Endonuclease G, a Candidate Human Enzyme for the Initiation of Genomic Inversion in Herpes Simplex Type 1 Virus*

Abstract

The herpes simplex virus type 1 (HSV-1)a sequence is present as a direct repeat at the two termini of the 152-kilobase viral genome and as an inverted repeat at the junction of the two unique components L and S. During replication, the HSV-1 genome undergoes inversion of L and S, producing an equimolar mixture of the four possible isomers. Isomerization is believed to result from recombination triggered by breakage at the asequence, a recombinational hot spot. We have identified an enzyme in HeLa cell extracts that preferentially cleaves the asequence and have purified it to near homogeneity. Microsequencing showed it to be human endonuclease G, an enzyme with a strong preference for G+C-rich sequences. Endonuclease G appears to be the only cellular enzyme that can specifically cleave the asequence. Endonuclease G also showed the predicted recombination properties in an in vitro recombination assay. Based on these findings, we propose that endonuclease G initiates thea sequence-mediated inversion of the L and S components during HSV-1 DNA replication.