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In the process of characterizing the requirements for expression of the essential immediate-early transcriptional activator (RTA) encoded by gene 50 of murine gammaherpesvirus 68 (MHV68), a recombinant virus was generated in which the known gene 50 promoter was deleted (G50pKO). Surprisingly, the G50pKO mutant retained the ability to replicate in permissive murine fibroblasts, albeit with slower kinetics than wild-type MHV68. 5-rapid amplification of cDNA ends analyses of RNA prepared from G50pKO-infected fibroblasts revealed a novel upstream transcription initiation site, which was also utilized during wild-type MHV68

Murine gammaherpesvirus 68 (␥HV68) infection of mice results in the establishment of a chronic infection, which is largely maintained through latent infection of B lymphocytes. Acute virus replication is almost entirelyBackground. Gammaherpesviruses are characterized by their ability to establish latency in lymphocytes. The establishment and maintenance of latency is important for persistence in the infected host, while virus reactivation is needed for transmission of the virus to new hosts and may also be required to maintain reservoirs of latently infected cells in the chronically infected host (2,12,15,33,34). The switch between latency and the lytic cycle for the human gammaherpesviruses, Epstein-Barr virus (EBV) and Kaposi's sarcoma-associated herpesvirus (KSHV), has been extensively characterized in established latently infected cell lines in vitro (11,27,42). Initiation of the EBV lytic cycle can be stimulated by several different reagents, including anti-immunoglobulin (anti-Ig), calcium ionophore, sodium butyrate, and the phorbol ester tetradecanoyl phorbol acetate (TPA) (27). KSHV reactivation can also be induced by stimulation with phorbol esters and sodium butyrate (3,19).Murine gammaherpesvirus 68 (␥HV68) is closely related to EBV and KSHV, and infection of mice with ␥HV68 provides a tractable animal model in which to study the pathobiology of

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