Evolutionary refinement of the ZIKV 5' UTR dictates a trade-off between RNA stability and promoter accessibility

Zika virus (ZIKV) has evolved from a sporadically circulating pathogen into a global health threat associated with severe neurological complications. While research has been focused on amino acid substitutions, the impact of non-coding RNA evolution on ZIKV fitness remains largely unexplored. Here, we characterize the structural and functional evolution of stem-loop A (SLA), the essential 5' terminal promoter for the viral polymerase. We identified four distinct evolutionary variants, ancestral (Anc), intermediate (Int), contemporary (Con), and alternative (Alt) that exhibit a progressive shift in thermodynamic stability. Notably, SLA^Con-dominant in recent outbreaks-displays increased binding affinity for the NS5 polymerase, whereas the divergent SLA^Alt variant markedly increases 5' exoribonuclease resistance at the cost of replicative efficiency. Although the SLA^Alt variant proved lethal in a contemporary ZIKV backbone, 5' rapid amplification of cDNA ends (RACE) and phylogenetic analysis confirmed its viability in nature. These findings reveal a previously unrecognized plasticity in the ZIKV promoter and demonstrate that non-coding architecture undergoes adaptive refinement to balance genome stability with kinetic accessibility. Ultimately, this work illustrates how the structural evolution of RNA serves as a key determinant of viral fitness and the emergence of isolate-specific replication strategies.