Restoration of viability to an Escherichia coli mutant deficient in the 5'----3' exonuclease of DNA polymerase I.

An Escherichia coli polA (Ex) mutant that is usually inviable at restrictive temperatures (43 degrees C) was found to grow normally at 43 degrees C when incubated in the presence of a membrane-containing fraction prepared from E. coli. This membrane fraction causes anaerobic conditions that are necessary but not sufficient for restoration of viability since some component present in the membrane fraction is also required for colony formation at 43 degrees C. The accumulation of small DNA fragments typical of aerobic growth of the polA(Ex) mutant was also seen under anaerobic conditions. The polA(Ex) strain was also much more sensitive than the isogenic wild-type strain to hydrogen peroxide.     

Physiological and nutritional responses of beef steers exposed to stable flies (Diptera: Muscidae).

Physiological and nutritional responses were measured in beef steers exposed to laboratory infestations of stable flies, Stomoxys calcitrans (L.). Calves (n = 6 per treatment) were infested with four levels of stable flies for three periods daily (15 min each period) for 14 d. Steers exposed to 0, 10, 20, or 30 stable flies consumed similar amounts of feed and water; they excreted similar amounts of feces and urine throughout the study. During the experiment, changes in body weight were not influenced by treatment. Heart and respiration rates, rectal temperatures, and circulating cortisol concentrations were similar among groups. Finally, nitrogen consumed, nitrogen lost in feces and urine, and nitrogen retained were unaffected by treatment. We suggest that intermittent exposure to these levels of stable flies, under the conditions of this study, does not evoke physiological and nutritional responses in beef steers.