Key resources table.

Reagent type (species) or resource	Designation	Source or reference	Identifiers	Additional information
Plasmid construct (E. coli)	pET30a-SsoMCM (C642A)	McGeoch et al. (2005)
Plasmid construct (E. coli)	pET30a-SsoMCM (C682A)	McGeoch et al. (2005)
Plasmid construct (E. coli)	pET30a-SsoMCM 1–612 (G452C)	This paper		Site- directed mutagenesis using primers listed in Materials and methods
Plasmid construct (E. coli)	pET30a-SsoMCM1–612 (S456C)	This paper		Site- directed mutagenesis using primers listed in Materials and methods
Expression strain	Rosetta 2	Novagen
Chemical compound	4-azidophenacyl bromide (APB)	Sigma-Aldrich	57018-46-9
Chemical compound	ATP	Invitrogen	51963-61-2
Chemical compound	1-(p-Bromoacetamidobenzyl) ethylenediamine N, N,N (Fe-BABE)	Dojindo	186136-50-5
Chemical compound	DNaseI	New England Biolabs	M0303S
Chemical compound	Streptavidin	Invitrogen	800-955-6288
Chemical compound	Cy3 succinimidyl ester	ThermoFisher	57757-57-0
Chemical compound	Cy3 maleimide	ThermoFisher	45-001-273
Sequence-based reagent	DNA primers and substrates	Sigma-Aldrich and IDT		Refer to Materials and methods
Software, algorithm	Kaleidagraph	www.synergy.com	V4.5