Abstract
Porcine circoviruses are circular single-stranded DNA viruses that infect swine and wild boars. Two species of porcine circoviruses exist. Porcine circovirus type 1 is non pathogenic contrary to porcine circovirus type 2 which is associated with the disease known as Post-weaning Multisystemic Wasting Syndrome. Porcine circovirus DNA has been shown to replicate by a rolling circle mechanism. Other studies have revealed similar mechanisms of rolling-circle replication in plasmids and single-stranded viruses such as Geminivirus. Three elements are important in rolling-circle replication: i) a gene encoding initiator protein, ii) a double strand origin, and iii) a single strand origin. However, differences exist between viruses and plasmids and between viruses. Porcine circovirus replication probably involves a "melting pot" rather than "cruciform" rolling-circle mechanism.This review provides a summary of current knowledge of replication in porcine circoviruses as models of the Circovirus genus. Based on various studies, the factors affecting replication are defined and the mechanisms involved in the different phases of replication are described or proposed.
Highlights
The members of the Circovirus genus in the Circoviridae family are animal viruses, most of which affect birds type 1 and type 2 porcine circoviruses (PCV-1 and PCV-2) affect swine and wild boars
This review provides a summary of current knowledge of replication in porcine circoviruses as models of the Circovirus genus
Open Reading Frame 1 (ORF1) is located on the positive strand and encodes the Rep and Rep' proteins involved in replication initiation
Summary
The members of the Circovirus genus in the Circoviridae family are animal viruses, most of which affect birds type 1 and type 2 porcine circoviruses (PCV-1 and PCV-2) affect swine and wild boars. Thereafter, a RCR initiator protein complex binds to the stem loop structure (figure 2c) and destabilizes the replication origin. Leading strand synthesis: initiation/elongation Unlike the cruciform rolling-circle model of the Geminiviruses, PCVs exhibit "melting pot" organization [6] This is described as a sphere of instability at the level of the stem-loop where the (+) and (-) strains are close together but not held by hydrogen bonds. Synthesis of the leading strand: termination A termination model of rolling circle replication based on pT181 plasmid studies was described in the nineties by Novick's team [30] This mechanism is based on nucleophilic attacks on a tyrosylphosphodiester bond by a free 3'OH and on catalytic activity of a tyrosine on the DNA (figure 6). The involvement of tyrosine in PCV replication suggests that the mechanism might be similar to that of the pT181 plasmid
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