The ribosomal L21 leader is a putative RNA regulatory element that was found upstream of the rplU-ysxB-rpmA genes encoding ribosomal protein L21, a protein of unknown function, and ribosomal protein L27 in Bacillus subtilis sand other Firmicutes. The expression of another ribosomal operon in B. subtilis, infC-rpmI-rplT, 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]. Similar transcription attenuation mechanism that can be proposed for the L21 leader has not been yet experimentally validated.