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SUMMARY: The size of the competent fraction in Bacillus subtilis cultures was calculated from the frequencies of observed and expected double transformants for the two unlinked markers adenine and indole, and compared with that obtained by light-microscopic autoradiography of samples exposed to [3H]thymidine labelled donor DNA. The two methods give similar results. The size of the competent fraction calculated from the frequencies of observed and expected double transformants is independent of the concentration of DNA and time of exposure to transforming DNA. The observed increase in the frequency of transformation with longer periods of exposure of the cultures to DNA was found to be linearly correlated with the amounts of DNA taken up by individual cells, as judged by the number of grains per labelled cell. No important heterogeneity within the competent fraction with respect to the amount of DNA absorbed per competent cell was observed. It was further demonstrated that under conditions in which competence develops (the competence regime), the transformation frequency and the size of the competent fraction increase proportionally, whereas the amount of DNA absorbed per competent cell does not change. This suggests that the efficiency of competent cells to integrate the donor markers is constant during the competence regime, and is, therefore, not a significant factor determining changes in transformability.

Autoradiographic Estimation of Competence and the Relationship between Competence and Transformability in Cultures of Bacillus subtilis