Unraveling the mystery of continuous and discontinuous transcription of coronaviruses In the past, human coronaviruses are common pathogens for mild upper respiratory tract infection. However, in recent years, new coronavirus pandemics, including severe acute respiratory syndrome (SRAS) and Middle East respiratory syndrome (MERS), claimed many lives with high mortality rates. As coronavirus pandemics pose an increasing threat in human society, drug development is an urgent issue. Before discussing the development of novel antiviral agents, a detailed understanding of coronavirus replication cycle is needed. Coronaviruses are enveloped viruses with positive-sense, single strand RNA genomes. The genome size is around 30000 base pairs, as one of the largest RNA viruses. After the viral particles enter the host cells, the genomic RNA is released into cytosol and translated into nonstructural viral proteins essential for viral replication, including the RNA polymerase. Several subgenomic RNAs with different lengths are then synthesized by the viral RNA polymerase, which predominantly encode the viral structural proteins, including spike (S), envelope (E), membrane (M), and nucleocapsid (N) proteins, for subsequent virion assembly and release. With merely one viral genomic RNA template, how can the RNA polymerase generate several subgenomic RNAs with different lengths? The production of subgenomic RNAs is not attributed by post-transcription splicing. Instead, it is generated by a base pairing of the specific body transcription regulatory sequence (TRS), which locates in front of each structural gene, with the complementary leader TRS at the 5’-UTR of viral genome. During the synthesis of negative-strand RNA, the viral RNA polymerase can recognize the body TRS and proceed the template switching event. This unique process is the so-called “discontinuous transcription”, a unique characteristic of coronaviruses. Whenever the polymerase encounters a TRS, it has to decide whether to stop for shorter subgenomic RNAs or to proceed for longer subgenomic RNAs or even the genomic RNA. However, the mechanism behind the decision-making was still poorly understood. In October 2014, Professor Yeh and Dr. Wu published their study on the mechanism for the transition from discontinuous to continuous transcription in coronaviruses, in Cell Host & Microbe. The research team first identified the glycogen synthase kinase-3 (GSK-3) in the host cells to be responsible for the viral nucleocapsid phosphorylation. GSK-3 inhibition selectively reduces the generation of genomic RNA and longer subgenomic RNAs, but not shorter subgenomic RNAs. Thus, they concluded that the phosphorylated nucleocapsid protein plays an important role in regulating the transition from discontinuous to continuous transcription. In addition, the research team also found that phosphorylated nucleocapsid allows recruitment of the cellular RNA helicase DDX1 to bind with the viral genome, which facilitates template readthrough and enables longer subgenomic RNA synthesis. This study sheds light on the key mechanism behind continuous and discontinuation RNA synthesis in coronaviruses. Since this mechanism is conserved among most species of coronaviruses, including SARS-CoV and MERS-CoV, it thus becomes a highly potential target for novel antiviral drug development. Reference Chia-Hsin Wu, Pei-Jer Chen, and Shiou-Hwei Yeh. Nucleocapsid phosphorylation and RNA helicase DDX1 recruitment enables coronavirus transition from discontinuous to continuous transcription. Cell Host Microbe, 16 (2014), pp. 462–472. DOI: 10.1016/ j.chom.2014.09.009. Professor Shiou-Hwei Yeh Department of Microbiology shyeh@ntu.edu.tw