4-Coumarate:CoA ligase (4CL), a key enzyme of phenylpropanoid metabolism in plants, is encoded in potato (Solanum tuberosum L.) by two structurally similar genes (St4cl-1, St4cl-2). Computer-based sequence analyses revealed similarities at the amino acid sequence level with other enzymes dependent on ATP for activation of aromatic carboxylic acids, e.g. some bacterial peptide synthetases. All these enzymes have a common seven amino acid sequence motif containing one cysteine residue. Using an assay on the basis of the polymerase chain reaction, we show that the mRNAs from both 4CL genes accumulate to equal levels in suspension-cultured cells and whole plant tissues, independent of various kinds of activating stimulus applied and of the overall transcriptional activity of the genes. The apparent lack of differential expression, together with the fact that both 4CL genes and proteins are nearly identical in structure, make it unlikely that 4CL isoforms in potato have specific roles in metabolic channeling. Constitutive in vivo footprints in the TATA-box proximal region of the St4cl-1 promoter define putative cis-acting elements which may be involved in the responses of the 4CL genes to various endogenous and exogenous stimuli.