Artificial ribozymes catalyze a variety of chemical reactions. Their structures and reaction mechanisms are largely unknown. We have analyzed a ribozyme catalyzing Diels-Alder cycloaddition reactions by comprehensive mutation analysis and a variety of probing techniques. New tertiary interactions involving base pairs between nucleotides of the 5' terminus and a large internal loop forming a pseudoknot fold were identified. The probing data indicate a preformed tertiary structure that shows no major changes on substrate or product binding. Based on these observations, a molecular architecture featuring a Y-shaped arrangement is proposed. The tertiary structure is formed in a rather unusual way; that is, the opposite sides of the asymmetric internal loop are clamped by the four 5'-terminal nucleotides, forming two adjacent two base-pair helices. It is proposed that the catalytic pocket is formed by a wedge within one of these helices.