Molecular dissection of the cauliflower mosaic virus translation transactivator

The cauliflower mosaic virus (CaMV) transactivator (TAV) is a complex protein that appears to be involved in many aspects of the virus life cycle. One of its roles is to control translation from the polycistronic CaMV 35S RNA. Here we report a molecular dissection of TAV in relation to its ability to enhance dicistronic translation in transient expression experiments. We have identified a protein domain that is responsible and sufficient for that activity. This 'MiniTAV domain' consists of only 140 of the 520 amino acids in the full-length sequence. A further domain located outside the MiniTAV, and therefore dispensable for transactivation, is probably involved in interactions with other molecules. This was identified by its ability to compete with wild-type TAV and some of its deletion mutants. We found, furthermore, that the TAV protein binds RNA. Two regions needed for RNA-binding properties were defined outside the MiniTAV domain and RNA binding seems not to be directly involved in the transactivation mechanism.