Introduction

Taxonomic Structure of the Family

Family	Geminiviridae
Genus	Mastrevirus
Genus	Curtovirus
Genus	Begomovirus

Virion Properties

Morphology

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

Physicochemical and Physical Properties

S_20w is approximately 70S.

Nucleic Acid

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

Proteins

Virions contain a single structural protein (CP; Mr 28-34 10³). All viruses make a replication associated protein (Rep) (Mr 41 10³) 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 thymidine 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).