Gene VI of cauliflower mosaic virus strains D4 and W260 is an important determinant of systemic infection in solanaceous species (Qiu and Schoelz, 1992). To investigate whether D4 and W260 share any sequences within gene VI that determine their solanaceous host range, we characterized more completely the regions of gene VI involved in systemic infection of Nicotiana bigelovii. We found that two domains within gene VI, which corresponded approximately to the 5' third and middle third of gene VI, influenced systemic infection of N. bigelovii. Exchange of these domains between D4, W260, and CaMV strain CM1841, a strain which is unable to systemically infect any solanaceous plant, revealed different virus combinations that could specify systemic infection of N. bigelovii when plants were grown under two different lighting conditions. Systemic infection of N. bigelovii by D4/CM1841 chimeric viruses required only the 5' third (domain 1) of gene VI of D4. In contrast, systemic infection of N. bigelovii by W260/CM1841 chimeric viruses required both the domain 1 and middle third of gene VI (domain 2) of W260, as well as two other regions primarily containing W260 genes I and II, and gene IV. The genetic requirements for systemic infection by chimeric viruses were not as stringent when plants were grown under low light conditions. Specifically, domain 2 of gene VI of D4 contained sequences sufficient for D4/CM1841 chimeric viruses to systemically infect N. bigelovii at low light intensity. We sequenced gene VI of strain W260 and compared differences in the deduced amino acid sequences between W260 and the previously published sequences of D4 and CM1841. There was only one amino acid position in domain 1 of gene VI, and no sites in domain 2, in which W260 and D4 agreed with each other and differed from CM1841. Consequently, the host range studies and sequence information indicate that different sequences within gene VI of CaMV strains W260 and D4 are responsible for systemic infection of N. bigelovii.