Genome Organization and Expression of Reverse Transcribing Elements: Variations and a Theme

Introduction. Until recently, reverse transcription was considered to be the sole prerogative of retroviruses. Over the last 4 or 5 years it has been recognized that members of two other virus groups, the hepadnaviruses and the caulimoviruses, undergo reverse transcription during replication. Furthermore, some vertebrate genetic elements, e.g. intracisternal A particle (IAP) genes (Ono et al., 1985), VL30 genes (Hodgson et al., 1983) and L1Md (Loeb et al., 1986), and transposable elements from other taxonomic groups [yeast Ty elements (Clare & Farabaugh, 1985; Hauber et al., 1985), Drosophila copia (Mount & Rubin, 1985) and copia-like elements (Saigo et al., 1984), Dictyostelium DIRS-1 element (Cappello et al., 1985), maize Bs1 element (Johns et al., 1985), and possibly maize Cin1 element (Shepherd et al., 1984)] have been shown to possess structural similarities to integrated retroviruses. The yeast Ty element transcript has recently been found to be contained within virus-like particles which have reverse transcriptase activity (Garfinkel et al., 1985; Mellor et al., 1985a).