As mentioned earlier, DNA pol III is the enzyme which carries out the replication of large E. coli chromosome. DNA pol I, because of the following properties, does not qualify as the enzyme for E. coli chromosome replication:
1) The polymerization rate (nucleotides added/sec) of this enzyme is 16-20 nucleotides/sec or approximately 600 nucleotides/min, which is too slow. It is slow by a factor of 100 or more to account for the rate at which the replication fork moves during bacterial chromosome replication.
2) DNA pol I has a very low processivity, 3-200 nucleotides added before polymerase dissociates.
3) A bacterial strain, isolated in 1969 by John Cairns, had a mutant DNA pol I gene, which produced the inactive enzyme. But, surprisingly this bacterial strain was viable. This clearly shows that even in the absence of active DNA pol I, E. coli can survive and replicate its chromosome. It means there is another DNA polymerase present to perform the function of DNA replication.