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Crystal structures of two classes of CCA enzymes. (a) The structure of the class I Archaeoglobus fulgidus enzyme (Protein Data Bank (PDB) code 1R8A). (b) The structure of the class II Bacillus stearothermophilus enzyme (PDB code 1M1W). Both enzymes consist of the head, neck, body, and tail domains, with different orientation of α helices and β sheets.
