Rep-Type endonuclease

Cress-type Rep endonucleases are sequence-specific DNA binding proteins that recognize an ssDNA hairpin origin of replication. It is involved in the initiation of rolling circle replication by cleaving a strand of circular dsDNA, thereby releasing a 3'OH strand for polymerase initiation.

Enzymatic reaction: All rep-type endonuclease belongs to the HUH-endonuclease superfamily. There are three class: Cress-type (Cressdnaviricota and Alphasatellitidae), Parvo -type (Piccovirales) and prokaryotic-type (Tubulavirales)

Tyr-endonuclease

Domain: Cress-type PFAM: PF02407
Parvo-type PFAM: PF01057
Ino-type PFAM: PF05144
Micro-type PFAM: PF05840

Mechanism : The active site consists of an Tyrosine, a triad HUH or HUG (Histidine, hydrophobic, Histidine/Glutamine) and a Mg+ ion. The Tyr acts as a nucleophile and attacks the Phosphate atom of ssDNA strand at the top of an hairpin, while the mg+ ion assists the process by stabilizing the reaction intermediates. The result releases a 3'OH ssDNA strand, while the the other 5'Phosphate strand remains bound to the protein.