4-Coumarate: coenzyme A ligase (4CL) is one of the key enzymes in phenylpropanoid metabolism leading to series of phenolics, including water-soluble phenolic acids, which are important compounds determining the medicinal quality of Danshen (Salvia miltiorrhiza Bunge), a traditional Chinese medicinal herb. To investigate the function of 4CL in the biosynthesis of water-soluble phenolic acid in Danshen, we have cloned two cDNAs (Sm4CL1 and Sm4CL2) encoding divergent 4CL members by applying nested reverse transcription-polymerase chain reaction (RT-PCR) with degenerate primers followed by 5'/3' rapid amplification of cDNA ends (RACE) (Note, these sequence data have been submitted to the GenBank database under accession numbers AY237163 and AY237164). Either of the coding regions was inserted into a pRSET vector and a kinetic assay was performed with purified recombinant proteins. The substrate utilization profile of Sm4CL1 was distinct from that of Sm4CL2. The K(m) values of Sm4CL1 and Sm4CL2 to 4-coumaric acid were (72.20 +/- 4.10) and (6.50 +/- 1.45) mu mol/L, respectively. These results, in conjunction with Northern blotting and other information, imply that Sm4CL2 may play an important role in the biosynthesis of water-soluble phenolic compounds, whereas Sm4CL1 may play a minor role in the pathway. Southern blotting analysis suggested that both Sm4CL1 and Sm4CL2 genes are present as a single copy and are located at different sites in the genome.