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Abstract:
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.
