The Elk-1 and Serum Response Factor binding sites in the Major Immediate-Early Promoter of the Human Cytomegalovirus are required for efficient viral replication in quiescent cells and compensate for inactivation of the NF-kB sites in proliferating cells [Caposio*, Luganini* co-first author]

The Major Immediate-Early Promoter (MIEP) region of human cytomegalovirus (HCMV) plays a critical role in the regulation of lytic and latent infections by integrating multiple signals supplied by the infecting virus, the type and physiological state of the host cell and its extracellular surroundings. The interaction of cellular transcription factors with their cognate binding sites, present at high densities within the enhancer element upstream from the MIEP core promoter, regulate the rate of IE gene transcription and thus affect the outcome of HCMV infection. We have previously shown that the NF-κB binding sites within the MIEP enhancer and cellular NF-κB activity induced by HCMV infection are required for efficient MIEP activity and viral replication in quiescent cells. We now show that inactivation of either Elk-1 or serum response factor (SRF) binding site within the enhancer also reduces MIEP activation and viral replication of recombinant HCMV viruses in quiescent fibroblasts. In these cells, we show that the expression of either Elk-1 or SRF is required for optimal IE gene expression, and that HCMV-stimulated activation of the MEK1/2-ERK1/2 signaling axis leads to Elk-1 transcriptional competency. Furthermore, the replication kinetics of recombinant viruses in which NF-κB, Elk-1 and SRF binding sites are all inactivated, demonstrate that the higher levels of Elk-1 and SRF binding to MIEP in proliferating cells can even compensate for a lack of HCMV-induced NF-κB-mediated MIEP transactivation. These observations highlight the importance of the combination of different MIEP binding sites to optimize IE gene expression in cells in different physiological states.