Cytoplasmic gene expression: lessons from poxviruses

In eukaryotic cells, the process of gene expression is confined to the nucleus and enabled by multisubunit RNA polymerases (RNAPs). Many viruses make use of the host cellular gene expression apparatus during infection, and hence transfer their genome at least transiently to the host nucleus. However, poxviruses have evolved a different strategy to propagate. Their double-stranded DNA genome is transcribed in the host cytoplasm by a virus-encoded RNAP (vRNAP), which is evolutionarily related to eukaryotic RNA polymerase II. In this Review, we highlight recent high-resolution structures of the poxviral transcription apparatus in different phases of action. These structures, along with biochemical data, now allow the definition of a comprehensive model of poxviral gene expression and its regulation.