A novel form of suppression due to an altered RNA polymerase

Transient reversal of RNA polymerase II active site closing controls fidelity of transcription elongation

To study fidelity of RNA polymerase II (Pol II), we analyzed properties of the 6-azauracil-sensitive and TFIIS-dependent E1103G mutant of rbp1 (rpo21), the gene encoding the catalytic subunit of Pol II in Saccharomyces cerevisiae. Using an in vivo retrotransposition-based transcription fidelity assay, we observed that rpb1-E1103G causes a 3-fold increase in transcription errors. This mutant showed a 10-fold decrease in fidelity of transcription elongation in vitro. The mutation does not appear to significantly affect translocation state equilibrium of Pol II in a stalled elongation complex. Primarily, it promotes NTP sequestration in the polymerase active center. Furthermore, pre-steady-state analyses revealed that the E1103G mutation shifted the equilibrium between the closed and the open active center conformations toward the closed form. Thus, open conformation of the active center emerges as an intermediate essential for preincorporation fidelity control. Similar mechanisms may control fidelity of DNA-dependent DNA polymerases and RNA-dependent RNA polymerases.

Genetic studies on the beta subunit of Escherichia coli RNA polymerase. I. The effect of known, single amino acid substitutions in an essential protein

The use of five different nonsense suppressors, in conjunction with a collection of 95 independent, spontaneously-occurring amber mutants affecting expression of rpoB, allows the generation of a maximum of 475 potential variants of the beta subunit of E. coli RNA polymerase, each carrying a known amino acid substitution at a particular site. The effect of these amino acid exchanges has been investigated in vivo. A significant majority (363/475) of substitutions lead to cellular death and altered properties--temperature sensitivity, apparently altered transcription termination and a changed stringent response--indicating that RNA polymerase function (unlike that of dispensable proteins) is extremely sensitive to such single site changes.