The reverse transcriptase gene of cauliflower mosaic virus is translated separately from the capsid gene

Cauliflower mosaic virus (CaMV) possesses start codons at the beginning of its reverse transcriptase (RT) gene (ORF V) suggesting that, unlike in retroviruses and retrotransposons, it is translated independently from the capsid gene (ORF IV). To test this hypothesis a mutational analysis of the CaMV ORF IV/V overlapping region was performed. Mutants in which both ORFs are separated by stop codons in all three reading frames are viable and stable, while mutations affecting the first two AUG codons of ORF V are either lethal or unstable, giving rise to true and second site reversions. Mutants in which the AUG codons were replaced by ACG or AAG reverted only slowly and ACG could direct the synthesis of small amounts of reporter enzyme in transfected plant protoplasts, showing that this codon can act in plant cells as a weak start codon. CaMV has apparently developed a strategy for translation of the RT gene different from that in retroviruses and retrotransposons, but similar to that of hepadnaviruses, another group of pararetroviruses. The separate translation of the RT gene as a common feature of pararetroviruses might reflect the difference in their life cycle in comparison with retroviruses.