Component of the molecular motor that translocates viral genomic DNA in empty capsid during DNA packaging. Forms a tripartite terminase complex together with TRM2 and TRM3 in the host cytoplasm. Once the complex reaches the host nucleus, it interacts with the capsid portal vertex. This portal forms a ring in which genomic DNA is translocated into the capsid. TRM1 carries an endonuclease activity that plays an important role for the cleavage of concatemeric viral DNA into unit length genomes. Associates with TRM2 and TRM3 to form the tripartite terminase complex. Interacts with portal protein. Found associated with the external surface of the viral capsid during assembly and DNA packaging, but seems absent in extracellular mature virions. Belongs to the herpesviridae TRM1 protein family.