This is the 2B and 2B-like sub-domain found in TraI (EC:5.99.1.2) a relaxase of F-family plasmids. It contains four domains; a trans-esterase domain that executes the nicking and covalent attachment of the T-strand to the relaxase, a vestigial helicase domain (carrying the 2B/2B-like sub-domain) that operates as an ssDNA-binding domain, an active 5' to 3' helicase domain, and a C-terminal domain that functions as a recruitment platform for relaxosome components. The 2B sub-domains in TraI are formed by residues 625-773 in the vestigial helicase domain and residues 1255-1397 in the active helicase domain. The 2B/2B-like sub-domain interacts with ssDNA where it contributes to the surface area where ssDNA bind. In other words the ssDNA-binding site is located in a groove between the 2B and 2B-like parts of the sub-domain. The sub-domain parts appear to act as clamps holding the ssDNA in place, resulting in the ssDNA being completely surrounded by protein. In previous studies, the 2B/2B-like sub-domain of the TraI vestigial helicase domain has been identified as translocation signal A (TSA) since it contains sequences essential for the recruitment of TraI to the T4S system. Thus, the 2B/2B-like sub-domain plays two major roles in relaxase function: (1) interacting with the DNA and possibly promoting high processivity and (2) mediating recruitment of the relaxosome to the T4S system [1].

1: Cryo-EM Structure of a Relaxase Reveals the Molecular Basis of DNA Unwinding during Bacterial Conjugation. Ilangovan A, Kay CWM, Roier S, El Mkami H, Salvadori E, Zechner EL, Zanetti G, Waksman G; Cell. 2017;169:708-721. PMID:28457609