terminase

Terminase is a multi-subunit enzyme complex essential for DNA packaging in tailed bacteriophages and some plasmids. It typically consists of a small subunit (TerS) and a large subunit (TerL). TerS binds to specific DNA sequences within concatemeric phage genomes, recognizing packaging signals and helping initiate assembly of the packaging motor. TerL provides the energy and catalytic activities required for DNA translocation and processing; it contains an ATPase motor domain and a nuclease domain.

During packaging, terminase assembles at a procapsid via interaction with the portal complex and forms the DNA translocation motor. TerL hydrolyzes ATP to drive the translocation of DNA into the capsid, while its nuclease activity makes the initial and/or terminal cuts to generate genome-length units. In pac-type systems, a start signal at pac triggers initiation, and a second cut terminates packaging once the capsid is full. In cos-type systems, specific cleavage at cos sites produces cohesive ends before or during packaging. The process is tightly coordinated with procapsid maturation and the portal–terminase interface.

Structure and evolution: TerL often contains separate domains for ATP hydrolysis and nuclease activity, whereas TerS specializes in DNA recognition and assembly of the motor. Terminases show diversity across caudoviruses (tailed phages) and related elements, with variations in subunit composition and signal recognition.

Function and significance: Terminases are central to the lifecycle of many phages, enabling genome maturation and assembly of infectious particles. They are widely studied as models of DNA translocation motors and, because of their specificity, are of interest in phage biology and potential biotechnological applications.