The ribosomal L20 leader is an RNA regulatory element that was found upstream of the Bacillus subtilis operon infC-rpmI-rplT encoding translation factor IF3 and the ribosomal proteins L35 and L20. The expression of this operon is autoregulated by a complex transcription attenuation mechanism involving the formation of alternative structures in the leader mRNA which depends upon the presence of ribosomal protein L20 [17289755]. L20 binds to a phylogenetically conserved domain and provokes premature transcription termination at the leader terminator. A regulatory model for L20 leader is based on molecular mimicry between the L20-binding sites on ribosomal and messenger RNA [17289755]. more

When rRNA is synthesized at high rate (i.e. rapid growth) L20 is titrated by rRNA leaving little protein available for binding to the operator. On the contrary, when the rRNA concentration decreases excess L20 will bind to the operator. This renders the nucleotides necessary for the long-range antiterminator interaction inaccessible thereby provoking premature transcription termination. Thus, the operator can also be viewed as an anti-antiterminator, a secondary structure trapped by the binding of L20, hence favoring termination.