Abstract
Incubation of a log-phase culture of a ultraviolet light-resistant, tryptophanrequiring strain of Escherichia coli (B/r WP2) for 40 min without tryptophan allows completion of the DNA replication cycle. When this occurs before ultraviolet light exposure an increase in ultraviolet light resistance of the DNA replication system of 2-fold (in terms of target number) over the log phase culture is produced. The projection to the ordinate of the relative rate values for inhibition of DNA synthesis after a series of ultraviolet light doses gave a target number of 4 for the log phase culture and 8 for the amino acid-starved culture. A comparable increase in resistance to the lethal effect of ultraviolet light was also seen with the same preirradiation treatment. The data suggest the possibility that the same mechanism is responsible for both the decreased ultraviolet light lethality and decreased inactivation of DNA replication. Both are presumably produced by completion of the DNA replication cycle during amino acid starvation, since lack of thymine with a thymine-requiring substrain prevents the development of resistance.
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