Latency is characteristic behavior of many viruses that persist in their hosts. During latency an intact viral genome resides in the cell, but most viral genes are silent. Diverse inducing stimuli, by which the environment interacts with the host cell, trigger the latent viral genome into replication. Experiments are described that led to the identification of a crucial Epstein-Barr virus (EBV) gene, called ZEBRA, which acts as a switch between latency and replication of this herpes virus. During latency of EBV, ZEBRA expression is repressed. Inducing stimuli cause synthesis of ZEBRA which, in turn, activates expression of several individual EBV early genes. The ZEBRA polypeptide is a site-specific DNA binding protein that is likely to function as a transcriptional transactivator. The recognition that a single viral product may control the latency-to-replication switch offers promise of a detailed understanding of a central process in virology.

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