rRNA (ribosome RNA) is the most abundant, accounting for about 80% of total cellular RNA, and the major structural component of the ribosome (more than 55% of the ribosome mass). In prokaryotes, the small 30S ribosomal subunit contains 16S rRNA. The large 50S subunit contains 5S and 23S rRNA. The 3′ end of the 16S rRNA (in the ribosome) recognizes the sequence of the Shine-Dalgarno sequence at the 5′ end of the mRNA. Bacterial 16S rRNA, 23S rRNA, and 5S rRNA genes are typically organized as co-transcriptional operons.