In the bipartite geminivirus tomato golden mosaic virus (TGMV), the activity of late gene promoters is up-regulated by the multifunctional viral protein AL2. Cis-acting sequences required for AL2-mediated promoter responses have not been well characterized. However, nucleotide sequence analysis has implicated a motif termed the conserved late element (CLE). The CLE is present in TGMV and many other begomoviruses, although it is not ubiquitous. Here we analysed the regulation of late gene expression in bean golden mosaic virus (BGMV), one of the begomoviruses which lacks the CLE. Transient reporter gene assays showed that BGMV late gene promoters were trans-activated in Nicotiana benthamiana protoplasts, both by the homologous BGMV AL2 protein and by the heterologous TGMV AL2 protein. The BGMV AL2 protein also trans-activated TGMV late gene promoters. Consistent with these results, we found that hybrid viruses with the late gene promoters exchanged between BGMV and TGMV were viable in planta.
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