How do cells determine RNA types?

Question

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?

Answer 1

Well remember that an mRNA, the RNA that is destined for the ribosome, has already been processed by the time it gets there. This is done by the splicing machinery, among other things (capping enzymes, for example). But the splicing machinery isn't smart in itself. Rather, the gene has the elements, just like it does to select the RNA Pol it needs, to direct the sort of modification the RNA needs to obtain. The image below depicts two types of RNA transcribed by RNA Pol II and how the processing elements let the modification machinery know how to handle them:

Source