One functional copy of the long terminal repeat gene specifying the immediate-early polypeptide IE 110 suffices for a productive infection of human foetal lung cells by herpes simplex virus.

The HSV-1/HSV-2 intertypic recombinant Bx1 (28-1) is heterotypic for the repeat sequences flanking the long unique region of the genome (IRL and TRL) and expresses both the immediate-early polypeptide IE 110 of HSV-1 and its functionally equivalent polypeptide IE 118 of HSV-2. The genome structures of five subclones of this recombinant and the immediate-early polypeptides they induce have been analysed. Subclone 14 lacked most of the IRL sequence, including the region from which part of the mRNA for IE 110 is transcribed, and expressed only the HSV-2 IE 118. Subclone 22 lacked almost all of TTL including the gene for IE 118 and induced only the HSV-1 IE 110. Since both subclones produced viable progeny in HFL cells it follows that expression of only one copy of the equivalent genes in TRL and IRL, here coding for the distinguishable polypeptides IE 110 or IE 118, is sufficient for successive complete cycles of virus replication.