Читать книгу Principles of Virology - Jane Flint, S. Jane Flint - Страница 296

Structures of RdRPs alone or in combination with RNA templates and products have been solved for a large number of (+), (–), and double-stranded RNA viruses. The information collected has had enormous impact on our understanding of the mechanisms of template and primer binding, NTP selection and binding, catalysis, and chain translocation. More recently, the use of cryo-electron microscopy has led to the resolution of very complicated assemblies of replication complexes at atomic detail. A spectacular example is the resolution of the structure of the L protein of vesicular stomatitis virus: the 3.8-Å-resolution density map could be used to build an atomic model for nearly all of the 2,109-amino-acid protein chain. Despite this abundance of structural information, many unsolved questions remain, including how uridylylation of VPg can be accomplished by a second RNA polymerase molecule; the role of oligomerization in RNA polymerase function; and how independent functional domains work together to ensure that a correct RNA product is produced. Additional structures are needed to detail the conformational movements that take place during the switch between initiation and elongation, and the changes that occur as the polymerase moves from an open to a closed conformation.

RNA viral genetic diversity, and the ability to undergo rapid evolution, is made possible by errors made during nucleic acid synthesis, as well as genome recombination and reassortment. The importance of polymerase errors is underscored by the dramatic decrease in poliovirus fitness caused by a single amino acid change in the polymerase that decreases error rate. A different amino acid change in the RNA polymerase, which increases error frequency, has a similar effect. These observations demonstrate that the mutational diversity of RNA viruses is almost precisely where it must be, determined in large part by the error frequency of the RNA polymerase.

Many host proteins that are required for viral RNA synthesis have been identified, but their precise functions remain obscure. We now have the ability to identify cell proteins that are associated with RNA polymerases and to determine the effect on RNA synthesis when they are removed. Lacking are structural and mechanistic insights into how these proteins participate in RNA synthesis.