Cauliflower mosaic virus: still in the news

Abstract

SUMMARY Taxonomic relationship: Cauliflower mosaic virus (CaMV) is the type member of the Caulimovirus genus in the Caulimoviridae family, which comprises five other genera. CaMV replicates its DNA genome by reverse transcription of a pregenomic RNA and thus belongs to the pararetrovirus supergroup, which includes the Hepadnaviridae family infecting vertebrates. Physical properties: Virions are non-enveloped isometric particles, 53 nm in diameter (Fig. 1). They are constituted by 420 capsid protein subunits organized following T= 7 icosahedral symmetry (Cheng, R.H., Olson, N.H. and Baker, T.S. (1992) Cauliflower mosaic virus: a 420 subunit (T= 7), multilayer structure. Virology, 16, 655-668). The genome consists of a double-stranded circular DNA of approximately 8000 bp that is embedded in the inner surface of the capsid. Viral proteins: The CaMV genome encodes six proteins, a cell-to-cell movement protein (P1), two aphid transmission factors (P2 and P3), the precursor of the capsid proteins (P4), a polyprotein precursor of proteinase, reverse transcriptase and ribonuclease H (P5) and an inclusion body protein/translation transactivator (P6). Hosts: The host range of CaMV is limited to plants of the Cruciferae family, i.e. Brassicae species and Arabidopsis thaliana, but some viral strains can also infect solanaceous plants. In nature, CaMV is transmitted by aphids in a non-circulative manner.

Cauliflower mosaic virus protein P6 is a suppressor of RNA silencing

“…CaMV gene VI encodes a 66 kDa polypeptide P6 which is essential for replication (Haas et al, 2002;Covey et al, 2000) and is also the major genetic determinant of symptom severity (Stratford & Covey, 1989). Transgene-mediated expression of P6 in Arabidopsis results in a chlorotic dwarf phenotype and alterations in leaf morphology (Zijlstra et al, 1996;Cecchini et al, 1997) reminiscent of those induced by silencing suppressors, e.g.…”

“…Several geminiviruses undergo silencing, and several also encode silencing suppressors (Bisaro, 2006;Cui et al, 2005;Trinks et al, 2005). Cauliflower mosaic virus (CaMV), the type member of the family Caulimoviridae, is a pararetrovirus with an 8 kbp dsDNA genome and replicates by reverse transcription (Haas et al, 2002). In Brassica napus, CaMV undergoes silencing (Al Kaff et al, 1998), and CaMVinfected Arabidopsis accumulate siRNAs that are derived mainly from the viral 35S RNA leader (Moissiard & Voinnet, 2006;Blevins et al, 2006).…”

“…CaMV gene VI encodes a 66 kDa polypeptide P6 which is essential for replication (Haas et al, 2002;Covey et al, 2000) and is also the major genetic determinant of symptom severity (Stratford & Covey, 1989 et al, 2003). This raises the question whether P6 might be suppressing silencing in infected GxA.…”

A putative antiviral role of plant cytidine deaminases

“…We hypothesize that deamination may take place mainly at the reverse transcription step. The CaMV genome is constituted by a single molecule of circular double-stranded DNA of 8 kbp 38 . The DNA of CaMV has three discontinuities, Δ1 in the negative-sense strand (or a strand), and Δ2 and Δ3 in the positive-sense strand (yielding the b and g strands).…”

“…The DNA of CaMV has three discontinuities, Δ1 in the negative-sense strand (or a strand), and Δ2 and Δ3 in the positive-sense strand (yielding the b and g strands). In short, the replication cycle of CaMV is as follows 38 : in the nucleus of the infected cell, the a strand is transcribed into 35S RNA, with terminal repeats, that migrates to the cytoplasm. Priming of the 35S RNA occurs by the annealing of the 3’ end of tRNA met to the primer-binding site (PBS) sequence, leading to the synthesis of the DNA a strand by the virus’ reverse transcriptase.…”