTION block lists exactly two
physical partners for kni: dCtBP (O46036) and gro/Groucho (P16371) (kni-uniprot.txt:97-98).
Partner identity is therefore curated input data, not something to be recovered from an abstract, so
none of these rows needs to be left UNDECIDED:
- PMID:10982842 (WITH/FROM = FB:FBgn0020496) documents the dCtBP interaction. dCtBP is a
corepressor, so the accurate MF is GO:0001222 transcription corepressor binding.
- PMID:19805071 (WITH/FROM = UniProtKB:P16371) documents the Groucho interaction (also a
corepressor) → GO:0001222.
- PMID:30995488 is a genome-wide Y2H interactome, but its WITH/FROM for kni is
UniProtKB:O46036 = dCtBP, not a sequence-specific TF — so GO:0140297 would be wrong here and
the correct term is GO:0001222. The paper itself places non-DNA-binding cofactors outside the
screened set.
PMID:30995488
- PMID:14605208 (Giot) is a general two-hybrid proteome map, but its WITH/FROM is likewise
UniProtKB:O46036 = dCtBP → GO:0001222.
PMID:14605208
- PMID:17972097 is an Arabidopsis ANGUSTIFOLIA vs dCtBP paper (abstract-only in cache) whose
WITH/FROM is UniProtKB:O46036 = dCtBP → GO:0001222. The abstract independently establishes
that dCtBP is "a transcriptional corepressor for ... DNA-binding repressors containing the short
amino acid motif, PXDLS", and kni is such a PXDLS repressor, so the assay context matches the
curated partner.
PMID:17972097
Note on Campli et al. 2026 (bioRxiv): flags kni as an adaptively-expanding "Family 11" TF at
metamorphic origins in Pancrustacea. This is comparative/phylogenomic project context only and is
not evidence for any D. melanogaster GO annotation; not used to support annotations here.
The reference_review blocks were not updated alongside the IPI action changes, so three still
described the superseded verdicts. Corrected for PMID:14605208, PMID:17972097 and
PMID:30995488; all three now record WITH/FROM = UniProtKB:O46036 (dCtBP, kni-uniprot.txt:97)
as the basis for MODIFY → GO:0001222. PMID:17972097 keeps correctness: UNVERIFIED, which is a
statement about the abstract-only cache, not about the partner.
The PMID:30995488 findings.statement had preserved the refuted inference ("Knirps partners
detected in this screen are transcription factors") in a machine-readable field; it now states only
what the paper reports. The screen-scope quote is likewise re-framed in the annotation reason:
read as evidence about this partner it argued against the very annotation it defended, since
dCtBP is among the partners the screen detected. It now serves its actual purpose — showing why the
screen's nominal TF-only scope was never a safe way to infer a partner's class.