A single missense mutation in the BR1 movement protein alters the host range of the squash leaf curl geminivirus

The bipartite geminiviruses causing squash leaf curl disease, SqLCV-E and SqLCV-R, have a broad host range that includes Nicotiana benthamiana, pumpkin, squash, and beans. We have characterized a naturally occurring mutant of the SqLCV-R B component (BRtd1) which has lost the ability to infect N. benthamiana, but still retains full infectivity for pumpkin and squash. The mutation responsible for this host range alteration was localized to the coding region of BR1, a gene essential for viral movement. Sequence analysis identified three missense mutations in the BR1 gene encoded by BRtd1. By site-directed mutagenesis, one of these missense mutations, a substitution of Cys for Arg at position 98 in the BR1 protein, has been identified as responsible for the altered host range properties of BRtd1. These results establish a role for the BR1 movement protein in determining the host range properties of SqLCV.