THE RELEASE ELEMENT OF THE YEAST POLYMERASE-I TRANSCRIPTION TERMINATOR CAN FUNCTION INDEPENDENTLY OF REB1P

The Saccharomyces cerevisiae polymerase I (poll) transcription termina tor utilizes a DNA-binding protein (Reb1p) as part of a signal that ca uses the polymerase to pause prior to release from the template. To st udy the release element of the terminator, independent of the Reb1p pa use signal, we have replaced the Reb1p binding site with the binding s ite for the lac repressor, which acts asa self-contained heterologous pause signal for poll. Release efficiency is maximal when the iac repr essor causes poll to pause in exactly the same position that Reb1p wou ld have caused it to pause, suggesting that poll must be precisely pos itioned for transcript release to occur. Mutational analysis shows tha t the release element is a region rich in T residues which codes for t he extreme 3' end of the transcript and which has no apparent ability to form hairpins when transcribed into RNA. We discuss possible mechan isms whereby this poll release element might function.