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Molecular and Cellular Biology, April 1999, p. 2672-2680, Vol. 19, No. 4
0270-7306/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.

Rpb7 Can Interact with RNA Polymerase II and Support Transcription during Some Stresses Independently of Rpb4

Ayelet Sheffer, Mazal Varon, and Mordechai Choder*

Department of Molecular Microbiology and Biotechnology, Faculty of Life Sciences, Tel-Aviv University, Tel-Aviv 69978, Israel

Received 2 September 1998/Returned for modification 30 October 1998/Accepted 5 January 1999

Rpb4 and Rpb7 are two yeast RNA polymerase II (Pol II) subunits whose mechanistic roles have recently started to be deciphered. Although previous data suggest that Rpb7 can stably interact with Pol II only as a heterodimer with Rpb4, RPB7 is essential for viability, whereas RPB4 is essential only during some stress conditions. To resolve this discrepancy and to gain a better understanding of the mode of action of Rpb4, we took advantage of the inability of cells lacking RPB4 (rpb4Delta , containing Pol IIDelta 4) to grow above 30°C and screened for genes whose overexpression could suppress this defect. We thus discovered that overexpression of RPB7 could suppress the inability of rpb4Delta cells to grow at 34°C (a relatively mild temperature stress) but not at higher temperatures. Overexpression of RPB7 could also partially suppress the cold sensitivity of rpb4Delta strains and fully suppress their inability to survive a long starvation period (stationary phase). Notably, however, overexpression of RPB4 could not override the requirement for RPB7. Consistent with the growth phenotype, overexpression of RPB7 could suppress the transcriptional defect characteristic of rpb4Delta cells during the mild, but not during a more severe, heat shock. We also demonstrated, through two reciprocal coimmunoprecipitation experiments, a stable interaction of the overproduced Rpb7 with Pol IIDelta 4. Nevertheless, fewer Rpb7 molecules interacted with Pol IIDelta 4 than with wild-type Pol II. Thus, a major role of Rpb4 is to augment the interaction of Rpb7 with Pol II. We suggest that Pol IIDelta 4 contains a small amount of Rpb7 that is sufficient to support transcription only under nonstress conditions. When RPB7 is overexpressed, more Rpb7 assembles with Pol IIDelta 4, enough to permit appropriate transcription also under some stress conditions.

* Corresponding author. Mailing address: Department of Molecular Microbiology and Biotechnology, Faculty of Life Sciences, Tel-Aviv University, Tel-Aviv 69978, Israel. Phone: (972) 36409030. Fax: (972) 36409407. E-mail: lcchoder{at}ccsg.tau.ac.il.

Molecular and Cellular Biology, April 1999, p. 2672-2680, Vol. 19, No. 4
0270-7306/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.


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