Purpose of PolyA in cloning - (May/07/2014 )

Hi everyone,
I had a beginner's question, new to all of this. While looking at plasmid constructs, I notice sometimes, when 2 proteins need to be co-expressed, they are cloned in as:

Protein A - IRES - Protein B

but other times as:

Protein A - IRES - Protein B - PolyA

What is the purpose of the poly A sequence? Does it boost up protein levels by stabilizing the mRNA? 

Thanks a lot!

Are you sure there is no polyA signal in the other construct? That's a place where polymerase detaches. I can't imagine what would be effect of ommiting it, trascripts stopping unexpectedly in different parts of construct?

PolyA is also one of the marks of eukyrote mRNA, I would even expect it to have other functions apart from mRNA stabilization.

Is't there a possibility that there is some non-standard polyA signal instead?