What is the mechanism of rRNA charging?

rRNA becomes charged through a coupling process. Aminoacyl-tRNA synthetases are enzymes that catalyze this process. First, an amino acid and ATP bind to the aminoacyl-tRNA enzyme, which catalyzes the coupling of that amino acid to form aminoacyl-AMP. Two phosphates become detached during the reaction. An uncharged tRNA molecule then binds to the enzyme. Aminoacyl-tRNA then transfers the amino acid from aminoacyl-AMP to tRNA to form aminoacyl-tRNA (aa-tRNA). The aa-tRNA and AMP are released from the enzyme in the process. The enzyme then returns to its original state for the coupling process to be repeated again. 

In the first step, the carboxyl group of the amino acid forms a covalent bond to the ATP molecules a-phosphate group, creating inorganic pyrophosphate (PPi) and producing a 5 aminoacyl adenylate intermediate. A nucleophilic attack on intermediate aminoacyl adenylate causes the attachment of aminoacyl group to the 3-OH of the tRN and release of an AMP molecule. The tRNA molecule becomes charged at this point.  The overall reaction is aa+ ATP +tRNA →aa-tRNA + AMP + PPi.