Mutation and trans-activation analyses suggest novel functional motifs for AC2 of the begomovirus Sri Lankan cassava mosaic virus.

This study was aimed to have a better understanding of the functional domains of the product of AC2, a protein having a trans-activation function on the late promoters of begomoviruses. The bipartite begomovirus Sri Lankan cassava mosaic virus (SLCMV) was chosen for this study. Infectious SLCMV clones carrying mutations in AC2 were analysed for infectivity, accumulation of viral DNA and trans-activation of the CP promoter in the experimental host Nicotiana benthamiana. The mutations were designed to express truncated AC2 proteins (containing N-terminal 36, N-terminal 101 or C-terminal 22 out of the 135 amino acid residues of the complete protein) or single amino acid replacements targeting the conserved "cysteine-rich" and the "Zn-finger" domains, implicated in trans-activation by AC2 of other begomoviruses. In the inoculated leaves, the DNA of all the above mutants, except the one expressing the C-terminal 22 amino acid residues accumulated at wild type levels. In newly-emerged leaves, only the DNA of the mutant expressing N-terminal 101 residues showed a delayed accumulation compared to the wild type and produced mild symptoms in the inoculated plants. The capability of trans-activation of CP promoter was abolished in all AC2 mutants except in the one expressing the N-terminal 101 residues, lacking the C-terminal trans-activation domain. The results suggest novel functional domains of the AC2 product in SLCMV, not described before.