The nature of the minimal 'selenocysteine insertion sequence' (SECIS) in Escherichia coli

The UGA codon, usually a stop codon, can also direct the incorporation into a protein of the modified amino acid selenocysteine. This UGA decoding process requires a cis-acting mRNA element called 'selenocysteine insertion sequence' (SECIS) that can form a stem-loop structure. In Escherichia coli the SECIS of the selenoprotein formate dehydrogenase (FdhH) mRNA has been previously described to consist of at least 40 nucleotides following the UGA codon. Here we determined the nature of the minimal SECIS required for the in vivo UGA-directed selenocysteine incorporation in E. coli. Our study is based on extensive mutational analysis of the fdhF SECIS DNA located in a lac'Z fusion. We found that the whole stem-loop RNA structure of the E. coli fdhF SECIS previously described is not required for the UGA-directed selenocysteine incorporation in vivo. Rather, only its upper stem-loop structure of 17 nucleotides is necessary on the condition that it is located in a proper distance (11 nucleotides) from the UGA codon. Based on these observations, we present a new model for the minimal E. coli SECIS.