Table 2 Factors involved in piRNA biogenesis in C. elegans

FKH-3

ForKHead transcription factor

Recognizing the Ruby motif; involved in type I 21 U-RNA biogenesis

N/A

N/A

[24, 45]

FKH-4

ForKHead transcription factor

Recognizing the Ruby motif; involved in type I 21 U-RNA biogenesis

N/A

N/A

[24, 45]

FKH-5

ForKHead transcription factor

Recognizing the Ruby motif; involved in type I 21 U-RNA biogenesis

N/A

N/A

[24, 45]

UNC-130

UNCoordinated 130

Binding to 21 U-RNA upstream regulatory sequence; required for piRNA transcription

N/A

FOXD4

[59]

SNPC-4

Small nuclear RNA activating protein complex

Binding to piRNA clusters; required for global piRNA expression

Lethal or sterile

SNAPC4

[59]

PRDE-1

Nuclear factor

Required for production of piRNA precursors from the Ruby motif; recruiting RNA polymerase II to the FKH motif

Mrt

N/A

[29, 60]

TOFU-3

Twenty-One-u Fouled Ups

Required for the production or stability of 21 U-RNA precursors; a SUMO-related protease; interacting with HP1a and regulating its localization to pericentric heterochromatin

Mrt

N/A

[29, 47]

TOFU-4

Twenty-One-u Fouled Ups

Required for the production or stability of 21 U-RNA precursors

Mrt

N/A

[29, 47]

TOFU-5

Twenty-One-u Fouled Ups

Required for the production or stability of 21 U-RNA precursors; having a DNA-binding domain structurally related to that of Myb and found in various transcription regulatory factors

Lethal or sterile

N/A

[29, 47]

TBP-1

TATA-box-binding protein

An additional factor required for TOFU-5 binding to piRNA clusters

N/A

TBP

TBPL2

[46]

PID-1

piRNA-induced silencing-defective 1

A potential nuclear import and export factor

Lethal

N/A

[56]

TOFU-1

Twenty-One-u Fouled Ups

Required for precursor RNA processing after its production

Mrt

N/A

[29, 47]

TOFU-2

Twenty-One-u Fouled Ups

Required for precursor RNA processing after its production

Mrt

N/A

[29, 47]

PARN-1

RNase

Required for piRNA 3′ end trimming and having a 3′-to-5′ exo-RNase activity

Mrt

PARN PNLDC1

[48]

TOFU-6

Twenty-One-u Fouled Ups

A component of PETISCO; required for 21 U biogenesis

N/A

N/A

[29, 47]

TOFU-7

Twenty-One-u Fouled Ups

Required for precursor RNA processing after its production

N/A

N/A

[29, 47]

HENN-1

HEN methyltransferase 1

Required for precursor RNA processing after its production

Sterile

HENMT1

[61, 62]

PID-3

(PICS-1)

piRNA induced silencing defective

A component of PETISCO; required for 21 U biogenesis; interacting with 21 U-RNA precursor

Mes

N/A

[57]

ERH-2

Enhancer of rudimentary homolog

A component of PETISCO; required for 21 U biogenesis; interacting with 21 U-RNA precursor

Mes

ERH

[57]

IFE-3

Homologs of eIF4E

A component of PETISCO; required for 21 U biogenesis; binding  to the 5′ cap of the 21 U precursor transcripts

Mes

EIF4E

EIF4E1B

[57]

EGO-1

Argonaute

RNA-dependent RNA polymerase

Sterile

N/A

[34, 63]

PRG-1

PIWI protein

21 U-RNA binding activity; combining  with piRNA to form RISC

Mrt

PIWIL1

PIWIL2

PIWIL3

PIWIL4

[19, 20]

CSR-1

Argonaute

Self- and non-self-transcript discrimination

Mrt

N/A

[54, 55]

TOST-1

Twenty-one U pathway antagonist

Inhibiting  21 U-driven silencing

Mes

N/A

[57]

EKL-1

Enhancer of ksr-1 Lethality

Encoding a Tudor-domain protein required for RNAi; transgene silencing; transgene-mediated cosuppression in the germline

N/A

N/A

[47, 64]

DRH-3

Dicer related helicase

A core component of the RdRp complex and essential for 22G-siRNA biogenesis

N/A

DHX58

[65]

RRF-1

Argonaute

RNA-dependent RNA polymerase

N/A

N/A

[66]

RDE-2

RNAi defective

Interacting with MUT-7; required for efficient RNAi

N/A

N/A

[67]

MUT-7

Mutator

Encoding a putative exoribonuclease; required for efficient RNAi

N/A

EXD3

[67]

RDE-8

RNAi defective

Cleaving target mRNAs to mediate silencing; generating  mRNA fragments to serve as template for RdRp

N/A

N/A

[68]

MUT-16

Mutator

Required for the accumulation of 22G-siRNA

N/A

N/A

[69]

SIMR-1

Tudor domain protein

Mediating the production of secondary siRNAs by the mutator complex

Mrt

N/A

[70]

HRDE-1

Nuclear Argonaute

Germline nuclear RNAi

Mrt

N/A

[71]

NRDE-3

Nuclear Argonaute

Somatic nuclear RNAi

Mrt

N/A

[71]

MET-2

H3K9me1/H3K9me2 methyltransferases

Required for 22G-siRNA formation

Mrt

SETDB1

SETDB2

[72]

SET-25

H3K9me3 methyltransferase

Heterochromatin maintenance, required for maintenance of piRNA-induced RNAi

N/A

N/A

[72]

SET-32

H3K9me3 methyltransferase

Silencing establishment; required for multigenerational inheritance of environmental RNAi and piRNA silencing

Mrt

N/A

[73]

HPL-1

Heterochromatin protein 1

Binding with H3K9me2/H3K9me3; required for piRNA inducetion of heritable small RNAs

Sterile

CBX1

CBX3

CBX5

[74]

HPL-2

Heterochromatin protein 2

Binding with H3K9me2/H3K9me3; required for piRNA inducetion of heritable small RNAs

Sterile

CBX3

[75]

WAGO

Argonaute

Argonaute protein in C. elegans required for 22G-siRNA after RNA-dependent RNA polymerase activity

N/A

AGO1

AGO2

AGO3

AGO4

[76]