The thermal inactivation of transforming activity at low ionic strength

Abstract

A sample of triply marked pneumococcal DNA has been thermally denatured at pH 7 and ionic strengths of 0·1 M and 3 × 10 −4 M . The inactivation temperatures for the markers have been compared to the hyperchromicity midpoint and the marker densities reported by Rolfe & Ephrussi-Taylor (1961). We find that: (1) at low ionic strength, the transition breadth for inactivation is much greater than at high μ ; (2) the inactivation midpoint temperatures at low μ are much greater than the ambient hyperchromicity T m , while at high μ , this difference is small; ( c ) the differences between inactivation midpoints for different markers are increased at low μ . We suggest that the inactivation of each marker involves the denaturation of CG-rich critical regions. The critical regions for low ionic strength denaturation are shorter than the critical regions for high ionic strength denaturation. They are richer in GC content and more heterogeneous in base composition. We interpret the increased transition breadth at low ionic strength as evidence for a series of partially denatured states for each marker molecule.