A Domain of Herpes Simplex Virus pU(L)33 Required To Release Monomeric Viral Genomes from Cleaved Concatemeric DNA

Monomeric herpesvirus DNA is cleaved from concatemers and inserted into preformed capsids through the actions of the viral terminase. The terminase of herpes simplex virus (HSV) is composed of three subunits encoded by U(L)15, U(L)28, and U(L)33. The U(L)33-encoded protein (pU(L)33) interacts with pU(L)28, but its precise role in the DNA cleavage and packaging reaction is unclear. To investigate the function of pU(L)33, we generated a panel of recombinant viruses with either deletions or substitutions in the most conserved regions of U(L)33 using a bacterial artificial chromosome system. Deletion of 11 amino acids (residues 50 to 60 or residues 110 to 120) precluded viral replication, whereas the truncation of the last 10 amino acids from the pU(L)33 C terminus did not affect viral replication or the interaction of pU(L)33 with pU(L)28. Mutations that replaced the lysine at codon 110 and the arginine at codon 111 with alanine codons failed to replicate, and the pU(L)33 mutant interacted with pU(L)28 less efficiently. Interestingly, genomic termini of the large (L) and small (S) components were detected readily in cells infected with these mutants, indicating that concatemeric DNA was cleaved efficiently. However, the release of monomeric genomes as assessed by pulsed-field gel electrophoresis was greatly diminished, and DNA-containing capsids were not observed. These results suggest that pU(L)33 is necessary for one of the two viral DNA cleavage events required to release individual genomes from concatemeric viral DNA. IMPORTANCE This paper shows a role for pU(L)33 in one of the two DNA cleavage events required to release monomeric genomes from concatemeric viral DNA. This is the first time that such a phenotype has been observed and is the first identification of a function of this protein relevant to DNA packaging other than its interaction with other terminase components.