Negative Feedback Speeds Transcriptional Response-Time In Human Cytomegalovirus

Upon infection of a cell, viruses initiate a complex gene-expression cascade that may result in productive/lytic infection, abortive infection, persistence, or latency depending on the state, cell-type, and environment of that particular cell. However, the regulatory networks underlying these diverse viral lifecycle fates are typically studied using experiments that are averaged over cell populations, potentially masking the dynamic behavior in individual cells. To understand how these diverse viral lifecycle fates are regulated at the single-cell level, we present a framework for quantitatively determining viral expression dynamics and regulatory circuit architectures in individual living cells using a simplified model of a herpesvirus signal transduction module: the human cytomegalovirus (HCMV) Major Immediate Early (MIE) circuit. The system utilizes time-lapse fluorescence video-microscopy of cells infected with recombinant viruses expressing fluorescent gene-products and quantitative modeling to analyze the resulting single-cell data. The combined computational-experimental approach revealed two previously unseen transient signal-processing characteristics of the MIE circuit: (i) negative feedback within the MIE circuit, counter-intuitively, speeds the gene-expression response-time of the essential viral transactivator gene-product IE2-86 (ii) transcriptional activators implicated in reactivation from latency (e.g. trichostatin A) generate a pulse in IE2-86 gene-expression in single-cells. As predicted by the model, mutational deletion of the MIE negative-feedback loop eliminated both the IE2-86 pulse and the accelerated response-time during viral infection. We propose that the negative-feedback architecture of the HCMV MIE circuit may allow the virus to respond quickly to external signals and outpace cellular innate defenses at the single-cell level.