26 DNA Base-Insertion Enzymes (Insertases)

Abstract

DNA base-insertion enzymes (insertases) are a class of enzymes that catalyze the reinsertion of a missing base into the appropriate apurinic or apyrimidinic sites (AP sites) in DNA. The base-deoxyribose bonds in DNA are susceptible to spontaneous cleavages that release free bases and lead to the formation of apurinic or apyrimidinic sites in the DNA. Spontaneous base release under physiological conditions proceeds slowly, but with a significant rate. The rate of depurination or depyrimidination is highly increased for some base derivatives that are formed in the DNA upon its exposure to radiation or chemical agents. Because AP sites in cellular DNA interfere with correct readout of the genetic information, the ability of cells to repair this type of damage in their DNA seems to be a vital need for their survival, and most cells have the ability to repair AP sites efficiently. This chapter summarizes experiments that led to the discovery of purine insertase enzymatic activities in cultured human fibroblasts and in E. coli . These activities insert missing purines directly and specifically into the appropriate apurinic sites, with no apparent need for rupture of the phosphodiester bond at the site or excision of undamaged nucleotides. The possible action of purine insertases in vivo offers an alternative pathway for the repair of apurinic sites in a single enzymatic step.