HOLE, THE GENE CODING FOR THE THETA SUBUNIT OF DNA-POLYMERASE-III OF ESCHERICHIA-COLI - CHARACTERIZATION OF HOLE MUTANT AND COMPARISON WITHA DNAQ (EPSILON-SUBUNIT) MUTANT

DNA polymerase III holoenzyme is a multiprotein complex responsible fo r the bulk of chromosomal replication in Escherichia coli and Salmonel la typhimurium. The catalytic core of the holoenzyme is an alpha epsil on theta heterotrimer that incorporates both a polymerase subunit (alp ha; dnaE) and a proofreading subunit (epsilon; dnaQ). The role of thet a is unknown. Here, we describe a null mutation of holE, the gene for theta. A strain carrying this mutation was fully viable and displayed no mutant phenotype. In contrast, a dnaQ null mutant exhibited poor gr owth, chronic SOS induction, and an elevated spontaneous mutation rate , like dnaQ null mutants of S. typhimurium described previously. The p oor growth was suppressible by a mutation affecting alpha which was id entical to a suppressor mutation identified in S. typhimurium. A doubl e mutant null for both holE and dnaQ was indistinguishable from the dn aQ single mutant. These results show that the theta subunit is dispens able in both dnaQ(+) and mutant dnaQ backgrounds, and that the phenoty pe of E mutants cannot be explained on the basis of interference with theta function.