The function of SECIS RNA in translational control of gene expression in Escherichia coli

The incorporation of selenocysteine into proteins is directed by specific UGA codons and mRNA secondary structures, designated SECIS elements. In bacteria, these elements are positioned within the reading frame of selenoprotein mRNAs immediately downstream of the triplet coding for selenocysteine, and they tether a complex of the selenocysteine-specific elongation factor SelB, GTP and selenocysteyl-tRNA(Sec) to the site of UGA decoding. A SECIS-like structure was identified in the 5' non-translated region of the selAB transcript, encoding selenocysteine synthase and SelB. It specifically binds to SelB and the formation of a SelB.GTP.selenocysteyl-tRNA(Sec) complex on the SECIS-like element represses expression of the downstream gene. This effect is abolished by mutations preventing formation of the complex. The regulatory pattern observed correlated with the levels of sel gene products. As quaternary complex formation on the SECIS-like element did not influence the transcription rate and only slightly reduced the level of selAB mRNA, it was concluded that the structure is involved in regulating translation initiation efficiency, thereby coupling selenocysteine biosynthesis to the availability of the trace element selenium.