Genetics, Vol. 160, 381-391, February 2002, Copyright © 2002

Structure/Function Analysis of the Saccharomyces cerevisiae Trf4/Pol {sigma} DNA Polymerase

Zhenghe Wanga, Irene B. Castañoa, Carrie Adamsa, Clemence Vua, David Fitzhugha, and Michael F. Christmana
a Department of Microbiology, University of Virginia, Charlottesville, Virginia 22908

Corresponding author: Michael F. Christman, E603, Boston University Medical Center, 715 Albany St., Boston, MA 02118., mfc{at}bu.edu (E-mail)

Communicating editor: M. LICHTEN

The Trf4p/Pol {sigma} DNA polymerase (formerly Trf4p/Pol {kappa}) couples DNA replication to the establishment of sister chromatid cohesion. The polymerase is encoded by two redundant homologs in Saccharomyces cerevisiae, TRF4 and TRF5, that together define a fourth essential nuclear DNA polymerase in yeast and probably in all eukaryotes. Here we present a thorough genetic analysis of the founding member of this novel family of DNA polymerases, TRF4. Analyses of mutants carrying 1 of 34 "surface-targeted" alanine scanning mutations in TRF4 have identified those regions required for Pol {sigma}'s essential function, for its role in DNA double-strand break repair, and for its association with chromosomes. The data strongly support the importance of the regions of predicted structural similarity with the Pol ß superfamily as critical for Trf4p/Pol {sigma}'s essential and repair functions. Surprisingly, five lethal mutations lie outside all polymerase homology in a C-terminal region. The protein possesses Mg2+-dependent 3' to 5' exonuclease activity. Cell cycle analysis reveals that Trf4p/Pol {sigma} associates with chromosomes in G1, S, and G2 phases, but that association is abolished coincident with dissolution of cohesion at the metaphase-to-anaphase transition.