Rescue of measles viruses from cloned DNA.

A system has been established allowing the rescue of replicating measles viruses (MVs) from cloned DNA. On one hand, plasmids were constructed from which MV antigenomic RNAs with the correct termini are transcribed by phage T7 RNA polymerase. On the other hand, helper cells derived from the human embryonic kidney 293 cell line were generated constitutively expressing T7 RNA polymerase together with MV nucleocapsid protein and phosphoprotein. Simultaneous transfection of the helper cells with the MV antigenomic plasmid and with a plasmid encoding the MV polymerase under direction of a T7 promoter led to formation of syncytia from which MVs were easily recovered. A genetic tag comprising three nucleotide changes was present in the progeny virus. As a first application of reverse genetics, a segment of 504 nucleotides from the 5' non-coding region of the fusion gene was deleted, leading to an MV variant whose replication behaviour in Vero cells was indistinguishable from that of the laboratory Edmonston B strain. Since no helper virus is involved, this system, in principle, should be applicable to the rescue of any member of the large virus order Mononegavirales, i.e. viruses with a nonsegmented negative-strand RNA genome.     

人类及动物RNA病毒的反向遗传系统

反向遗传系统可以对RNA病毒直接进行遗传操作,为RNA病毒的分子生物学研究提供了一种强大的工具。在过去20年,特别是自90年代中期第一例负链RNA病毒感染性克隆构建成功以来,动物RNA病毒的分子生物学研究取得了长足的进展,这很大程度上归功于各种动物RNA病毒反向遗传系统的建立。这里系统总结了人类及动物非反转录RNA病毒中各类代表性成员在建立反向遗传系统时的方案设计、遇到的困难及研究者如何克服这些困难。分类讨论到的代表性病毒种属有脊髓灰质炎病毒、冠状病毒（包括SARS病毒）、黄病毒、野田村病毒、流感病毒、传染性法氏囊病病毒以及呼肠孤病毒等。     

Evolution of mutational robustness in an RNA virus

Mutational (genetic) robustness is phenotypic constancy in the face of mutational changes to the genome. Robustness is critical to the understanding of evolution because phenotypically expressed genetic variation is the fuel of natural selection. Nonetheless, the evidence for adaptive evolution of mutational robustness in biological populations is controversial. Robustness should be selectively favored when mutation rates are high, a common feature of RNA viruses. However, selection for robustness may be relaxed under virus co-infection because complementation between virus genotypes can buffer mutational effects. We therefore hypothesized that selection for genetic robustness in viruses will be weakened with increasing frequency of co-infection. To test this idea, we used populations of RNA phage φ6 that were experimentally evolved at low and high levels of co-infection and subjected lineages of these viruses to mutation accumulation through population bottlenecking. The data demonstrate that viruses evolved under high co-infection show relatively greater mean magnitude and variance in the fitness changes generated by addition of random mutations, confirming our hypothesis that they experience weakened selection for robustness. Our study further suggests that co-infection of host cells may be advantageous to RNA viruses only in the short term. In addition, we observed higher mutation frequencies in the more robust viruses, indicating that evolution of robustness might foster less-accurate genome replication in RNA viruses.