Using overlapping and mutant oligonucleotides as probes, gel mobility assays and competition experiments identified a sequence from -47 to -32 bp upstream of the LIM2 CAP site, which a lens protein complex bound with high affinity which appeared to bind only to the "sense" strand of the double-stranded DNA molecule. This sequence consisted of a string of four guanine residues followed by seven other nucleotides (AACCTAA) and followed by another four guanines, i.e. GGGGAACCTAAGGGG, called the Hsu element. Promoter-CAT constructs containing this sequence or mutations of the sequence indicated that the Hsu element is located within the basal promoter, and is essential for expression of the LIM2 gene. The trans factors binding to the Hsu element are present throughout development, and appear to be lens-specific. Since the LIM2 gene promoter does not contain a classic TATA box, the Hsu element may serve as the site for binding the RNA polymerase complex.