A NOVEL YEAST PROTEIN INFLUENCING THE RESPONSE OF RNA-POLYMERASE-II TO TRANSCRIPTIONAL ACTIVATORS

A sensitive lit vitro crosslinking technique using a photoactive deriv ative of the chimeric activator LexA-E2F-1 was used to identify yeast proteins that might influence the response of RNA polymerase II to tra nscriptional activators. We found that a novel yeast protein, Xtc1p, c ould be covalently crosslinked to the activation domain of LexA-E2F-1 when this derivatized activator was bound to DNA upstream of an activa tor-responsive RNA polymerase II promoter. Because affinity chromatogr aphy experiments showed that Xtc1p also bound directly and specificall y to the activation domains of E2F-1, the viral activator VP16, and th e yeast activator GaI4p and copurified with the RNA polymerase II holo enzyme complex, Xtc1p may modulate the response of RNA polymerase LI t o multiple activators, Consistent with this notion, yeast strains dele ted for the XTC1 gene exhibited pleiotropic growth defects, including temperature sensitivity,,galactose auxotrophy, and a heightened sensit ivity to activator overexpression, as well as an altered response to t ranscriptional activators in vivo.