RNA polymerase I is located in the nucleolus and transcribes the large ribosomal RNA precursor (pre-rRNA), which is processed into the 28S, 18S, and 5.8S rRNA components of ribosomes. RNA polymerase II transcribes mRNA (and snRNA, miRNA); RNA polymerase III transcribes tRNA, 5S rRNA, and other small RNAs. This division of labour is unique to eukaryotes; prokaryotes have a single RNA polymerase.