Ultraviolet Light Induced Mutation and Deoxyribonucleic Acid Replication in Bacteria.

HE terminal process through which the genetic change induced by ultraviolet Tlight (UV) is incorporated into the genetic apparatus is the initial postirradiation deoxyribonucleic acid (DNA) replication ( HAAS and DOUDNEY 1959; DOUDNEY 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 establishment of the photochemical modification which results in mutation with subsequent 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 agreement 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.