Rpb7 can interact with RNA polymerase II and support transcription during some stresses independently of Rpb4

Rpb4l and Rpb7 are two yeast RNA polymerase II (Pol II) subunits whose mech anistic roles have recently started to be deciphered. Although previous dat a suggest that Rpb7 can stably interact with Pol II only as a heterodimer w ith Rpb4, RPB7 is essential for viability, whereas RPB4 is essential only d uring some stress conditions. To resolve this discrepancy and to gain a bet ter understanding of the mode of action of Rpb4, we took advantage of the i nability of cells lacking RPB4 (rpb4 Delta, containing Pol II Delta 4) to g row above 30 degrees C and screened for genes whose overexpression could su ppress this defect. We thus discovered that overexpression of RPB7 could su ppress the inability of rpb4 Delta cells to grow at 34 degrees C (a relativ ely mild temperature stress) but not at higher temperatures. Overexpression of RPB7 could also partially suppress the cold sensitivity of rpb4 Delta s trains and fully suppress their inability to survive a long starvation peri od (stationary phase). Notably, howe, er, overexpression of RPB4 could not override the requirement for RPB7. Consistent with the growth phenotype, ov erexpression of RPB7 could suppress the transcriptional defect characterist ic of rpb4 Delta cells during the mild, but not during a more severe, heat shock. We also demonstrated, through two reciprocal coimmunoprecipitation e xperiments, a stable interaction of the overproduced Rpb7 with Pol II Delta 4. Nevertheless, fewer Rpb7 molecules interacted with Pol II Delta 4 than with wild-type Pol II, Thus, a major role of Rpb4 is to augment the interac tion of Rpb7 with Pol II. We suggest that Pol II Delta 4 contains a small a mount of Rph7 that is sufficient to support transcription only under nonstr ess conditions. When RPB7 is overexpressed, more Rpb7 assembles with Pol II Delta 4, enough to permit appropriate transcription also under some stress conditions.