Bipartite geminivirus host adaptation determined cooperatively by coding and noncoding sequences of the genome

Bipartite geminiviruses are small, plant-infecting viruses with genomes com posed of circular, single-stranded DNA molecules, designated A and B. Altho ugh they are closely related genetically, individual bipartite geminiviruse s frequently exhibit host-specific adaptation. Two such viruses are bean go lden mosaic virus (BGMV) and tomato golden mosaic virus (TGMV), which are w ell adapted to common bean (Phaseolus vulgaris) and Nicotiana benthamiana, respectively. In previous studies, partial host adaptation was conferred on BGMV-based or TGMV-based hybrid viruses by separately exchanging open read ing frames (ORFs) on DNA A or DNA B. Here we analyzed hybrid viruses in whi ch all of the ORFs on both DNAs were exchanged except for AL1, which encode s a protein with strictly virus-specific activity. These hybrid viruses exh ibited partial transfer of host-adapted phenotypes. In contrast, exchange o f noncoding regions (NCRs) upstream from the AR1 and BR1 ORFs did not confe r any host-specific gain of function on hybrid viruses. However, when the e xchangeable ORFs and NCRs from TGMV were combined in a single BGMV-based hy brid virus, complete transfer of TGMV-like adaptation to N. benthamiana was achieved. Interestingly, the reciprocal TGMV-based hybrid virus displayed only partial gain of function in bean. This may be, in part, the result of defective virus-specific interactions between TGMV and BGMV sequences prese nt in the hybrid, although a potential role in adaptation to bean for addit ional regions of the BGMV genome cannot be ruled out. (C) 2000 Academic Pre ss.