DNA polymerase I (Pol I) of E. coli consists of three functional parts (domains): an N-terminal domain with 5 ´ to 3 ´ exonuclease activities required for removal of the RNA primer, a central domain responsible for 3 ´ to 5 ´ exonuclease proofreading, and a C-terminal domain with polymerase activity. Pol I is thought to simultaneously remove RNA primers and fill in the gaps that result. A group of proteins known as RNaseH also have 5 ´ to 3 ´ exonuclease activity and can thus remove RNA primers. However, they lack the other two functions observed for Pol I. Predict the ability of the following mutants to replicate DNA:
(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 and a strain lacking all RNaseH proteins.
Explain your reasoning for each.