The influence of culture age on induction of mutation and mitotic recombination by MNNG and EMS in yeast Saccharomyces cerevisiae D7

280 95 Matija~evi6, Z., J. Franeki6, V. Ba~un-DruZina and M. Ala6evi6, Faculty of Food and Biotechnology, Pierottijeva 6, 41000 Zagreb (Yugoslavia) The influence of culture age on induction of mutation and mitotic recombination by M N N G and EMS in yeast Saccharomyces cerevisiae D7 Among other factors (pH, concentration of agents, etc.) which have to be considered in mutagenicity testing, the level of particular genetic event is significantly influenced by the growth phase of microbial test culture. The induction of reverse mutation at ilvl, gene conversion at trp5 and mitotic crossing-over at ade2 were measured in a population of yeast cells taken from the exponential and stationary phase after treatment with the two standard chemical mutagens M N N G and EMS. The yields of gene convertants and especially retromutants are increased in exponential phase culture by both mutagens. On the other hand, the induction of chromosomal aberrations together with crossing-over is better expressed in stationary-phase cells. Among the genetic effects examined mutagenic effects were the most dose dependent.

96 McConville, M., D. McGregor and D. Prentice, Inveresk Research International Ltd., Musselburgh, EH21 7UB (Great Britain) Simple new method for assessing toxicity in the Salmonella/activation assay A method was devised for assessing the toxicity of compounds in the Salmonella/activation assay. This method might be useful for evaluating the mutagenic potential of bacteriocidal compounds. The microcolonies formed by non-mutant, his- bacteria which form the background 'lawn' on a typical Salmonella/activation test plate are counted by linking a microscope, on which a video camera is mounted, to an automated colony counter. After focusing on a randomly selected area of 'lawn', his- microcolonies can be viewed on a T.V. monitor and automatically counted. The method gives an accurate count of the numbers of microcolonies, provided the number of viable bacteria per plate does not greatly exceed 1.5 x 10 7. The relationship between numbers of his +-revertant colonies and surviving bacteria has been studied using combinations of phthalic anhydride and 2-nitrofluorene. Here, the numbers of his +-revertant colonies are directly proportional to the number of surviving bacteria, but it is not yet known if this applies to toxic mutagens in general in this assay.