Evidence suggests that, in prokaryotes, the peptidyl transferase reaction is performed by the large subunit 23S rRNA, whereas proteins probably have a greater role in eukaryotic ribosomes. Most of the proteins lie close to, or on the surface of, the 30S subunit, arranged peripherally around the rRNA . The small subunit ribosomal proteins can be categorised as primary binding proteins, which bind directly and independently to 16S rRNA; secondary binding proteins, which display no specific affinity for 16S rRNA, but its assembly is contingent upon the presence of one or more primary binding proteins; and tertiary binding proteins, which require the presence of one or more secondary binding proteins and sometimes other tertiary binding proteins.
The small ribosomal subunit protein S18 is known to be involved in binding the aminoacyl-tRNA complex in Escherichia coli , and appears to be situated at the tRNA A-site. Experimental evidence has revealed that S18 is well exposed on the surface of the E. coli ribosome, and is a secondary rRNA binding protein . S18 belongs to a family of ribosomal proteins  that includes: eubacterial S18; metazoan mitochondrial S18, algal and plant chloroplast S18; and cyanelle S18.