Specific effects of heating of transformable streptococci on their ability to discriminate between homospecific, heterospecific, and hybrid deoxyribonucleic acid.

Heating competent bacteria of the Challis strain of Streptococcus at a temperature of 48 C causes them to lose their transformability and mainfest a slight retardation of growth rate without loss of viability. The heat-induced loss of transformability is due to diminution in the ability of the bacteria to bind deoxyribonucleic acid (DNA) irreversibly. Another effect of heat is upon a step in the transformation process subsequent to binding, a step in which DNA molecules will compete if they multiply infect an unheated cell. Despite the reduction in irreversible binding exhibited by heated cells, competition between DNA molecules to transform these cells is decreased. Neither of these sites affected by heat exhibits any specificity with regard to origin of DNA. Since heat treatment causes a relative stimulation of transformation by heterospecific DNA, a third effect of heat must be envisaged. The amount of heat-induced stimulation is dependent upon the amount of heterospecific material in the transforming DNA. Linkage of heterospecific markers is increased as a consequence of heating the recipients. Transformation by markers of different transforming efficiency in homospecific DNA is also affected by heat treatment in a differential manner. Taken together, these results point to a heat-sensitive intracellular mechanism that recognizes DNA base sequences during transformation. The effect of heat upon discrimination against heterospecific DNA has been found to occur also in the pneumococcus and in Bacillus subtilis.