**4.1 Nucleoprotein (N)**

The RNA genome is wrapped by N (391 amino acids) to create a nuclease-resistant, helical RNP complex called nucleocapsid (NC), and it functions as the template for both replication and transcription [22, 23]. RSV virus genome for replication does not follow the "rule of six" [24], which is common to most paramyxoviruses [25]. The three-dimensional (3D) crystal structure revealed a decameric, ribonucleoprotein complex of N protein and RNA with 3.3 A° resolution and suggested N protein can function as a helicase to separate temporary double-stranded RNA during RNA synthesis [23]. As a decameric structure, every N subunit has a core region comprising two domains, N-terminal and C-terminal, which are linked by a hinge region and the RNA genome turns inside a basic surface groove located at the interface of N-terminal/C-terminal; specifically, every N subunit interacts with seven ribonucleotides of RNA [23].

## **4.2 RNA-dependent RNA polymerase (L)**

The L protein (2165 amino acids) has three enzymatic domains including RNAdependent RNA polymerase (RdRp) domain, polyribonucleotidyl transferase domain which is essential for capping located in its N-terminal, and methyltransferase domain which is necessary for cap methylation located in C-terminal [26–29]. Viral mRNA undergoes a post-transcriptional modification before translation and methyltransferase plays a significant role by catalyzing the methylation of cap structure at both N7- and 2′-O-positions because N7-methylation is vital for viral RNA translation and 2′-O-methylation is important for hiding viral RNA from the innate immunity system [30].
