The use of viscoelastometry to determine survival curves for intact genomes

Viscoelastometry enables one to determine both size (Mr) and number concentration (L1) of intact genome molecules in solution. Comparison of four parameters, corrected for shear stress, as functions of 60Co gamma-ray dose showed that (1) the principal retardation time (tau 11,0) remained constant, indicating that intact genomes (bacteriophage T4c) were being measured; (2) the principal recoil (gamma 11,r,0) decreased with dose directly proportionately to (and determining) L1; (3) both the total recoil (gamma r,0) and the recoil area (Ar,0), under conditions of high solvent viscosity decreased with dose almost as sensitively as gamma 11,r,0. The DNAD37 was 540 +/- 25 Gy and the biological PFUD37 was 410.1 +/- 4.5 Gy yielding 75.9 +/- 3.6% of inactivating events explicable by one double-strand break (DSB) per genome. This value is comparable to Freifelder's [Virology 36, 613-619 (1981)] value of 86% for phage T4r48+, and the Frankenberg-Schwager et al. (Br. J. Cancer, in press) value of 0.84 DSB/cell/lethal event in diploid mutant rad54-3 yeast under conditions restrictive for DSB repair. Therefore, T4c may be an excellent model system for DNA damage repair studies with relevance to pro- and eukaryotes. Viscoelastometry, working near its lower size limit, provided precise estimates of the proportion of genomes lacking DSBs. It is not subject to the molecular deformation upper size limitations of the other biophysically understood size measurement methods (e.g., sedimentation rotor speed dependence). Therefore, it should be the method of choice for the study of genomes larger than those of the T even bacteriophages.