ULTRAVIOLET LIGHT INDUCED MUTATION AND DEOXYRIBONUCLEIC ACID REPLICATION IN BACTERIA

Abstract

HE terminal process through which the genetic change induced by ultraviolet Tlight (UV) is incorporated into the genetic apparatus is the initial postirradia-tion deoxyribonucleic acid (DNA) replication (HAAS and DOUDNEY 1959; DOUD-NEY and HAAS 1960). It has been suggested that two discrete radiation effects are prerequisite to incorporation of the " potential mutation " into DNA (DOUDNEY, KADA and HAAS 1960; DOUDNEY 1961). These effects are (1) blockage of DNA synthesis by UV, causing a requirement for ribonucleic acid (RNA) and protein synthesis to restore the UV-damaged DNA synthetic system and (2) the estab-lishment of the photochemical modification which results in mutation with sub-sequent DNA replication. Through a concurrent examination of the kinetics of recovery of DNA synthesis in bacteria following various amounts of exposure to UV and the determination of mutation frequency response at the same exposure levels, it has been possible to obtain evidence to support this hypothesis. A study of distribution of the subunits of DNA obtained from UV-exposed bacteria using the cesium chloride density gradient technique has demonstrated that DNA replication following UV exposure is ' Lsemiconservative " and therefore in agree-ment with the results of MESELSON and STAHL (1958) with non-UV exposed bacteria. As a result of these findings, certain modifications of the general model for UV-induced mutation suggested by DOUDNEY (1961) are presented here. Organisms: Escherichia coli strain WP2 (tryptophan requiring) is used for the mutation study and the investigation of the recovery of DNA synthesis after UV exposure. This strain was isolated by WITKIN (1956) from E. coli strain B/r and supplied to this laboratory some years ago. In the UV-induced mutation study, reversion of the tryptophan requirement is followed. The parent prototrophic organism, E. coli strain B/r, is used in the studies of distribution of DNA subunits with replication. Culture growth procedures: A small inoculum of bacteria is taken from a 24 hour nutrient agar slant culture and added to minimal medium (DOUDNEY and HAAS 1958). With strain WP2, all growth media are supplemented with DL-1 This iniestigation was supported in part by U. S Atomic Energy Commisslon contract AT-(4&1)-2139