Multiple Lytic Origins of Replication Are Required for Optimal Gammaherpesvirus Fitness In Vitro and In Vivo

Abstract

An unresolved question in herpesvirus biology is why some herpesviruses contain more than one lytic origin of replication (oriLyt). Using murine gammaherpesvirus 68 (MHV-68) as model virus containing two oriLyts, we demonstrate that loss of either of the two oriLyts was well tolerated in some situations but not in others both in vitro and in vivo. This was related to the cell type, the organ or the route of inoculation. Depending on the cell type, different cellular proteins, for example Hexim1 and Rbbp4, were found to be associated with oriLyt DNA. Overexpression or downregulation of these proteins differentially affected the growth of mutants lacking either the left or the right oriLyt. Thus, multiple oriLyts are required to ensure optimal fitness in different cell types and tissues.

Author Summary

Herpesviruses show two stages in their life cycle: lytic replication and latency. Lytic DNA replication is initiated at a defined site on the viral genome, the so-called lytic origin of replication (oriLyt). While some herpesviruses have a single oriLyt, others have multiple oriLyts. Why some herpesviruses need more than one oriLyt is not known. This study demonstrates that the presence of multiple oriLyts enables gammaherpesviruses to efficiently establish infection in different cell or tissue types and during different phases of the viral life cycle. Depending on the cell type, different cellular proteins were found to be associated with oriLyt DNA, and overexpression or downregulation of these proteins differentially affected the growth of viruses containing only a single oriLyt. Thus, multiple oriLyts ensure optimal viral fitness in different cell types and tissues.