Initiation of translation of the cauliflower mosaic virus genome from a polycistronic mRNA: evidence from deletion mutagenesis.

Mutants with deletions in the open reading frame (ORF) VII to I region of the cauliflower mosaic virus genome were constructed in vitro and tested for infectivity by inoculation of plants. Sequences within ORF VII and the intergenomic region between ORFs VII and I are not required for viral infectivity. Nevertheless, the location of translation initiation and termination codons within this region governs the infectivity of the virus. Firstly, translation beginning at the initiation codon of ORF VII should reach an in-phase termination codon before the initiation codon of ORF I. Secondly, an initiation codon should be located in the vicinity of the sequence homologous to methionine tRNA, the proposed primer binding site for reverse transcription. Results suggest translation from a polycistronic mRNA by a ;relay race' mechanism whereby ribosomes after passing a termination codon re-initiate protein synthesis at the nearest AUG codon.