We recently identified several ESTs that bind to the fragile X mental retardation protein (FMRP) in vitro. To determine whether they interacted in vivo we performed three-hybrid screens in a Saccharomyces cerevisiae histidine auxotroph. We demonstrate that two of the ESTs support growth on histidine and transduce beta-galactosidase activity when co-expressed with FMRP under selective growth conditions. In contrast, the iron response element (IRE) RNA does not. Likewise, the ESTs do not support growth or transduce beta-galactosidase activity when co-expressed with the iron response element binding protein (IRP). Each EST is relatively small and has 40% identity with a sequence in FMR1 mRNA harboring FMRP binding determinants. Interestingly, while neither the ESTs contain a G-quartet structural motif they do contain U-rich sequences that are found in mRNA with demonstrated in vitro binding and in vivo association with FMRP. This indicates that U-rich elements comprise another motif recognized by FMRP. (C) 2003 Elsevier Science (USA). All rights reserved.