Mutants of Diplococcus pneumoniae that Lack Deoxyribonucleases and Other Activities Possibly Pertinent to Genetic Transformation

Mutants of Diplococcus pneumoniae that lacked the two major deoxyribonucleases of the cell—one an endonuclease, the other an exonuclease preferentially active on native deoxyribonucleic acid (DNA)—were obtained. The development of a method for detecting mutant colonies, based on the binding of methyl green to DNA, facilitated isolation of the mutants. Neither enzyme was essential for growth of the cells, for repair of ultraviolet damage, or for any phase of DNA-mediated transformation. Residual deoxyribonuclease activity in the double mutant corresponded to an exonuclease, approximately one-fifth as active as the major exonuclease, that attacked native and denatured DNA equally well. This activity appeared to be associated with the DNA-polymerase enzyme. A mutant that apparently lacked a cell wall lytic enzyme was also fully transformable. A mutant strain that was four times more sensitive to ultraviolet light than the wild type also transformed normally. Recipient cells of this strain were deficient in the repair of ultraviolet-irradiated transforming DNA. Mutants were found which, unlike the wild type, integrated donor markers only with high efficiency, thereby indicating that a particular cellular component that is susceptible to loss by mutation, such as an enzyme, is responsible for low integration efficiency.