SOME BIOCHEMICAL ASPECTS OF THE POSTIRRADIATION MODIFICATION OF ULTRAVIOLET-INDUCED MUTATION FREQUENCY IN BACTERIA

Abstract

Postirradiation treatments blocking RNA or protein syntesis lower markedly the ultraviolet-induced mutation frequency (reversion of tryptophan requirement) in Escherichia coli strain WP2. Treatments of less than 20 min reduce the mutation frequency to a minimal level without significant modification of the time or rate of subsequent synthesis of RNA, DNA, and protein. Therefore the mutation frequency decline process apparently is an active process, probably enzymatically mediated, and not dependent on delay in postirradiation macromolecular synthesis. The progressive loss of susceptibility of the potential mutation to these treatments is closely correlated with the progression of ribonucleic acid snythesis in the culture. A relation exists between the amoumt of RNA synthesized at the time of chloramphenicol addition, the relative rate of DNA synthesis in the presence of chloramphenicol, and the induced mutation frequency. Apparently only when the RNA and protein necessary for genetic replication are synthesized prior to chloramphenicol addition will DNA be formed. When the RNA and protein involved have not been formed, the potentiality for mutation is lost through the active decline process. Decline in induced mutation frequency is promoted by 5-hydroxyuridine after a considerable lag. This decline is correlated with the doubling of RNA in the culture, suggesting thatmore » the analogue causes the decline through incorporation into the RNA. However, loss of susceptibility to the 5-hydroxyuridine-promoted reduction in mutation frequency occurs prior to measurable RNA synthesis and is correlated with the mutation stabilization process. Icubation with 5hydroxyuridine of sufficient duration to prevent all sensitive mutations does not delay DNA synthesis. Several lines of evidence indicate that DNA synthesis is the terminal event in ultraviolet-induced mutation. However, delay of DNA synthesis does not modify ultraviolet-induced mutation frequency in the absence of the active decline process unless drastic treatments are employed which grossly alter cellular metabolism. Loss of mutation photoreversibility is correlated with DNA synthesis in the culture. Dinitrophenol (but not chloramphenicol) blocks the loss of mutation photoreversibility. (auth)« less