No, this was not the question ww...
The sequence with the 3 letters is the output of a prediction algorithm, which has 3 labels only (in this case I, M and O).
The sequence with the hyphens is the same sequence but refers to the amino-acid sequence instead (so yes, there are 20 letters in that case).
If you remove the hyphens, you will see that both sequences are of the same length, so what I wanted is to replace each letter of the amino-acid sequence with the respective predicted label of it.
Thanks to everyone for answering, now my code runs extremely faster!