During infection, many RNA viruses, including respiratory syncytial virus (RSV), form specialized biomolecular condensates, viral factories (VFs), where viral transcription and replication occur^1,2. Paradoxically, high protein concentrations are typically required for condensate nucleation³, yet attaining sufficient protein levels in infection is thought to require VFs for viral transcription and replication. Here, to uncover how viruses solve this paradox to establish VFs, we visualized early infection of RSV in real time with single genomic viral ribonucleoprotein (vRNP) resolution. Our results reveal that VFs are nucleated from infecting vRNPs rather than de novo in the cytoplasm. VF nucleation further requires in-virion pre-assembly of viral protein–protein interaction networks on vRNPs to form ‘pre-replication centres’ (PRCs). PRCs are potent condensate nucleation seeds due to their efficient recruitment and retention of viral proteins. The high affinity of PRCs also results in increased association of the viral polymerase and its co-factors, allowing efficient viral transcription even in the absence of VFs. Together, these activities create a feed-forward loop that drives rapid VF formation. PRC assembly depends on in-virion viral protein levels and is highly heterogeneous among virions, explaining cell-to-cell heterogeneity in infection progression, and identifying heterogeneous virions as an important origin of infection heterogeneity. Together, our results show that in-virion pre-assembly of PRCs kick-starts viral condensate nucleation upon host-cell entry and explains cell-to-cell heterogeneity in RSV infection.

Respiratory syncytial virus (RSV) is the most prevalent cause of acute lower respiratory infection in young children. Currently, the first RSV vaccines are approved by the FDA. Recently, N6-methyladenosine (m⁶A) RNA methylation has been implicated in the regulation of the viral life cycle and replication of many viruses, including RSV. m⁶A methylation of RSV RNA has been demonstrated to promote replication and prevent anti-viral immune responses by the host. Whether m⁶A is also involved in viral entry and whether m⁶A can also affect RSV infection via different mechanisms than methylation of viral RNA is poorly understood. Here, we identify m⁶A reader YTH domain-containing protein 1 (YTHDC1) as a novel negative regulator of RSV infection. We demonstrate that YTHDC1 abrogates RSV infection by reducing the expression of RSV entry receptor CX3C motif chemokine receptor 1 (CX3CR1) on the cell surface of lung epithelial cells. Altogether, these data reveal a novel role for m⁶A methylation and YTHDC1 in the viral entry of RSV. These findings may contribute to the development of novel treatment options to control RSV infection.