Photoreactivation of Escherichia coli K12 (IFO 3301), in the presence or absence of yeast extract (YE), was investigated after inactivation by low-pressure UV lamp. An endonuclease sensitive site (ESS) assay was used to determine the UV-induced pyrimidine dimers in the genome of E. coli, while a colony-forming ability (CFA) test was also used to examine the survival ratio of E. coli. The YE solution reduced the CFA recovery at a final concentration of 125 mg/L. A dialysis of the YE solution indicated that the YE fraction (with nominal molecular weight >1,000 and <3,500) was effective at repressing the CFA recovery. Interestingly, the repair of ESS was equivalent regardless of the presence of the YE dialysate, while the CFA recovery was significantly repressed in the presence of YE. It was, therefore, suggested that YE components, probably with molecular weights of 1,000-3,500, were effective at repressing the CFA recovery of E. coli without affecting the ESS repair during photoreactivation.

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