The viral immediate-early transactivator Rta/Orf50 is necessary and sufficient to initiate Kaposi's sarcoma-associated herpesvirus/human herpesvirus 8 (KSHV/HHV-8) reactivation from latently infected cells. Since Rta/Orf50 is conserved among all known gamma-2-herpesviruses, we investigated whether the murine gamma-68-herpesvirus (MHV-68) and rhesus monkey rhadinovirus (RRV) homologs can functionally substitute for KSHV Rta/Orf50. (i) Our comparison of 12 KSHV promoters showed that most responded to all three Rta/Orf50proteins, but three promoters (vGPCR, K8, and gB) responded only to the KSHV Rta/Orf50 transactivator. Overall, the activation of KSHV promoters was higher with KSHV Rta than with the RRV and MHV-68 Rta. (ii) Only the primate Rta/Orf50 homologs were able to interfere with human p53-depedent transcriptional activation. (iii) Transcriptional profiling showed that the KSHV Rta/Orf50 was more efficient than it's homologs in inducing KSHV lytic transcription from the latent state. These results suggest that the core functionality of Rta/Orf50 is conserved and independent of its host, but the human protein has evolved additional, human-specific capabilities.