16 A Reverse Transcriptase for Cauliflower Mosaic Virus State of the Art, 1992

Abstract

Three papers appeared in 1983 proposing the involvement of reverse transcriptase in the replication of the plant DNA virus, cauliflower mosaic virus (CaMV) (Guilley et al. 1983; Hull and Covey 1983; Pfeiffer and Hohn 1983). This was soon after reverse transcription had been demonstrated for the hepadnavirus, duck hepatitis B virus (DHBV) (Summers and Mason 1982). The proposed replication scheme was to put together pieces of a puzzle that had been appropriated one by one during the last 10 years. As key predictions of the model have been since confirmed, it now finds general acceptance. Some details that still remain obscure are discussed below.

To place the subject in historical context, a major effort in plant research in 1975–1985 had been devoted to the development of gene vectors. Most of the emphasis was put on the T-DNA delivery system of the soil bacterium, Agrobacterium tumefaciens, but the alternative search for viral vectors was not without logic. Although the overwhelming majority of plant viruses have a plus-strand RNA genome, making them difficult to use as vectors, the following three classes are the few exceptions: the caulimoviruses (from cauliflower mosaic), the geminiviruses, and the recently discovered badnaviruses (from bacilliform DNA). Caulimoviruses have an 8-kb circular double-stranded genome packaged in an icosahedrical virion particle; they are transmitted by aphids naturally (Fig. 1A) but can also be propagated mechanically. These viruses induce formation of characteristic inclusion bodies in the cytoplasm of infected cells where virions accumulate in large numbers (Fig. 1). For general...