Coronaviruses (CoVs) stand out among RNA viruses because of their unusually large genomes (∼30 kb) associated with low mutation rates. CoVs code for nsp14, a bifunctional enzyme carrying RNA cap guanine N7-methyltransferase (MTase) and 3'-5' exoribonuclease (ExoN) activities. ExoN excises nucleotide mismatches at the RNA 3'-end in vitro, and its inactivation in vivo jeopardizes viral genetic stability. Here, we demonstrate for severe acute respiratory syndrome (SARS)-CoV an RNA synthesis and proofreading pathway through association of nsp14 with the low-fidelity nsp12 viral RNA polymerase. Through this pathway, the antiviral compound ribavirin 5'-monophosphate is significantly incorporated but also readily excised from RNA, which may explain its limited efficacy in vivo. The crystal structure at 3.38 Å resolution of SARS-CoV nsp14 in complex with its cofactor nsp10 adds to the uniqueness of CoVs among RNA viruses: The MTase domain presents a new fold that differs sharply from the canonical Rossmann fold.

Foreign DNA molecules and chromosomal fragments are generally eliminated from proliferating cells, but we know little about how mammalian cells prevent their propagation. Here, we show that dividing human and canine cells partition transfected plasmid DNA asymmetrically, preferentially into the daughter cell harboring the young centrosome. Independently of how they entered the cell, most plasmids clustered in the cytoplasm. Unlike polystyrene beads of similar size, these clusters remained relatively immobile and physically associated to endoplasmic reticulum-derived membranes, as revealed by live cell and electron microscopy imaging. At entry of mitosis, most clusters localized near the centrosomes. As the two centrosomes split to assemble the bipolar spindle, predominantly the old centrosome migrated away, biasing the partition of the plasmid cluster toward the young centrosome. Down-regulation of the centrosomal proteins Ninein and adenomatous polyposis coli abolished this bias. Thus, we suggest that DNA clustering, cluster immobilization through association to the endoplasmic reticulum membrane, initial proximity between the cluster and centrosomes, and subsequent differential behavior of the two centrosomes together bias the partition of plasmid DNA during mitosis. This process leads to their progressive elimination from the proliferating population and might apply to any kind of foreign DNA molecule in mammalian cells. Furthermore, the functional difference of the centrosomes might also promote the asymmetric partitioning of other cellular components in other mammalian and possibly stem cells.foreign DNA | asymmetric cell division | centrosome | endoplasmic reticulum | Ninein G enerally, noncentromeric DNA molecules are mitotically instable in eukaryotes. This results in their apparent disappearance from an ever-increasing proportion of the progeny of an affected cell (e.g., 1-3). Endogenous sources of such DNA are recombination byproducts [double minutes, extrachromosomal ribosomal (r)DNA circles (ERCs) and other DNA circles (3-6)] or mitotic defects generating noncentromeric chromosomal fragments and cytoplasmic micronuclei (1, 7). Exogenous sources are DNA of pathogens or DNA, typically plasmids, artificially introduced into cells. For the latter, decades of work established that plasmid-born protein expression is transient, persisting only for a few cell cycles (8). This finding is consistent with plasmid DNA being somehow eliminated through divisions. Thus, some mechanisms seem to prevent the propagation of foreign DNA and extrachromosomal DNA in proliferating eukaryotic cells. However, how this is achieved is unclear.In animal cells, DNA sensors mediate the early detection of exogenous DNA, such as DNA of invading pathogens and artificially introduced DNA (9-11). Both in leukocytes and nonprofessional immune cells, these can trigger innate immune responses, such as cytokine production, autophagy, and apoptosis (9). However, what happens to the DNA molecules themselves over time is unclear. When microi...