In Vitro- and In Vivo-Generated Defective RNAs of Satellite Panicum Mosaic Virus Define cis -Acting RNA Elements Required for Replication and Movement

Abstract

Satellite panicum mosaic virus (SPMV) depends on its helper virus, panicum mosaic virus (PMV),Four plant satellite viruses have been identified in association with helper viruses from natural infections, including satellite tobacco necrosis virus (STNV), satellite panicum mosaic virus (SPMV), satellite maize white line mosaic virus, and satellite tobacco mosaic virus (STMV) (4, 23). Satellite viruses are valuable tools for dissecting sequence or structural features of RNA molecules that are essential for biological and biochemical functions, such as replication, symptom induction, and spread, without perturbing the helper virus genome. These "molecular parasites" can also help us to understand the intricately coordinated interactions between helper virus-encoded proteins, the satellite virus RNAs, and the host cells (15,23).The SPMV genome is composed of a single-stranded, plussense RNA of 824 nucleotides (nt). Four open reading frames (ORFs) were originally identified, two on the viral plus-sense strand and two on the viral complementary strand (8). An ORF from nt 88 to 561 on the viral sense RNA encodes a 17.5-kDa capsid protein (CP) (8). The 5Ј untranslated region (UTR) is composed of 87 nt preceding the SPMV CP start codon, and the 3Ј UTR extends from nt 562 to 824. Although strong secondary structures were predicted in the 5Ј and 3Ј UTRs and implicated in the replication of the SPMV genome (8), cis-acting sequences essential for the replication and/or systemic movement of SPMV have not been experimentally identified.SPMV depends on its helper virus, panicum mosaic virus (PMV), for replication and systemic spread (8,21). PMV has the unusual characteristic of supporting two distinct types of subviral entities, SPMV and satellite RNAs, which is reflected in the naturally occurring viral complex in infected St. Augustinegrass lawns along the Gulf Coast of the United States (3). Sequence analysis revealed that the 4,326-nt singlestranded plus-sense RNA of PMV encodes six ORFs (28). The 5Ј-proximal p48 and p112 read-through proteins provide the core components of the PMV replicase complex, which is also proposed to be essential for the replication of SPMV and the satellite RNAs (28). A 26-kDa CP and three other smaller proteins (p15, p8, and p6.6) have been implicated in local and systemic movement of PMV (27). RNA transcripts produced in vitro from the full-length cDNA clones of PMV and SPMV are infectious on millet host plants (9,21,28). In the present study, a comprehensive set of SPMV mutants was generated and analyzed for the ability to infect millet protoplasts and plants. Infectivity assays with these mutants mapped the cis-acting elements that are required for SPMV replication and movement. Furthermore, the characterization of an infectious cDNA clone derived from a defective SPMV RNA (D-RNA), which accumulated de novo in the plants coinoculated with PMV and SPMV CP-deficient mutants, also defined the cis-acting elements required by SPMV for its viability in plants. MATERIALS AND METHODSHost plants and RN...

The Capsid Protein of Satellite Panicum Mosaic Virus Contributes to Systemic Invasion and Interacts with Its Helper Virus

“…The capsid protein also elicits symptoms on a nonhost plant, Nicotiana benthamiana (22), a feature that may be related to the unconventional role of SPMV CP in regulating a suppressor of gene silencing (22). SPMV CP is not essential for replication and systemic movement of the satellite virus RNA in millet plants (21). However, the absence of CP expression stimulated the accumulation of SPMV-defective interfering RNAs, suggesting an additional role of the CP in maintaining SPMV RNA integrity (24).…”

“…The CP is used to assemble 16-nm spherical satellite virus particles (19,21). As expected, SPMV CP has a high affinity for binding SPMV RNA, as shown by gel mobility shift assays (6).…”

“…Several cis-active elements on the SPMV RNA are required for SPMV replication and capsid protein translation (21). In particular, nt 63 to 104 on the 5Ј-untranslated region (UTR) were associated with host-specific spread of the SPMV.…”

Characteristics of the monocistronic genome of extra small virus, a virus-like particle associated with Macrobrachium rosenbergii nodavirus: possible candidate for a new species of satellite virus

“…They involve a small size virus (satellite virus) and a larger virus (helper virus). One of the most studied co-operations concerns the association of panicum mosaic satellite virus (PMSV) and panicum mosaic virus (PMV) (Qiu & Scholthof, 2000, 2001. It has been demonstrated that the smaller partner (satellite virus) needs the larger one for its replication, but that it is the satellite virus that induces severe chlorosis on proso millet plant (Qiu & Scholthof, 2001).…”

Molecular Plant-Microbe Interactions That Cut the Mustard

“…These questions relate to host range determinants and reiterate the particular importance of host proteins. For example, studies with satellite panicum mosaic virus have shown that a small RNA element conveys a host-specific contribution to virus movement (Qiu and Scholthof, 2000), perhaps through its interaction with a host factor. Research with Tomato Bushy stunt virus on various plants showed that certain amino acids of a movement-associated protein need to be retained for activity in all its hosts but other residues have host-dependent activities (Chu et al, 2000).…”