MORPHOLOGY 

Virions are geminate (about 18 x 30 nm), apparently consisting of two incomplete icosahedra (T=1) with a total of 22 pentameric capsomers. 

 

Figure 1: (Right) Typical geminiviruses consist of two joined quasi-isometric subunits, with a characteristic "waist" constriction, and pointed ends: purified particles of the species Maize streak virus stained with uranyl acetate. The bar represents 50 nm. (Left) Capsids are constructed as shown: triangles superimposed on negatively-stained virion indicate organisation of capsomers. 

PHYSICOCHEMICAL AND PHYSICAL PROPERTIES 

S20w is approximately 70. 

NUCLEIC ACID 

Geminate virions are presumed to contain a single molecule of circular single-stranded DNA, 2.5-3.0 kb in size. This may be one or two distinct species molecule, depending on genus, for a total genome size of 2.5 - 5.2 kb. 

PROTEINS 

Virions contain a single structural protein (coat protein; Mr 28-34 x 10exp3). All viruses make a Rep protein (Mr = 41 x 103) which initiates rolling-circle replication. No other proteins have homologues throughout the family. 

LIPIDS 

None reported. 

CARBOHYDRATES 

None reported. 

GENOME ORGANIZATION AND REPLICATION 

Their genome may be composed of one component (all mastreviruses and curtoviruses, some begomoviruses) or two components (most begomoviruses). Coding regions diverge in both virion sense (=genomic strand) and complementary sense strands, from an intergenic region. Replication occurs through a double-stranded replicative intermediate via a rolling circle mechanism. The ssDNA synthesis is initiated at the 3' thimidine residue after single-strand cleavage of the virion-sense strand by the virus-encoded Rep protein at the indicated position within an absolutely conserved TAATATT/AC sequence, in the loop of a potential stem-loop structure within the intergenic region. Transcription of the viral genome is bidirectional with independently-controlled transcripts initiating within the intergenic region (IR).