Kaposi's sarcoma-associated herpesvirus (KSHV) is an oncogenic virus that causes lifelong
infection in humans by establishing latency after primary
infection.
Latent infection is a prerequisite for both
persistent infection and the development of KSHV-associated
cancers. While viral lytic genes are transiently expressed after primary
infection, their expression is significantly restricted and concomitant with the binding of host epigenetic repressors
Polycomb Repressive Complex 1 and 2 (PRC1 and PRC2) to lytic genes. PRC1 and PRC2 mediate the repressive histone marks H2AK119ub and H3K27me3, respectively, and maintain
heterochromatin structure on KSHV lytic genes to inhibit their expression. In contrast to PRC2, little is known about the recruitment and role of PRC1 factors on the KSHV genome following de novo
infection. Thus, the goal of this study was to examine the function of PRC1 factors in the establishment of KSHV latency. To address this question, we performed an
shRNA screen targeting 7 different components of the canonical and non-canonical PRC1 complexes during primary KSHV
infection. We found that RYBP, a main subunit of the non-canonical PRC1 complexes, is a potent repressor of KSHV lytic genes that can bind to the viral genome and inhibit lytic genes as early as 4 hours post
infection. Surprisingly, our ChIP analyses showed that RYBP binds to lytic viral gene promoters in a PRC1-independent manner, does not affect PRC1 activity on the KSHV genome, and can reduce the level of histone marks associated with transcription elongation. Our data also suggest that RYBP can repress the viral lytic cycle after primary
infection by inhibiting the transcription elongation of the lytic cycle inducer KSHV gene RTA. Based on our results we propose that RYBP uses a PRC1-independent mechanism to block
KSHV RTA expression thereby promoting the establishment of KSHV latency following de novo
infection.