Induced variations in microorganisms I. The mutagenic effect of ultraviolet light on Rhizobium trifolii

Abstract

The mutagenic effect of ultraviolet light on a strain ofRhizobium trifolii T5 was studied. A gradual build-up of radioresistance in the population of the wild cells was observed as a result of cyclic UV irradiation, although there, was no definite indication for a plateau of maximal resistance of the population even after 20 cycles of irradiation. The radio-resistance was built up sooner in a population cycled at a low dose of irradiation (3 sec) than at a high dose (10 sec). The fluctuation test indicated that UV acted as an inducing agent. The frequency of radioresistant cells in a radiation cycled population was about 6.8×10^-6 as against c.4×10^-6 in the nonirradiated population. Five, mutants were examined in detail, in which radioresistance in two was accompanied by resistance to streptomycin also. The mutants did not differ drastically from the wild, strain in their biochemical properties, salt tolerance and clover infectivity. No UV induced auxotrophic mutants were detected.