Allosterically controlled ribozymes. Allosterically activated ribozyme (top) and allosterically inhibited ribozyme (bottom) [20, 21]. The allosteric ribozyme is composed of two components (left of the dashed arrow), a oligonucleotide binding site (OBS) and a ribozyme part. The two components are covalently bound and form a single nucleic acid molecule (centre). Upon binding an effector oligonucleotide (E) the conformation of the binding site changes and affects the conformation of the ribozyme component. The latter conformational change will activate (top) or inhibit (bottom) the catalytic activity of the ribozyme part. The same scheme can also be realised with deoxyribozymes. The scissors symbolically represent the cleavage reaction of the ribozyme.