(1) a strain with a mutant gene encoding Pol I such that it no longer has polymerase activity (but retains both types of nuclease activities);
(2) a strain without RNaseH proteins;
(3) a strain with a mutant gene encoding Pol I such that it no longer has 5´ to 3´ exonuclease activities (but retains 3´ to 5´ nuclease and polymerase activities);
(4) a strain with the mutant Pol I described in (5)and a strain lacking all RNaseH proteins.
DNA polymerase 3 is required for leading and lagging strand replication, whereas DNA polymerase 1 is required for removing RNA primers from fragments and replacing them with the required nucleotides. These enzymes cannot be substituted for one another because they perform different functions.
During DNA replication, repair, and recombination, DNA polymerase I fills gaps in the DNA. Editing and proofreading are also performed by DNA polymerase II, mostly on the lagging strand. The major replicative enzyme is DNA polymerase III.