REPLICATIVE STRATEGY OF BANANA BUNCHY TOP VIRUS
C HORSER, G HAFNER, R WANITCHAKORN, R HARDING and J DALE
Centre for Molecular Biotechnology, Queensland University of Technology, GPO Box 2434, Brisbane, Queensland 4001, Australia
Background and objectives
Results and conclusions
We have demonstrated that the major gene of BBTV DNA-1 encodes a replication-associated protein (Rep) based on (i) the presence of both dNTP binding and rolling circle replication motifs; and (ii) the ability of the expressed protein to nick and ligate within the loop sequence that is conserved between all components. Importantly, we have identified multimeric forms of DNA-1 in infected plants. We have also demonstrated that BBTV DNA-3 encodes the virus coat protein based on N-terminal sequencing of the coat protein and the reaction of the expressed protein with antisera generated against BBTV virions.
BBTV DNA-5 encodes a 18.87-kDa protein which contains a LXCXE motif that is associated with retinoblastoma-like binding proteins. We hypothesize that this protein is involved in switching infected cells to S phase early in the infection process to facilitate viral DNA replication.
We have not identified functions for the small gene of DNA-1 or the genes of DNAs 2, 4 and 6, and have not yet identified proteins involved in the inter- and intracellular movement of BBTV DNA.
Based on our results, we hypothesize that BBTV replicates by a rolling circle mechanism and propose that following inoculation into a banana cell and uncoating, BBTV genomic ssDNA is converted into transcriptionally active dsDNA forms by a host DNA polymerase and the various genes expressed. It is probable that the first of the expressed genes is that encoded by DNA-5 which initiates the switching of the infected cell to S phase, facilitating rolling circle DNA replication, directed by the viral Rep protein. The process of cell-to-cell and long-distance movement of BBTV in bananas is yet to be defined.