REAGENT or RESOURCE	SOURCE	IDENTIFIER
Bacterial and virus strains
BL21-CodonPlus (DE3)-RIL Competent Cells	Agilent Technologies	Cat#230245
MAX Efficiency DH5α Competent Cells	Thermo Fisher Scientific	Cat#18258012
Chemicals, peptides, and recombinant proteins
Dithiothreitol	Gold Biotechnology	Cat#DTT10
TCEP	Sigma	Cat#646547
BSA	Sigma	Cat#A7030
Kanamycin	GoldBio	Cat#K-120
Chloramphenicol	GoldBio	Cat#C-105
LB Broth, Miller	Thermo Fisher Scientific	Cat#BP1426-2
LB Agar, Miller	Thermo Fisher Scientific	Cat#BP9724-500
IPTG	GoldBio	Cat#I2481
Imidazole	Sigma	Cat#I5513
ADP	Sigma	Cat#A2754
AMPPNP	Roche	Cat#10102547001
NP-40 (alternative)	Millipore	Cat#492016
Quick Start™ Bradford 1× Dye Reagent	Bio-Rad	Cat#500-0205
Ni-NTA beads	QIAGEN	Cat#30210
cOmplete™, Mini, EDTA-free Protease Inhibitor Cocktail	Roche	Cat#11836170001
Tris-HCI Buffer, pH 7.5 (1 M)	Thermo Fisher Scientific	Cat#15567027
BamHI	NEB	Cat#R0136S
HindIII	NEB	Cat#R0104S
KOD Hot Start Master Mix	Merck	Cat#71842-3
Ulp1 protease	This paper	N/A
Critical commercial assays
JCSG + suite kit	NeXtal	Cat#130920
PEG ion kit	Hampton	Cat#HR2-139
QuikChange II Site-Directed Mutagenesis Kit	Agilent Technologies	Cat#200524
QIAquick Gel Extraction Kit	QIAGEN	Cat#28706X4
QIAprep Spin Miniprep Kit	QIAGEN	Cat#27106X4
Deposited data
Crystal structure of DDX21-AMPPNP-ssRNA	This paper	PDB: 6L5N
Crystal structure of DDX21-apo	This paper	PDB: 6L5L
Crystal structure of DDX21-ADP	This paper	PDB: 6L5O
Crystal structure of DDX21-AMP	Chen et al. (2020)	PDB: 6L5M
Oligonucleotides
DDX21 WT primer – forward 5′-gagaaca gattggtggatccttctctaattttcccatatctg-3′	This paper	N/A
DDX21 WT primer – reverse 5′-ggccgcaag cttgtcgacggagctcttatattcgtttgaacttaattc-3′	This paper	N/A
DDX21 del-loop primer – forward 5′-tgaacag gtggacctgattaaaacggcaataactgtgg-3′	This paper	N/A
DDX21 del-loop primer – reverse 5′-ccacagttattgccgttttaatcaggtccacctgttca-3′	This paper	N/A
U10 RNA: 5′-UUUUUUUUUU-3′	This paper	N/A
U15 RNA: 5′-UUUUUUUUUUUUUUU-3′	This paper	N/A
Recombinant DNA
Plasmid: pET28a(+)_DDX21 (188–563, WT)	This paper	N/A
Plasmid: pET28a(+)_DDX21 (188–563, del 401–405)	This paper	N/A
pFGET19_Ulp1	Guerrero et al. (2015)	Addgene Plasmid #64697
pSMT3 vector	Gift from Dr. Chris Lima, Sloan Kettering Institute	N/A
Software and algorithms
XDS	Kabsch (2010)	https://xds.mr.mpg.de/
HKL2000	Otwinowski and Minor (1997)	https://www.hkl-xray.com/download-instructions-hkl-2000
PHASER	McCoy et al. (2007)	https://www.phaser.io
REFMAC5	Murshudov et al. (2011)	https://www.ccp4.ac.uk/html/refmac5.html
Phenix	Adams et al., 2010	http://www.phenix-online.org/
Coot	Emsley et al., 2010	https://www2.mrc-lmb.cam.ac.uk/personal/pemsley/coot/
Other
Amicon® Ultra-15 Centrifugal Filter Unit (10 KDa MWCO)	Millipore	Cat#UFC901096
Amicon® Ultra-15 Centrifugal Filter Unit (30 KDa MWCO)	Millipore	Cat#UFC903096
Nanodrop 2000	Thermo Scientific	ND-2000
Novex™ 4%–20% Tris-Glycine Mini Gels	Thermo Fisher Scientific	Cat#EC6021BOX
Econo-Column Chromatography Column	Bio-Rad	Cat#7374151
D-Tube Dialyzers (15 mL, MWCO 3.5 kDa)	Millipore	Cat#71742
250 mL Glass Narrow-Mouth Erlenmeyer Flask	Thermo Fisher Scientific	Cat#FB500250
2 L Glass Narrow-Mouth Erlenmeyer Flask	Thermo Fisher Scientific	Cat#FB5006000
New Brunswick Innova 44 shaker	Eppendorf	Cat#M1282-0004
APV Laboratory Homogenizers	SPX FLOW	APV-1000
Tube Roller Mixer	AliExpress	Cat#MX-T6-S
Magnetic stirrer	Corning	PC-410
AKTA pure 25 M protein purification system	Cytiva	Cat. #29018226
Monolith NT.Automated Premium Capillary Chips	NanoTemper Technologies	Cat#MO-AK005
Monolith NT.Automated	NanoTemper Technologies	https://nanotempertech.com/zh_cn/monolith-automated/
Superdex 200 10/300 GL	Cytiva	Cat#17517501
HiLoad 16/600 Superdex 200 prep grade	Cytiva	Cat#28989335
T100 Thermal Cycler	Bio-Rad	Cat#1861096
Eppendorf 5810R Centrifuge	Eppendorf	Cat#5811000065
Eppendorf 5430R Centrifuge	Eppendorf	Cat#022620509
Sorvall™ RC 6 Plus Centrifuge	Thermo Scientific	Cat#46910
SS-34 Fixed-Angle Rotor	Thermo Fisher Scientific	Cat#28020TS
F10-6 × 500y Fixed-Angle Rotor	Thermo Fisher Scientific	Cat#78510TS
A-4-62 Rotor	Eppendorf	Cat#022638203
FA-45-30-11 Rotor	Eppendorf	Cat#022638009