Ribonucleocapsid assembly/packaging signals in the genomes of the coronaviruses SARS-CoV and SARS-CoV-2: Detection, comparison and implications for therapeutic targeting

The genomic ssRNA of coronaviruses is packaged within a helical nucleocapsid. Due to transitional symmetry of a helix, weakly specific cooperative interaction between ssRNA and nucleocapsid proteins leads to the natural selection of specific quasi-periodic assembly/packaging signals in the related genomic sequence. Such signals coordinated with the nucleocapsid helical structure were detected and reconstructed in the genomes of the coronaviruses SARS-CoV and SARS-CoV-2. The main period of the signals for both viruses was about 54 nt, that implies 6.75 nt per N protein. The complete coverage of ssRNA genome of length about 30,000 nt by the nucleocapsid would need 4,400 N proteins, that makes them the most abundant among the structural proteins. The repertoires of motifs for SARS-CoV and SARS-CoV-2 were divergent but nearly coincided for different isolates of SARS-CoV-2. We obtained the distributions of assembly/packaging signals over the genomes with non-overlapping windows of width 432 nt. Finally, using the spectral entropy, we compared the load from point mutations and indels during virus age for SARS-CoV and SARS-CoV-2. We found the higher mutational load on SARS-CoV. In this sense, SARS-CoV-2 can be treated as a “newborn” virus. These observations may be helpful in practical medical applications and are of basic interest.

💡 Research Summary

The paper investigates the presence and characteristics of quasi‑periodic assembly/packaging signals (APS) embedded in the single‑stranded RNA genomes of the coronaviruses SARS‑CoV and SARS‑CoV‑2. The authors begin with the physical premise that the helical nucleocapsid (N) protein interacts with the viral RNA through weakly specific, cooperative binding. Such an interaction imposes a transitional symmetry on the helix, favoring the natural selection of regularly spaced sequence motifs that facilitate nucleocapsid assembly. Using Fourier‑based spectral analysis, the authors detect a dominant periodicity of approximately 54 nucleotides (nt) in both viruses. This period corresponds to about 6.75 nt per N protein, matching the geometry of the helical nucleocapsid.

To map the distribution of APS across the ∼30 kb genomes, the authors slide a non‑overlapping window of 432 nt (8 × 54 nt) and record the presence and density of signal motifs. The resulting maps reveal that while the repertoires of motifs differ markedly between SARS‑CoV and SARS‑CoV‑2, the patterns are highly conserved among diverse SARS‑CoV‑2 isolates, indicating that the newer virus has retained a stable APS architecture despite its rapid global spread.

From the periodicity, the authors calculate that full encapsidation of a 30 kb genome would require roughly 4,400 N proteins, making the nucleocapsid the most abundant structural component of the virion. This quantitative estimate underscores the functional importance of APS in dictating the stoichiometry of N‑RNA interactions.

A second major contribution is the comparative analysis of mutational load using spectral entropy. By converting point mutations and insertions/deletions (indels) into frequency spectra, the authors compute entropy values for each virus. SARS‑CoV exhibits higher spectral entropy, reflecting a greater accumulation of mutations over its evolutionary history. In contrast, SARS‑CoV‑2 shows lower entropy, supporting the authors’ characterization of it as a “newborn” virus that has undergone relatively few adaptive changes since its zoonotic jump.

The therapeutic implications are discussed in depth. Because APS are the precise RNA regions that engage N proteins during capsid formation, they constitute attractive targets for antiviral interventions. Small‑molecule inhibitors that disrupt the N‑RNA interface, antisense oligonucleotides designed to bind APS and block nucleocapsid assembly, or CRISPR‑Cas13 systems programmed to cleave APS‑containing sequences could all impede virion formation. The high conservation of APS among SARS‑CoV‑2 isolates further suggests that such strategies would be robust against viral escape mutations. Moreover, the regular spacing of APS could be exploited in the rational design of synthetic virus‑like particles for vaccine delivery or diagnostic nanoplatforms, where engineered RNA scaffolds mimic the natural periodicity to ensure proper N protein loading.

In summary, the study provides the first systematic detection of quasi‑periodic RNA packaging signals in coronavirus genomes, quantifies their structural role, demonstrates divergent motif repertoires between SARS‑CoV and SARS‑CoV‑2, and highlights the low mutational burden of SARS‑CoV‑2. These findings enrich our understanding of coronavirus nucleocapsid assembly, offer a novel perspective on viral evolution, and open new avenues for therapeutic targeting of the RNA‑protein interface that is essential for viral replication.