Post-transcriptional gene silencing suppressor activity of two non-pathogenic alphasatellites associated with a...

Alphasatellites and betasatellites are begomovirus-associated single-stranded circular DNA molecules. Two distinct alphasatellites, Gossypium darwinii symptomless alphasatellite and Gossypium mustelinium symptomless alphasatellite, were previously isolated from Gossypium davidsonii and G.mustelinium. Here we show that the replication-associated proteins (Rep: a rolling-circle replication initiator protein) encoded by these alphasatellites interact with the Rep and C4 proteins encoded by their helper begomovirus, Cotton leaf curl Rajasthan virus (CLCuRaV), in a yeast two-hybrid assay. Both the alphasatellite-encoded Reps were found to have strong gene silencing suppressor activity, in contrast to the betasatellite-encoded betaC1 and CLCuRaV-encoded C2, C4 and V2 proteins. The presence of alphasatellites maintained suppression of gene silencing in the youngest, actively growing tissue of CLCuRaV-betasatellite-infected plants. This is the first demonstration of a rolling-circle replication initiator protein with suppressor of gene silencing activity and provides a possible explanation for the selective advantage provided by the association of alphasatellites with begomovirus-betasatellite complexes.