RNA based viral silencing suppression in plant pararetroviruses

Abstract

The 35S promoter of cauliflower mosaic virus and that of other plant pararetroviruses gives rise to an RNA, which is both a pre-genome and a polycistronic mRNA. The 600 nucleotide long very structured leader of this RNA is also transcribed separately. The resulting 8S RNA is then converted to a double strand giving rise to a huge set of siRNAs, which suppress silencing. In this Mini-Review I discuss how this versatile stretch of 600 nts constitutes a masterpiece of evolution.

Keywords: POL II; caulimovirus; shunting; siRNA; silencing; silencing suppression; translation; viroid.

FIGURE 1

Cauliflower Mosaic Virus (CaMV). (a) Positions of promoters (bent arrows), primer binding site (cloverleaf), polyadenylation signal (!) on the CaMV DNA, and the positions of the CaMV individual CaMV RNAs. (b) The 8 kb long circular CaMV DNA, its transcripts and its coding regions [ORF VII, no obvious function; MOV, movement protein; ITF, insect transmission factor; VAP, virion associated protein; GAG, capsid protein, PR/RT-RH, protease and reverse transcriptase fused coding region (POL); TAV, transactivator/viroplasmin]. (c) The CaMV RNA leader with its compact secondary structure. Capital letters correspond to small ORFs. Special features are indicated. The roundish arrow symbolizes the shunt process. (d) Nuclear entry of open circular CaMV DNA. (e) Structure of pospiviroid. (f) Suboptimal folding of Rice Tungro Bacilliform Virus (RTBV) 8S RNA revealing spatial vicinity of the facultative polyadenylation signal and the fall-off site.

FIGURE 2

siRNA mapping along caulimoviruses. For each virus the relative amounts of its si-RNAs (black marks for CaMV, black, and red marks for RTBV, blue and red marks for BSV-GF) are plotted against the whole genome (Blevins et al., 2011; Rajeswaran et al., 2014a,b). The respective leader regions are presented below as black lines. The 8S RNAs of CaMV and RTBV are shown as green lines, as well as the corresponding 100 nt RNA of BSV (“8S RNA”). The 35S RNA is shown only for CaMV (blue line).