Positive strands to the rescue again: a segmented negative-strand RNA virus derived from cloned cDNAs.

Abstract

The very large group of negative-strand RNA viruses* includes some of the most serious and notorious pathogens of great medical and economic importance (1). These viruses are subdivided into those with segmented, negative-strand genomic RNAs [the Orthomyxoviridae (e.g., influenza viruses), Bunyaviridae (>300 viruses, including Bunyamwera), and Arenaviridae (e.g., Lassa fever)] and those with nonsegmented, negative-strand RNA genomes [Rhabdoviridae (e.g., vesicular stomatitis virus (VSV) and rabies), Paramyxoviridae (e.g., measles and respiratory syncytial virus), and Filoviridae (Marburg and Ebola viruses). The paper by Bridgen and Elliott (2) in this issue of the Proceedings is an important milestone because it reports the first complete recovery of a segmented, negative-strand RNA virus, the Bunyamwera virus, starting from DNA copies of the genome segments. Interestingly, in achieving this recovery, the authors applied a methodology that was developed for the nonsegmented viruses rather than the helper virus methodology developed for the segmented influenza virus. This system will now allow detailed molecular genetic analysis of all aspects of bunyavirus replication and could prove important in vaccine development as well. The rescue of Bunyamwera virus from DNA has its roots in a technical revolution that has occurred in negative-strand RNA virology over the past several years. Understanding of the rescue system requires a brief history of the baroque methods already applied to recovery of other negative-strand RNA viruses from DNA copies. Analysis of both positive- and negative-strand RNA viruses using directed mutagenesis requires that one be able to recover the virus from a DNA copy of the RNA genome. This breakthrough came early with positive-strand RNA viruses, where the genomic RNA itself acts as mRNA and is infectious. Plasmid DNAs encoding positive-strand RNA genomes of the bacteriophage Qβ or poliovirus yielded RNAs that would regenerate complete infectious viruses (3, 4), and this has now been achieved …