Frequency and Mechanism of Expansion of the C‐terminal Domain of Yeast RPB1

Abstract

The carboxyl-terminal domain (CTD) of the largest subunit of RNA polymerase II, RPB1, is made up of tandem heptapeptide repeats whose residues are heavily post-translationally modified, helping to orchestrate eukaryotic transcription. Severe truncation of the CTD adversely affects growth; at least 8 of the 26 wild-type repeats are necessary for survival in yeast. Through construction of truncated CTDs by Recursive Directional Ligation (RDL), this study seeks to develop a novel and precise system to monitor CTD expansion. In addition, we calculate the mutational frequency associated with truncation repair, and explore the role of DNA repair processes in CTD expansion. CTDs constructed by RDL were found to be an appropriate experimental system for CTD truncation, with evidence of growth deficiency and expansion that is consistent with previous studies. Suppressor mutations were found to appear at a defined frequency among truncated strains, and this frequency tends to increase when DNA repair proteins are deleted. We propose that CTD expansion may be a natural mechanism for repairing damage at the essential RPB1 gene.