The roles of eighteen baculovirus late expression factor genes in transcription and DNA replication.

A set of 18 plasmid subclones of the Autographa californica nuclear polyhedrosis virus genome supports expression from a late viral promoter in transient expression assays (J. W. Todd, A. L. Passarelli, and L. K. Miller, J. Virol. 69:968-974, 1995). Using this set of plasmids, we have assigned a role for each of the 18 genes required for optimal late gene expression with respect to its involvement at the levels of transcription, translation, and/or DNA replication. RNase protection analyses demonstrated that all of the known late expression factor genes (lefs) affected the steady-state level of reporter gene RNA. Thus, none of the lefs appeared to be specifically involved in translation. A subset of the lefs supported plasmid replication; ie-1, lef-1, lef-2, lef-3, p143, and p35 were essential for plasmid replication, while ie-n, lef-7, and dnapol had stimulatory effects. The predicted sequence of lef-7 suggests that it is a homolog of herpesvirus single-stranded DNA-binding protein (UL29). The role of p35 in plasmid replication appears to be suppression of apoptosis, because p35 could be functionally replaced in the replication assay by either Cp-iap or Op-iap, two heterologous baculovirus genes which suppress apoptosis by a mechanism which appears to differ from that of p35. Thus, one or more of the replication-related lefs or the process of plasmid replication appears to induce cellular apoptosis. Our results indicate that the remaining lefs, lefs 4 through 11, p47, and 39K (pp31), function either at the level of transcription or at that of mRNA stabilization.