Nuclear sites of herpes simplex virus type 1 DNA replication and transcription colocalize at early times postinfection and are largely distinct from RNA processing factors.

We have visualized the intracellular localization of herpes simplex virus (HSV) type 1 replication and transcription sites in infected HeLa cells by using direct labelling methods. The number of viral transcription foci increases in a limited way; however, the number of replication sites increases in a near-exponential manner throughout infection, and both replication and transcription sites are found buried throughout the nuclear interior. Simultaneous visualization of viral transcription and replication foci shows that the two processes colocalize at early times, but at later times postinfection, there are additional sites committed solely to replication. This contrasts with the situation in adenovirus-infected cells in which, throughout replication, sites of transcription are adjacent to but do not colocalize with sites of viral DNA replication. The data for an increase in HSV transcription sites suggest an initial phase of replication of input genomes which are then transcribed. Sites of HSV replication colocalize with viral DNA replication and packaging proteins but are largely distinct from the punctate distribution of small nuclear ribonucleoprotein particles. Very high multiplicities of infection have shown an upper limit of some 18 viral transcription foci per nucleus, suggesting cellular constraints on transcription site formation. Use of virus replication mutants confirms that the labelled foci are sites of viral RNA and DNA synthesis; in the absence of viral DNA replication functions, no replication foci and only a limited number of transcription foci were present. Absence of a packaging function had no apparent effect on transcription or replication site formation, illustrating that DNA packaging is not a prerequisite for ongoing DNA synthesis. Further, the essential HSV protein IE63 is required for efficient replication site formation at later times postinfection but is not required for transcription foci formation.