Fig. 1.

Two types of mRNA structures. Messenger RNA is an intermediate step between DNA genome and protein expression. Eukaryotic pre-mRNA undergoes several maturation steps en route from the nuclear to the cytoplastic ribosome for translation, including 5’ capping, 3’ cleavage and polyadenylation, and splicing. A The structure of conventional mRNA vaccines is based on classical eukaryotic mRNA, containing a 5’ cap, untranslatable regions (UTR), an open-reading frame (ORF), and a poly(A) tail. mRNA vaccines request further modifications to reduce undesired immune responses and improve the anti-tumor immunogenicity of the vaccines. B The general structure of alphavirus-derived SAM vaccine is characterized by its self-amplifying subgenomic structure, which permits two ORFs in a single mRNA strand to encode a replication complex and gene of interest in sequence, driven by ssRNA virus-derived subgenomic promoters