The SARS-Cov2 coronavirus's genome was released, and is now available on Genbank.  Looking at it...


    1 attaaaggtt tataccttcc caggtaacaa accaaccaac tttcgatctc ttgtagatct
   61 gttctctaaa cgaactttaa aatctgtgtg gctgtcactc ggctgcatgc ttagtgcact
  121 cacgcagtat aattaataac taattactgt cgttgacagg acacgagtaa ctcgtctatc
  ...
29761 acagtgaaca atgctaggga gagctgccta tatggaagag ccctaatgtg taaaattaat
29821 tttagtagtg ctatccccat gtgattttaa tagcttctta ggagaatgac aaaaaaaaaa
29881 aaaaaaaaaa aaaaaaaaaa aaa

  
  Wuhan seafood market pneumonia virus isolate Wuhan-Hu-1, complete genome, Genbank


Geeze, that's a lot of a nucleotides---I don't think that's just random.  I would guess that it's either an artifact of the sequencing process, or there is some underlying biological reason.

Question: Why does the SARS-Cov2 coronavirus genome end in 33 a's?

Good observation! The 3' poly(A) tail is actually a very common feature of positive-strand RNA viruses, including coronaviruses and picornaviruses.
For coronaviruses in particular, we know that the poly(A) tail is required for replication, functioning in conjunction with the 3' untranslated region (UTR) as a cis-acting signal for negative strand synthesis and attachment to the ribosome during translation. Mutants lacking the poly(A) tail are severely compromised in replication. Jeannie Spagnolo and Brenda Hogue report:

The 3′ poly (A) tail plays an important, but as yet undefined role in Coronavirus genome replication. To further examine the requirement for the Coronavirus poly(A) tail, we created truncated poly(A) mutant defective interfering (DI) RNAs and observed the effects on replication. Bovine Coronavirus (BCV) and mouse hepatitis Coronavirus A59 (MHV-A59) DI RNAs with tails of 5 or 10 A residues were replicated, albeit at delayed kinetics as compared to DI RNAs with wild type tail lengths (>50 A residues). A BCV DI RNA lacking a poly(A) tail was unable to replicate; however, a MHV DI lacking a tail did replicate following multiple virus passages. Poly(A) tail extension/repair was concurrent with robust replication of the tail mutants. Binding of the host factor poly(A)- binding protein (PABP) appeared to correlate with the ability of DI RNAs to be replicated. Poly(A) tail mutants that were compromised for replication, or that were unable to replicate at all exhibited less in vitro PABP interaction. The data support the importance of the poly(A) tail in Coronavirus replication and further delineate the minimal requirements for viral genome propagation.
Spagnolo J.F., Hogue B.G. (2001) Requirement of the Poly(A) Tail in Coronavirus Genome Replication. In: Lavi E., Weiss S.R., Hingley S.T. (eds) The Nidoviruses. Advances in Experimental Medicine and Biology, vol 494. Springer, Boston, MA

Yu-Hui Peng et al. also report that the length of the poly(A) tail is regulated during infection:

Similar to eukaryotic mRNA, the positive-strand coronavirus genome of ~30 kilobases is 5’-capped and 3’-polyadenylated. It has been demonstrated that the length of the coronaviral poly(A) tail is not static but regulated during infection; however, little is known regarding the factors involved in coronaviral polyadenylation and its regulation. Here, we show that during infection, the level of coronavirus poly(A) tail lengthening depends on the initial length upon infection and that the minimum length to initiate lengthening may lie between 5 and 9 nucleotides. By mutagenesis analysis, it was found that (i) the hexamer AGUAAA and poly(A) tail are two important elements responsible for synthesis of the coronavirus poly(A) tail and may function in concert to accomplish polyadenylation and (ii) the function of the hexamer AGUAAA in coronaviral polyadenylation is position dependent. Based on these findings, we propose a process for how the coronaviral poly(A) tail is synthesized and undergoes variation. Our results provide the first genetic evidence to gain insight into coronaviral polyadenylation.
Peng Y-H, Lin C-H, Lin C-N, Lo C-Y, Tsai T-L, Wu H-Y (2016) Characterization of the Role of Hexamer AGUAAA and Poly(A) Tail in Coronavirus Polyadenylation. PLoS ONE 11(10): e0165077

This builds upon prior work by Hung-Yi Wu et al, which showed that the coronaviral 3' poly(A) tail is approximately 65 nucleotides in length in both genomic and sgmRNAs at peak viral RNA synthesis, and also observed that the precise length varied throughout infection. Most interestingly, they report:

Functional analyses of poly(A) tail length on specific viral RNA species, furthermore, revealed that translation, in vivo, of RNAs with the longer poly(A) tail was enhanced over those with the shorter poly(A). Although the mechanisms by which the tail lengths vary is unknown, experimental results together suggest that the length of the poly(A) and poly(U) tails is regulated. One potential function of regulated poly(A) tail length might be that for the coronavirus genome a longer poly(A) favors translation. The regulation of coronavirus translation by poly(A) tail length resembles that during embryonal development suggesting there may be mechanistic parallels.
Wu HY, Ke TY, Liao WY, Chang NY. Regulation of coronaviral poly(A) tail length during infection. PLoS One. 2013;8(7):e70548. Published 2013 Jul 29. doi:10.1371/journal.pone.0070548

It's also worth pointing out that poly(A) tails at the 3' end of RNA are not an unusual feature of viruses. Eukaryotic mRNA almost always contains poly(A) tails, which are added post-transcriptionally in a process known as polyadenylation. It should not therefore be surprising that positive-strand RNA viruses would have poly(A) tails as well. In eukaryotic mRNA, the central sequence motif for identifying a polyadenylation region is AAUAAA, identified way back in the 1970s, with more recent research confirming its ubiquity. Proudfoot 2011 is a nice review article on poly(A) signals in eukaryotic mRNA.
