L36
The L36 residue is the smallest protein in the 50S subunit. Within the amino acid structure of the protein is a sequence of three cysteins and one histadine, which possess the capabilities of forming a tetrahedral structure with zinc ions. Thus, the L36 protein is an example of a zinc ribbon domain. Considering that other residues, including TFIIB transcription factors, possess similar zinc ribbon structural elements, Hard et al. propose that L36 represents an evolutionary ancient transcriptional element. The structure shown above was determined from the L36 residue of the organism Thermus thermophilus. (Hard et al. 2000)