An analysis of the complete sequence of a sugarcane bacilliform virus genome infectious to banana and rice

The genome of sugarcane bacilliform virus (ScBV), a badnavirus, consists of a circular dsDNA. The complete sequence of a cloned infective ScBV genome is reported here. The genome is 7568 bp in size and possesses a number of features suggesting that ScBV is a pararetrovirus. A tRNA(Met) binding site that may serve as a primer for minus-strand synthesis is present. The plus-strand of the ScBV genome contains three open reading frames (ORFs) which are capable of encoding proteins with calculated M, values of 22K, 13K and 215K. The 215K protein has regions with similarity to the RNA-binding domains, aspartic proteases and replicases of retroelements. In addition, the 215K protein also has a region with restricted similarity to the intercellular transport proteins of plant viruses. Comparisons with the other sequenced badnaviruses, Commelina yellow mottle (CoYMV) and rice tungro bacilliform (RTBV) viruses, indicate that the arrangement of the ORFs in these viruses is conserved. Located next to the putative RNA-binding domain is a cysteine-rich region that is unique to the badnaviruses. When the molecular relationships of a portion of the reverse transcriptases of plant pararetroviruses were determined, two badnaviruses, CoYMV and ScBV, form one distinct cluster, whereas three caulimoviruses, cauliflower mosaic virus, carnation etched ring virus and figwort mosaic virus, form a second cluster. The badnavirus RTBV and the caulimovirus soybean chlorotic mottle virus occupy intermediate positions between the clusters. When introduced by Agrobacterium-mediated inoculation, a construct containing 1.1 copies of the cloned ScBV genome is infectious to both rice and banana.