30S

The 30S subunit is the small component of the prokaryotic ribosome, forming the 70S ribosome together with the 50S large subunit. The name derives from its sedimentation coefficient, not its exact mass. In bacteria and many archaea, the 30S contains the 16S ribosomal RNA and about 21 ribosomal proteins, designated S1 through S21.

Structure and components: The 16S rRNA folds into domains that include the decoding center, which monitors codon-anticodon pairing, and regions that interact with the Shine-Dalgarno sequence in bacterial mRNAs to align the start codon. The associated ribosomal proteins stabilize the rRNA structure and participate in contacts with mRNA and tRNA during translation.

Function: During translation, the 30S subunit binds mRNA and the initiator tRNA at the start codon with the help of initiation factors. It coordinates codon recognition and the selection of aminoacyl-tRNAs, ensuring fidelity before peptide bond formation proceeds on the assembled 70S ribosome. It also contributes to movements of tRNA and mRNA during elongation.

Antibiotic relevance: The 30S subunit is a common target for antibiotics. Aminoglycosides (for example, streptomycin, gentamicin) bind the decoding center and can cause misreading of the genetic code. Tetracyclines block entry of aminoacyl-tRNA, while spectinomycin inhibits translocation by binding to the 30S.

Evolution and terminology: The 30S subunit is conserved in bacteria and archaea; its eukaryotic counterpart is the 40S subunit. The 70S ribosome in bacteria is formed by the association of 30S and 50S during protein synthesis.