This domain is found in Protelomerase from Klebsiella phage phiKO2 (TelK) and similar proteins from tailed bacteriophages and bacterial prophages mainly from Gammaproteobacteria. TelK is a hairpin telomere resolvase with DNA cleavage-rejoining activity that resolves the catenated circular dimer produced from the bidirectional replication of a chromosome. The core of the TelK monomer consists of an N-terminal domain and a catalytic domain connected by a long alpha- helical linker. However, there are two extensions of the protein: a C-terminal DNA binding domain named the stirrup (Pfam:PF20818) and an N-terminal domain. The latter has an insertion termed the muzzle (this entry), which folds into two long alpha-helices. This protruding element is packed against the DNA-binding catalytic and N-terminal domains of the opposite subunit in each dimer [1].

1: An interlocked dimer of the protelomerase TelK distorts DNA structure for the formation of hairpin telomeres. Aihara H, Huang WM, Ellenberger T; Mol Cell. 2007;27:901-913. PMID:17889664