The ribosomal S15 leader is an RNA regulatory element that was found upstream of the Escherichia coli gene rpsO encoding the small ribosomal protein S15.The expression of rpsO is autoregulated through the formation of a pseudoknot in the leader mRNA [8955900]. This secondary RNA structure has an alternative structure, a combination of three hairpins, that are responsible for translational regulation of the S15 protein expression. The 30S ribosomal subunit binds efficiently to the pseudoknot because the Shine-Dalgarno sequence is presumed to be fully exposed. When the concentration of free S15 is low, the pseudoknot opens, the initiation complex is formed and translation can proceed. more

In the presence of S15, the pseudoknot stability is increased drastically by S15 binding and pseudoknot transconformation and translation initiation is blocked.