Cassava mosaic disease

The distinct quasi-isometric, geminate (twin) particles of CMGs measure ca. 30 x 20 nm, and the coat protein has a molecular weight of ca. 30 kDa. The particles each contain one molecule of circular, single-stranded DNA (Mr ca. 0.92 x 10(sup)6), and the genome consists of two circular molecules of similar sizes. In leaf tissue, the virus particles accumulate mainly in the nuclei of phloem parenchyma and of cortical and epidermal cells (Bock and Harrison, 1985). Altogether, the two genomic components of CMGs (DNA-A and DNA-B) contain eight genes (open reading frames). The DNA-A harbours two overlapping virion-sense and four overlapping complementary-sense ORFs whereas the DNA-B encodes two oppositely oriented non-overlapping ORFs. Homologous ORFs are also found in other whitefly-transmitted geminiviruses and encode proteins of Mr 10 kDa. The DNA-A genes include those encoding the virus particle protein and others involved in DNA replication. Those in DNA-B influence virus cell-to-cell movement within the plant. The virus coat protein is implicated in vector transmission (Lazarowitz, 1992). Between the two genomes lies a shared ca. 200-nucleotide sequence referred to as 'common region' that also carries the replication initiation TAATATT ? AC nanonucleotide.