The synthesis of eukaryotic selenoproteins involves the recoding of an internal 
UGA codon as a site for selenocysteine incorporation. This recoding event is 
directed by a selenocysteine insertion sequence in the 3'-untranslated region. 
Because UGA also functions as a signal for peptidyl-tRNA hydrolysis, we have 
investigated how the rates of translational termination and selenocysteine 
incorporation relate to cis-acting elements in the mRNA as well as to 
trans-acting factors in the cytoplasm. We used cis-elements from the 
phospholipid glutathione peroxidase gene as the basis for this work because of 
its relatively high efficiency of selenocysteine incorporation. The last two 
codons preceding the UGA were found to exert a far greater influence on 
selenocysteine incorporation than nucleotides downstream of it. The efficiency 
of selenocysteine incorporation was generally much less than 100% but could be 
partially enhanced by concomitant overexpression of the tRNA(Sec) gene. The 
combination of two or three UGA codons in one reading frame led to a dramatic 
reduction in the yield of full-length protein. It is therefore unlikely that 
multiple incorporations of selenocysteine are processive with respect to the 
mode of action of the ribosomal complex binding to the UGA site. These 
observations are discussed in terms of the mechanism of selenoprotein synthesis 
and its ability to compete with termination at UGA codons.