Ribosomal protein L21 is one of the proteins from the large ribosomal subunit. In Escherichia coli, L21 is known to bind to the 23S rRNA in the presence of L20. It belongs to a family of ribosomal proteins which, on the basis of sequence similarities, groups:

• Eubacterial L21.
• Marchantia polymorpha chloroplast L21.
• Cyanelle L21.
• Spinach chloroplast L21 (nuclear-encoded).

Eubacterial L21 is a protein of about 100 amino-acid residues, the mature form of the spinach chloroplast L21 has 200 residues. As a signature pattern, we selected a conserved region located in the C-terminal section of these proteins.