[subroutine]

The number of adenine repeats that confer functional properties is quite variable but it definitely needs to be more than "just 1". I've seen anywhere from 25-250 used in designed plasmids. The exact number people use in their engineered sequence is based on a number of factors, not all of them scientific in nature (e.g. companies charge per basepair synthesize a bespoke polypeptide; e.g. you copied the sequence from a previous clone into ApE and that sequence used 30 repeats and worked fine).

[kevviiinn]

Iirc there's a protein that adds 12-15 slowly then another protein comes in and adds another 200+ when it detects the start of the tail. Not sure about the details about how or why it stops tho. At least when considering mRNA getting prepped for nuclear export

[subroutine]

You are probably right, I'm not an expert on this process. But there's likely a difference between the innate eukaryotic cell polyadenylation process vs. how coronavirus accomplishes polyadenylation because coronavirus rna never enters the nucleus.