T4 is a bacteriophage of E. coli. The activities of T4 DNA polymerase are very similar to Klenow fragment of DNA polymerase I - it functions as a 5' -> 3' DNA polymerase and a 3' -> 5' exonuclease, but does not have 5' -> 3' exonuclease activity.
In general, T4 DNA polymerase is used for the same types of reactions as Klenow fragment, particularly in blunting the ends of DNA with 5' or 3' overhangs. There are however, two differences between the two enzymes that have practical signficance:
- The 3' -> 5' exonuclease activity of T4 DNA polymerase is roughly 200 times that of Klenow fragment, making it preferred by many investigators for blunting DNAs with 3' overhangs.
- While Klenow fragment will displace downstream oligonucleotides as it polymerizes, T4 DNA polymerase will not. This attribute makes T4 DNA polymerase the more efficient choice for certain types of oligonucleotide mutagenesis reactions.