I was reading about the different types of RNA polymerases, and I am confused as to how a cell determines which type of RNA it is transcribing.
According to this nature article:
RNA pol I transcribes ribosomal RNAs (rRNAs), RNA pol II transcribes RNAs that will become messenger RNAs (mRNAs) and also small regulatory RNAs, and RNA pol III transcribes small RNAs such as transfer RNAs (tRNAs)
The system as to how the different types of RNA polymerase are selected makes sense to me, the core promoter's sequence determines the affinity of the polymerase to itself.
But, how does RNA polymerase II determine if the transcript is a regulatory RNA or a messenger RNA? Can someone explain that process to me or point me to where I would find it?
Another question: When RNA polymerases I and III transcribe their RNA, what prevents the cell from processing them the same way as mRNA and sending it off to a ribosome with a poly-a tail?