A ribosomal protein L21 leader is a ribosomal protein leader autoregulatory structure that regulates mRNAs containing a gene that encodes ribosomal protein L21. An RNA motif was predicted to function as an L21 leader in a bioinformatics study, and is found in B. subtilis and other low-GC Gram-positive bacteria within the phylum Bacillota. It is located in the 5â² untranslated regions of mRNAs encoding ribosomal protein L21, a protein of unknown function, and ribosomal protein L27 (rplU-ysxB-rpmA).