BACKGROUND: In the C. albicans retrotransposon Tca2, the gag and pol ORFs are 
separated by a UGA stop codon, 3' of which is a potential RNA pseudoknot. It is 
unclear how the Tca2 gag UGA codon is bypassed to allow pol expression. However, 
in other retroelements, translational readthrough of the gag stop codon can be 
directed by its flanking sequence, including a 3' pseudoknot.
RESULTS: The hypothesis was tested that in Tca2, gag stop codon flanking 
sequences direct translational readthrough and synthesis of a gag-pol fusion 
protein. Sequence from the Tca2 gag-UGA-pol junction (300 nt) was inserted 
between fused lacZ and luciferase (luc) genes in a Saccharomyces cerevisiae dual 
reporter construct. Although downstream of UGA, luc was expressed, but its 
expression was unaffected by inserting additional stop codons at the 3' end of 
lacZ. Luc expression was instead being driven by a previously unknown minor 
promoter activity within the gag-pol junction region. Evidence together 
indicated that junction sequence alone cannot direct UGA readthrough. Using 
reporter genes in C. albicans, the activities of this gag-pol junction promoter 
and the Tca2 long terminal repeat (LTR) promoter were compared. Of the two 
promoters, only the LTR promoter was induced by heat-shock, which also triggers 
retrotransposition. Tca2 pol protein, epitope-tagged in C. albicans to allow 
detection, was also heat-shock induced, indicating that pol proteins were 
expressed from a gag-UGA-pol RNA.
CONCLUSION: This is the first demonstration that the LTR promoter directs Tca2 
pol protein expression, and that pol proteins are translated from a gag-pol RNA, 
which thus requires a mechanism for stop codon bypass. However, in contrast to 
most other retroelement and viral readthrough signals, immediate gag 
UGA-flanking sequences were insufficient to direct stop readthrough in S. 
cerevisiae, indicating non-canonical mechanisms direct gag UGA bypass in Tca2.