What is the motivation behind the AAAAGCAUAU GACUAAAAAA of the mRNA SARS-CoV-2 vaccine when encoding its polyadenylated ending?
To enhance stability and translation efficiency according to {1}:

The 5′UTR (TEV) [8,21] and 3′ UTR (F-I) of this construct have been shown to enhance stability and translation efficiency, as has the 100-nucleotide poly(A) tail interrupted by a short linker (A30LA70, where L = GCAUAUGACU) [22]


References:

{1}
Stadler, C., Bähr-Mahmud, H., Celik, L. et al. Elimination of large tumors in mice by mRNA-encoded bispecific antibodies. Nat Med 23, 815–817 (2017). https://doi.org/10.1038/nm.4356 https://www.nature.com/articles/nm.4356
{21} Gallie, D.R., Tanguay, R.L. &amp; Leathers, V. Gene 165, 233–238 (1995).
{22} Vallazza B, Petri S, Poleganov MA, Eberle F, Kuhn AN, Sahin U. Recombinant messenger RNA technology and its application in cancer immunotherapy, transcript replacement therapies, pluripotent stem cell induction, and beyond. Wiley Interdiscip Rev RNA. 2015 Sep-Oct;6(5):471-99. doi: 10.1002/wrna.1288. Epub 2015 Jun 9. PMID: 26061157. https://pubmed.ncbi.nlm.nih.gov/26061157/