We have isolated a cDNA for a cytochrome P450, cinnamate 4-hydroxylase (C4H), of Arabidopsis thaliana using a C4H cDNA from mung been as a hybridization probe. The deduced amino acid sequence is 84.7% identical to that of mung bean C4H and therefore was designated CYP73A5. The CYP73A5 protein was expressed in insect cells using the baculovirus expression system and when reconstituted with lipid and NADPH-cytochrome P450 reductase resulted in C4H activity with a specific activity of 68 nmol min-1 nmol-1 P450. Southern blot analysis revealed that CYP73A5 is a single-copy gene in Arabidopsis. C4H (CYP73A5) expression was apparently coordinated in Arabidopsis with both PAL1 and 4CL in response to light and wounding. Although the light induction of CHS followed a time course similar to that observed with C4H, no induction of CHS was detected upon wounding. On the other hand, the C4H expression patterns exhibited no significant coordination with those of PAL2 and PAL3. A C4H promoter region of 907 bp contained all of the three cis-acting elements (boxes P, A, and L) conserved among the PAL and 4CL genes so far reported as controlling expression.