Source of the nonlinear dependence of bacteriophage SP82 transfection on deoxyribonucleic acid concentration

Extracts of competent cells of Bacillus subtilis exhibited nuclease activity on radioactively labeled cell deoxyribonucleic acid (DNA). The activity was not decreased when Mg was omitted from the reaction mixture but was decreased to zero by addition of ethylenediaminetetraacetic acid. Of the other metals tested, only Ca increased nuclease activity more than Mg. Addition of 1.5 x 10(-3)m Ca to transfection mixtures increased the nonlinearity of the relationship between number of transfectants and DNA concentration. The inferred role of the Ca-activated nuclease was checked by showing that the ultraviolet enhancement of transfection is reversed by addition of Ca. It was also shown, by testing in three different laboratories, that the Ca ion in water is the likely source of the different nonlinear relationships found in different laboratories.