Transcript leader regions of two Saccharomyces cerevisiae mRNAs contain internal ribosome entry sites that function in living cells

In higher eukaryotes, translation of some mRNAs occurs by internal initiati on. It is not known, however, whether this mechanism is used to initiate th e translation of any yeast mRNAs. In this report, we identify naturally occ urring nucleotide sequences that function as internal ribosome entry sites (IRESes) within the 5' leader sequences of Saccharomyces cerevisiae YAP1 an d p150 mRNAs. When tested in the 5' untranslated regions of monocistronic r eporter genes, both leader sequences enhanced translation efficiency in veg etatively growing yeast cells. Moreover, when tested in the intercistronic region of dicistronic mRNAs, both sequences were shown to contain IRESes th at functioned in living cells. The activity of the p150 leader was much gre ater than that of the YAP1 leader. The second cistron was not expressed in control dicistronic constructs that lacked these sequences or contained the 5' leader sequence of the CLN3 mRNA in the intercistronic region. Further analyses of the p150 IRES revealed that it contained several nonoverlapping segments that were able independently to mediate internal initiation. Thes e results suggested a modular composition for the p150 IRES that resembled the composition of IRESes contained within some cellular mRNAs of higher eu karyotes. Both YAP1 and p150 leaders contain several complementary sequence matches to yeast 18S rRNA. The findings are discussed in terms of our unde rstanding of internal initiation in higher eukaryotes.