Heterologous complementation by geminivirus AL2 and AL3 genes.

The geminivirus group is diverse and contains viruses which can be placed into three distinct subgroups on the basis of their genome organization and biological properties. However, most dicot-infecting gemiviruses possess AL1, AL2, and AL3 open reading frames. AL1 encodes the only viral protein that is absolutely required for replication, AL2 codes for a protein that transactivates the expression of virion sense promoters, and AL3 specifies a protein that enhances viral DNA replication. In the studies presented here, we examined the functional specificity of the AL1, AL2, and AL3 specifies gene products of subgroup II and subgroup III geminiviruses. Surprisingly, we found that all viruses tested were able to produce a gene product which complemented the reduced DNA replication phenotype of a tomato golden mosaic virus (TGMV) AL3 mutant. We also found that all bipartite subgroup III viruses tested produced a protein that could transactivate the virion sense promoters of a TGMV AL2 mutant, although a subgroup II virus did not. In contrast, the replication activity of AL1 protein proved to be virus specific. The data are discussed with regard to the functions of these proteins in viral replication and their practical significance for the development of crop protection strategies.