Radiation Genetics I N Microorganisms and Evolutionary Considerations 1

Recent knowledge of UV-resis tance mechanisms in microorganisms is reviewed in perspective, with emphasis on E . coli. Dark-repair genes are classified into “excision” and “tolerance” (ability to produce a normal copy of DNA from damaged DNA). The phenotype of DNA repair is rather common among the microorganisms compared, and yet their molecular mechanisms are not universal. In contrast, DNA photoreactivation is the simplest and the most general among these three repair systems. It is proposed that DNA repair mechanisms evolved in the order: photoreactivation, excision repair, and tolerance repair. The UV protective capacity and light-inducible RNA photoreactivation possessed by some plant viruses are interpreted to be the result of solar UV selection during a rather recent era of evolution. HE high sensitivity of the living organism to radiation is a fundamental Tphenomenon and is due to the interaction of high penetrating energy with the genetic material indispensable and unique to living things. The number of DNA lesions required for inactivation of a genome, either by X or UV, increases with increase in genome complexity (TERZI 1961 ; KONDO 1964). This progressive radioresistance is believed to be mostly ascribable to DNA repair (HOWARDFLANDERS 1968). Our knowledge of the biological effects of radiation at the molecular level has recently increased considerably ( SETLOW and SETLOW 1972) and yet irradiation of a living thing with ionizing radiation often still present more questions than it answers. This is partly due to the complexity of the DNA damage induced (TOWN, SMITH and KAPLAN 1973). In contrast, the major cause of UV effects is known to be pyrimidine dimers. Therefore, the complexity of UV genetics in microorganisms must be mostly ascribed to the complex genetic apparatus. All living things are now believed, on the basis of ample evidence, to be descendants of a common ancestor (DAYHOFF 1972). Therefore, I believe thct radiation genetics in microorganisms is worthwhile reviewing from the evolutionary standpoint in order to make a framework into which diverse characteristics may be put together (KONDO 1972; RADMAN, ROMMELACRE and ERRERA 1973). Supported by grants from the Ministry of Education, Japan and the Toray Science Foundation. Abbreuiations: X (X or y rays); UV (ultraviolet); PR (photoreactivation); Exc (Excision); Inc (Incision); Rec (recombinational[al]); TDHT (5-thyminyl-5,6-dihydrothumine); dimers (cyclobutane-type pyrimidine d i m e r s in DNA); Pol (DNA polymerase [activity]); Exo (DNA exonuclease [activity]); superscripts “S”, “R’, “-”, e.g., U V S or UVB (UV-sensitive or -resistant for plaqueor colony-forming ability) ; Exc(excisionless). Genetics 78: 1W-161 September, 1974