Molecular Plant Pathology – Pathogen Profiles

Brome mosaic virus, good for an RNA virologist’s basic needs 

C. Cheng Kao and K. Sivakumaran
Department of Biology, Indiana University, Bloomington, IN 47405, USA

Taxonomic relationship: Type member of the Bromovirus genus, family Bromoviridae. A member of the alphavirus-like supergroup of positive-sense single-stranded RNA viruses. 
Physical properties: Virions are nonenveloped icosahedrals made up of 180 coat protein subunits (Fig. 1). The particles are 26 nm in diameter and contain 22% nucleic acid and 78% protein. The BMV genome is composed of three positive-sense, capped RNAs: RNA1 (3.2 kb), RNA2 (2.9 kb), RNA3 (2.1 kb) (Fig. 2). 
Viral proteins: RNA1 encodes protein 1a, containing capping and putative RNA helicase activities. RNA2 encodes protein 2a, a putative RNA-dependent RNA polymerase. RNA3 codes for two proteins: 3a, which is required for cell-to-cell movement, and the capsid protein. The capsid is translated from a subgenomic RNA, RNA4 (1.2 kb). 
Hosts: Monocots in the Poacea family, including Bromus inermis, Zea mays and Hordeum vulgare, in which BMV causes brown streaks. BMV can also infect the dicots Nicotiana benthamiana and several Chenopodium species. In N. benthamiana, the infection is asymptomatic while infection of Chenopodium can cause either necrotic or chlorotic lesions. 

Electron micrograph of brome mosaic virus. The average diameter of each particle is 26 nm.