InterPro domain: IPR011856

This superfamily represents a structural domain found in three types of endonucleases: TsnA endonuclease (N-terminal) [ 1 ], Hjc-type resolvase [ 2 ], and tRNA-intron endonuclease (C-terminal) ( 3.1.27.9 ) [ 3 ]. These domains have a 3-layer alpha/beta/alpha topology, which is similar in structure to a motif found in several restriction endonucleases.

TsnA endonuclease is a catalytic component of the Tn7 transposition system. Tn7 transposase is composed of four proteins: TnsA, TnsB, TnsC and TsnD. DNA breakage at the 5' end of the transposon is carried out by TnsA, and breakage and joining at the 3' end is carried out by TnsB. TnsC is the molecular switch that regulates transposition. The N-terminal domain of TnsA is catalytic.

Hjc is a type of Holliday junction resolvase. The Holliday junction is an essential intermediate of homologous recombination, comprising four-stranded DNA complexes that are formed during recombination and related DNA repair events. During homologous recombination, genetic information is physically exchanged between parental DNAs via crossing single strands of the same polarity within the four-way Holliday structure. Hjc is an archaeal endonuclease, which specifically resolves the junction DNA to produce two separate recombinant DNA duplexes. This process is terminated by the endonucleolytic activity of resolvases, which convert the four-way DNA back to two double strands.

tRNA-intron endonucleases cleave pre-tRNA producing 5'-hydroxyl and 2',3'-cyclic phosphate termini, and specifically removing the intron. The splicing of transfer RNA precursors is similar in Eucarya and Archaea. In both kingdoms an endonuclease recognises the splice sites and releases the intron, but the mechanism of splice site recognition is different in each kingdom.

1. The carboxy-terminal portion of TnsC activates the Tn7 transposase through a specific interaction with TnsA. EMBO J. 23, 2972-81
2. Crystal structure of the archaeal holliday junction resolvase Hjc and implications for DNA recognition. Structure 9, 197-204
3. Crystal structure and evolution of a transfer RNA splicing enzyme. Science 280, 279-84