This is an alpha and beta domain found in polyvalent proteins of conjugative element or often in the neighbourhood of one [2]. The KfrB domain has been speculated to play a role in discrimination of self from non-self in plasmid conjugation systems [2].

1: Polyvalent Proteins, a Pervasive Theme in the Intergenomic Biological Conflicts of Bacteriophages and Conjugative Elements. Iyer LM, Burroughs AM, Anand S, de Souza RF, Aravind L; J Bacteriol. 2017; [Epub ahead of print] PMID:28559295

2: Multiple enzymatic activities of ParB/Srx superfamily mediate sexual conflict among conjugative plasmids. Maindola P, Raina R, Goyal P, Atmakuri K, Ojha A, Gupta S, Christie PJ, Iyer LM, Aravind L, Arockiasamy A; Nat Commun. 2014;5:5322. PMID:25358815