Role of AP endonuclease in DNA breakage and cell inactivation of Escherichia coli subjected to mild heat (52 degrees C)

Visual quantification of DNA double-strand breaks in bacteria

In this paper, we describe a method for the visualization of double-strand breaks in a single electrostretched Escherichia coli DNA molecule. We also provide evidence that electrostretched or migrated DNA under neutral microgel electrophoresis conditions is made up of individual chromosomes. Using the neutral microgel electrophoresis technique, DNA migration (stretching) was measured and the number of DNA double-strand breaks were counted following exposure of E. coli cells to 0, 12.5, 25, 50, or 100 rad of X-rays. The use of an intense fluorescent dye, YOYO and custom-made slides have helped us in visualizing individual bacterial DNA molecules. Bacterial DNA appears similar in structure compared to electrostretched DNA from human lymphocytes. We were able to detect changes in DNA migration (stretching) induced by an X-ray dose as low as 12.5 rad and an increase in the number of DNA breaks induced by a dose as low as 25 rad. The extent of DNA migration and number of breaks were directly correlated to X-ray dosage.

Effect of the DNA replication velocity on the response of Salmonella typhimurium to mild heating

Changes in the DNA replication velocity of Salmonella typhimurium following mild heat stress (52 degrees C) were studied independently of the major physiological parameter of growth rate, using thymine-requiring mutant strains derived from Salm. typhimurium LT2. The isolated mutant strains BM1 or BM2, grown either as batch or chemostat cultures, showed a greater sensitivity to 52 degrees C heat stress when grown on a minimal medium containing near-limiting concentrations of thymine, compared with growth in the presence of excess thymine. Radiolabelling experiments provided evidence for alterations in the velocity of DNA replication upon growth on different thymine concentrations, independent of the growth rate. Thus, replicating DNA was implicated as the major site of injury after mild moist heat stress.

Elevation of the heat resistance of Salmonella typhimurium by sublethal heat shock

The survival of Salmonella typhimurium after a standard heat challenge at 55 degrees C for 25 min increased by several orders of magnitude when cells grown at 37 degrees C were pre-incubated at 42 degrees, 45 degrees or 48 degrees C before heating at the higher temperature. Heat resistance increased rapidly after the temperature shift, reaching near maximum levels within 30 min. Elevated heat resistance persisted for at least 10 h. Pre-incubation of cells at 48 degrees C for 30 min increased their resistance to subsequent heating at 50 degrees, 52 degrees, 55 degrees, 57 degrees or 59 degrees C. Survival curves of resistant cells were curvilinear. Estimated times for a '7D' inactivation increased by 2.6- to 20-fold compared with cells not pre-incubated before heat challenge.

Genetic control of heat resistance and thermotolerance by recA and uvrA in E. coli K12

Several recA and uvrA derivatives of E. coli K12 AB1157 develop a transient increase in heat resistance, i.e. induced thermotolerance after a brief exposure to 43.5 degrees C (less than 1 h). Thermotolerance was identified from the appearance of an inflection in the survival curve or from the loss of heat resistance in the presence of chloramphenicol (CAM) or rifampicin. Heat resistance and induced thermotolerance were enhanced by recA and uvrA gene functions and their contribution was roughly as follows: AB1157 (recA+ uvrA+) greater than AB2463 (recA- uvrA+) greater than AB1886 (recA+ uvrA-) greater than AB2480 (recA- uvrA-). In heat resistance, uvrA and recA contributed approximately equally and their effects were additive. Induced thermotolerance developed sooner and was maintained at a higher level in the presence of uvrA as compared with recA. Since uvrA-dependent excision repair is scheduled prior to recA-dependent (postreplication) repair, induction of thermotolerance may be linked to DNA repair. Although recA and uvrA play a distinct role, they are not essential, and thermotolerance can develop in the absence of either one or both of these gene functions. Furthermore, since thermotolerance can be induced in recA mutants (AB2463 and AB2480), its biochemical pathway must be different from that of the recA-dependent SOS system.