Graduate Thesis Or Dissertation
 

Recovery from ultraviolet light-induced damage in Micrococcus radiodurans

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  • The fortification of a growth medium for Micrococcus radiodurans with N-Z -Case (a pancreatic digest of casein) resulted in a decrease in pigmentation and resistance to ultraviolet-irradiation. The decreased resistance was exhibited on the survival curve as a reduction in the extrapolation number from ~300 to ~20 with little change in the slope of the exponential portion of the curve. The possibility that the decreased UV resistance resulted from a decrease in a protective or screening substance was ruled out by measuring the UV-induced conversion of thymine into thymine-containing dimers. Resistant and sensitive cells exposed to the same incident dose contained equivalent percentages of thymine-containing dimers. Recovery, defined as the repair of sub-lethal damage and a return to normal multiplicity, was determined using paired dose studies. A bimodal recovery curve was obtained for both resistant and sensitive populations irradiated to the same survival level. The difference in the UV inactivation response expressed by the resistant and sensitive cultures was concluded to result from differences in recovery rates; full recovery, in both cell populations irradiated to equivalent survival levels, occurred in the same amount of time, even though the resistant cells had received approximately twice the dose and had suffered almost twice the damage as the sensitive cells. A UV-induced lag in the completion of cytokinesis, with permanent inhibition at higher doses, was observed in M. radiodurans.
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