What is the expected composition of RNA in the RNA-Seq?

Question

[bulovic.ana]

What is the expected composition of RNA in the results of RNA-Seq? The assumption is that no rRNA depletion was performed.

I've come across information stating that the composition of a typical eukaryote cell is:

~80% rRNA, ~15% tRNA, and the rest mRNA. (http://www.ncbi.nlm.nih.gov/books/NBK21729/)

Why is this so? As I understand, only the RNA molecules which which will constitute the ribosome in their RNA form are called ribosomal RNA, not those that code for ribosomal proteins. Where, then, does this rRNA abundance come from? Why aren't there more mRNA that code for ribosomal proteins?

Answer 1

[seidel]

Consider the concentration of ribosomes in the cell, and that they are nucleo-protein complexes. The ribosomal proteins are made by translation of mRNAs - an amplification process: many copies of a protein can be synthesized from a single mRNA molecule. However, the ribosomal complex contains a stoichiometric ratio of proteins and nucleic acid components (i.e. rRNAs). Thus the rRNAs have to be produced in high abundance directly. Thus the rDNA loci, which codes for the rRNAs, are often amplified many many times in the cell. For instance in yeast, there are roughyl 150 copies of the loci, in higher organisms it can be thousands.

[bulovic.ana]

It all makes perfect sense, funny I never thought of it like that. Thanks a lot.