Human dihydrofolate reductase (DHFR) is a critical target in cancer chemotherapy. Previous studies showed that an 82-nt RNA fragment within the DHFR mRNA protein-coding region functions as a DHFR cis-acting response element. In this study, we further investigated the key elements contained within this sequence that are required for the DHFR mRNA-DHFR protein interaction. Using enzymatic foot-printing assays and RNA-binding experiments, we isolated a 27-nt sequence (DHFR27, corresponding to nts 407-433), which bound with high affinity and specificity to human DHFR to form a ribonucleoprotein complex. In vivo transient transfection experiments using a luciferase reporter system revealed that DHFR27 RNA could repress the luciferase expression in a DHFR-dependent manner when placed upstream of luciferase mRNA. This work provides new insights into the essential molecular elements that mediate RNA-protein interactions. (c) 2008 Published by Elsevier Inc.