GC content partly predicted 2 or more RNA polymerases in eukaryotes??

[exutu]

Hello.
I'm reading in my molecular biology book by Robert Weaver, reading about the eukaryot RNA polymerase. This is a book which uses a fair amount of its time on the experiments which lead to the knowledge presented. It starts out with 3 reasons for why there probably should be at least 2 RNA polymerase:
(and we all know that there is 3 nuclear RNA polymerases, at least that is what I'm told )
"...(1) They have a different base composition from that of other nuclear genes. For example, rat rRNA genes have a GC content of 60%, but the rest of the DNA has a GC content of 40 %"
the two others are that the rRNA genes is unusually repetitive and that they are in different compartments (nucleolus and nucleoplasm).
I'm not sure I understand why the different GC content lead to believe that there are more than one RNA polymerase. Is it because the different polymerases are optimized for different GC content (although I don't know how that mechanism would work, I would love to know it) or something completely different?
Hope you can help me

[candywhy]

Hope u can find what u want...
wish all the best to ur new book