Selective repurposing of the eukaryotic DNA replication machinery by a plant virus

Technical Summary: Selective Repurposing of the Eukaryotic DNA Replication Machinery by a Plant Virus

Problem Statement
Geminiviruses, which cause devastating crop diseases worldwide, strictly depend on host factors to replicate their circular single-stranded (ss) DNA genomes. While it is established that the virus-encoded Replication-associated protein (Rep) recruits host machinery and catalyzes nicking and ligation during rolling-circle replication (RCR), the precise molecular composition of the viral replisome remains largely unknown. Furthermore, it is unclear whether these viruses utilize a replication strategy analogous to the host's bidirectional replication fork (involving both leading and lagging strand synthesis) or if they employ a distinct mechanism. A major obstacle to resolving this has been the essential nature of many host replication genes, which precludes the generation of conventional loss-of-function mutants, and the scarcity of in vivo experimental approaches to study viral replication.

Methodology
The authors employed a multi-faceted approach combining proximity labeling, reverse genetics, and chromatin immunoprecipitation to dissect the geminiviral replisome:

1. TurboID-based Proximity Labeling (PL): To identify proteins in the immediate vicinity of Rep during infection, the authors fused Rep from Tomato yellow leaf curl virus (TYLCV) and Abutilon mosaic virus (AbMV) to TurboID. They validated that C-terminal tagging preserved Rep function (complementation of Rep-null mutants) and nuclear localization. PL was performed in planta using Nicotiana benthamiana, followed by mass spectrometry (MS) to identify biotinylated host proteins.
2. Virus-Induced Gene Silencing (VIGS): To assess the functional requirement of candidate host proteins, the authors used Tobacco rattle virus (TRV) to silence specific genes encoding replication factors (e.g., RFC1, POLD1, POLD3, PCNA, TOPO1, RPA70A, FEN1, LIG1).
3. Replication Assays: The impact of silencing was evaluated using:
   • A transgenic 2IR-GFP reporter system where GFP expression depends on Rep-mediated DNA replication.
   • Local and systemic infection assays with TYLCV, AbMV, Chickpea chlorotic dwarf virus (CpCDV, a mastrevirus), and the nanovirus Pea necrotic yellow dwarf virus (PNYDV).
4. Chromatin Immunoprecipitation (ChIP-qPCR): To confirm physical association with the viral genome, FLAG-tagged, silencing-resistant versions of the host proteins were expressed in silenced plants. ChIP was performed to test binding to the viral intergenic region (origin of replication) in the presence or absence of Rep.
5. Comparative Analysis: The study extended the analysis to different geminivirus genera (Mastrevirus) and the CRESS DNA family Nanoviridae to test for conservation of the replication mechanism.

Key Results

• Identification of the Rep Proxiome: TurboID-PL identified a core set of host DNA replication-related proteins proximal to both TYLCV and AbMV Rep. These included the clamp loader RFC (subunits RFC1-5), DNA polymerase δ subunits (POLD1 and POLD3), the sliding clamp PCNA, topoisomerase I (TOPO1), and the ssDNA-binding protein RPA70A.
• Functional Requirement: VIGS-mediated silencing of RFC1, POLD1, POLD3, PCNA, and TOPO1 significantly reduced Rep-mediated GFP reporter accumulation and drastically impaired viral accumulation in both local and systemic infections for TYLCV and AbMV.
• Physical Association: ChIP-qPCR confirmed that RFC1, POLD1, POLD3, PCNA, TOPO1, and RPA70A physically bind to the viral DNA replicon. Crucially, the recruitment of these factors (particularly RFC1, POLD3, TOPO1, RPA70A, and PCNA) to the viral origin was dependent on the presence of Rep.
• Replication Mode (Leading vs. Lagging Strand): Silencing of FEN1 (flap endonuclease 1) and LIG1 (DNA ligase I)—factors essential for lagging-strand Okazaki fragment processing—had no effect on viral replication. This suggests geminiviruses do not utilize classical lagging-strand synthesis.
• Polymerase Swapping: Despite the dispensability of lagging-strand factors, the study confirms the requirement for DNA polymerase δ (typically associated with lagging-strand synthesis in eukaryotes). This indicates a "polymerase swap" where Pol δ mediates a continuous, leading-strand-like RCR.
• Conservation: The requirement for RFC1, POLD1, POLD3, and PCNA was conserved across different geminivirus genera (including the mastrevirus CpCDV) and extended to the nanovirus PNYDV, suggesting a conserved mechanism across CRESS DNA viruses.

Significance and Claims
The paper claims to provide the first comprehensive catalog of host proteins proximal to geminiviral Rep during infection and to define the molecular composition of the viral replisome. The authors propose a model where geminiviruses (and likely other CRESS DNA viruses) selectively co-opt core host replication factors (RFC, PCNA, RPA, TOPO1) to facilitate unidirectional rolling-circle replication.

Key mechanistic insights include:

1. Selective Repurposing: The virus repurposes the host bidirectional replication fork machinery but bypasses lagging-strand specific processing components (FEN1, LIG1).
2. Polymerase Specificity: The virus utilizes DNA polymerase δ, normally a lagging-strand polymerase, to drive continuous leading-strand synthesis, potentially leveraging its high processivity in the absence of Okazaki fragments.
3. Rep-Dependent Recruitment: The viral Rep protein is the central orchestrator that recruits these host factors to the viral genome.

The authors conclude that these findings shed light on the molecular mechanism of geminiviral DNA replication and identify specific host dependencies (such as the interaction between Rep and Pol δ components) that could be targeted for engineering antiviral resistance in crops. They note that natural polymorphisms in these host factors (e.g., POLD1) have already been associated with resistance in some crops, validating the potential of targeting this host-virus interface.