Brome mosaic virus, BMV
Brome mosaic virus (ANU site)

Brome mosaic (picture) is a small (27nm, 86S) virus with icosahedral symmetry (T=3),. The virus consists of 3 types of particles which are physically virtually indistinguishable. The most dense contains RNA1 (3234 nucleotides), the least dense contains RNA2 (2865 nucleotides) and the middle density contains RNA3 (2114 nucleotides) and RNA4 (876 nucleotides). BMV is a cation (most viruses are anions) under conditions of stability (below pH 6.5).

BMV was discovered in the 1940s. By the 1950s it was known to be among the smallest viruses. In the 1970s it was shown to have a divided genome. In the 1980s the genome was sequenced and it was, along with TMV, the first virus for which infectious clones were available.

BMV contains 4 RNAs. RNA1 encodes P1 (methyltransferase and helicase), RNA2 encodes P2 (RNA polymerase), RNA3 encodes p3a (movement) and the capsid protein. RNA4 is a mRNA for the capsid protein.

BMV is cosmopolitan and found virtually wherever wheat is grown.

Natural hosts
BMV normally infects Bromus inermis and other grasses. It is occasionally found in corn (Zea mays). In Lincoln it occasionally infects (systemically) Chenopodium hybridum. In Louisiana it is often found in Commelina in lawns.

Natural transmission
BMV is usually found where there is heavy foot and/or machinery traffic. Natural spread appears to be largely mechanical. In the few instances where it systemically infected maize, corn flea beetle populations were high. Beetles are known to transmit in the laboratory.

BMV is one of the few grass viruses that infects dicots. It readily infects common test plants such as Chenopodium species. BMV infects most maize varieties necrotically and often kills (e.g Golden Cross Bantam). Interestingly 5-10% of maize varieties (including Va35, T8 and W540) become systemically infected. The necrotic reaction is inherited as a single dominant gene. Commercial maize varieties which become infected (few) lack the necrotic gene. BMV infects Nicotiana benthamiana (mild symptoms).

The dsRNA pattern is very similar to that of cucumber mosaic, but distinct from other viruses. DsRNA is harder to isolate than for CMV, it is detectable only briefly, early in infection

Virus isolation
BMV is acid stable and easily isolated by ultracentrifugation or PEG precipitation from acidic (pH 4.5 or below) buffers (commonly acetate). Avoid neutral pH since BMV swells above pH 6.5 and becomes unstable.

Capsid protein
BMV has among the smallest capsid proteins. The BMV cp migrates just above TMV cp on my SDS polyacrylamide gels. It can easily be detected by SDS gel electrophoresis of total Bromus inermis proteins