Notes on Genus: Alphanecrovirus
Type member: Tobacco necrosis virus A
Necroviruses, (named from tobacco necro
sis virus) are members of the family Tombusviridae
. Until 2013, these were included in a single genus but the high level of sequence diversity in the polymerase, and (two) movement protein sequences led to the genus being divided into 2 genera. Like other members of the family, necroviruses have icosahedral particles and small, monopartite ss RNA genomes. In common with the genera Machlomovirus
, necroviruses have a small (27-30 kDa) coat protein but the necrovirus genome is smaller (<4 kb). In natural infections necroviruses are typically restricted to roots and transmitted by the fungus Olpidium brassicae
Virions isometric (icosahedral), 28nm in diameter. 32 capsomeres per nucleocapsid.
Monopartite, linear, single stranded positive sense RNA about 4000 nucleotides long. 3' terminus has no poly(A) tract. 5' terminus has a ppApGpUp...sequence.
Genus Genomic Organization
There are 5 (or 6) ORFs. ORFs1 and 2 (a readthrough of 1) are believed to be a polymerase. The small ORFs 3 and 4 are probably translated from the larger of two subgenomic RNAs and the coat protein (ORF5) from a smaller subgenomic.
Type Member Genomic Organization
The RNA is translated by means of several open reading frames (ORFs) into 6 products:
|1 ||23 kDa||Replication protein|
|2 ||82 kDa||Polymerase readthrough|
|3 ||8 kDa||Movement protein|
|4 ||6 kDa||Putative movement protein|
|5 ||30 kDa||Coat protein|
|6 ||7 kDa||unknown function|