The Autographa californica Nuclear Polyhedrosis Virus Homologous Region 1a: Identical Sequences Are Essential for DNA Replication Activity and Transcriptional Enhancer Function

Hr1a consists of two 30-bp imperfect palindrome sequences separated by 58 bp and each palindrome contains a naturally occurring EcoRI site at its core. Plasmid subclones of the hr1a-containing AcMNPV Hin dIII-N fragment were examined for their ability to replicate in virus-infected (Spodoptera frugiperda) Sf9 cells, and to stimulate transcription when linked in cis with a 39K gene promoter-β-glucuronidase fusion and cotransfected into cells along with a plasmid (ple-1) containing the gens encoding the trans-acting factor 1E-1. Only those plasmids containing hr1a underwent infection-dependent replication and were able to stimulate transcription. Sequences mapping to the left of hr 1a were required for maximal levels of replication. A single palindrome from hr1a a was sufficient for supporting plasmid replication and for stimulating transcription, although both activities were more efficient when both palindromes were present. Plasmids containing only one-half of a palindrome or disruptions of the central EcoRI core either did not replicate, or replicated very poorly, and did not exhibit enhanced transcriptional activity from the 39K gene promoter. Gel retardation experiments showed that labeled hr1a-containing DNA fragments had retarded migration after incubation with extracts from cells transfected with ple-1. Supershift experiments using polyclonal antibodies to IE-1 indicated that IE-1 is a component of the protein complex bound to hr1a. Fragments containing disruptions of the EcoRI-core still bound IE-1, as shown by gel retardation assays, indicating that IE1 binding alone is not sufficient to allow replication and transcriptional enhancement.