The UGA codon, which usually acts as a stop codon, can also direct the incorporation into a protein of the amino acid selenocysteine. This UGA decoding process requires a cis-acting mRNA element called the selenocysteine insertion sequence (SECIS), which can form a stem-loop structure. In Escherichia coli, selenocysteine incorporation requires only the 17-nucleotide-long upper stem-loop structure of the fdhF SECIS. This structure carries a bulged nucleotide U at position 17. Here we asked whether the single bulged nucleotide located in the upper stem-loop structure of the E. coli fdhF SECIS is involved in the in vivo interaction with SelB. We used a genetic approach, generating and characterizing selB mutations that suppress mutations of the bulged nucleotide in the SECIS. All the selB suppressor mutations isolated were clustered in a region corresponding to 28 amino acids in the SelB C-terminal subdomain 4b. These selB suppressor mutations were also found to suppress mutations in either the loop or the upper stem of the E. coli SECIS. Thus, the E. coli SECIS upper stem-loop structure can be considered a "single suppressible unit," suggesting that there is some flexibility to the nature of the interaction between this element and SelB.