The cDNA which encodes lycopene cyclase, CrtL, was cloned from tomato (Lycopersicon esculentum cv. VF36) and tobacco (Nicotiana tabacum cv. Samsun NN) and functionally expressed in Escherichia coli. This enzyme converts lycopene to beta-carotene by catalyzing the formation of two beta-rings at each end of the linear carotene. The enzyme interacts with half of the carotenoid molecule and requires a double bond at the C-7,8 (or C-7,8') position. Inhibition in E. coli indicated that lycopene cyclase is the target site for the inhibitor MPTA, 2-(4-methylphenoxy)tri-ethylamine hydrochloride. The primary structure of lycopene cyclase in higher plants is significantly conserved with the enzyme from cyanobacteria but different from that of the non-photosynthetic bacteria Erwinia. mRNA of CrtL and Pds, which encodes phytoene desaturase, was measured in leaves, flowers and ripening fruits of tomato. In contrast to genes which encode enzymes of early steps in the carotenoid biosynthesis pathway, whose transcription increases during the 'breaker' stage of fruit ripening, the level of CrtL mRNA decreases at this stage. Hence, the accumulation of lycopene in tomato fruits is apparently due to a down-regulation of the lycopene cyclase gene that occurs at the breaker stage of fruit development. This conclusion supports the hypothesis that transcriptional regulation of gene expression is a predominant mechanism of regulating carotenogenesis.