Effect of Salmonella assay negative and positive carcinogens on intrachromosomal recombination in S-phase arrested yeast cells.

A wide variety of carcinogens including Ames assay (Salmonella) positive as well as Salmonella negative carcinogens induce intrachromosomal recombination (DEL recombination) in Saccharomyces cerevisiae. We have shown previously that the Salmonella positive carcinogens, ethyl methanesulfonate (EMS), methyl methanesulfonate (MMS) and 4-Nitroquinoline-N-oxide (4-NQO, and the Salmonella negative carcinogens, safrole, benzene, thiourea, carbon tetrachloride, and urethane, induced DEL recombination in growing, in G1 and in G2 arrested yeast cells. Since we found interesting differences in response between dividing and arrested cells, we wanted to find out whether these differences were due to the difference between cell division versus cell cycle arrest or to any particular cell cycle phase. In the present paper we incubated cells in the presence of hydroxyurea (HU) for cell cycle arrest in S-phase and exposed them to the above carcinogens, and plated them onto selective medium to determine DEL and interchromosomal recombination (ICR) frequencies. It was surprising that carbon tetrachloride had no effect on DEL recombination or ICR in HU treated cells even though it induced DEL recombination in G1 and G2 arrested as well as dividing cells. Further experiments are in agreement with the interpretation that carbon tetrachloride was responsible for prematurely pushing G1 cells into S-phase. The consequence of this may be replication on a damaged template which may be responsible for the action of carbon tetrachloride. EMS, MMS, 4-NQO and urethane were more recombinagenic in HU treated cells than in previous experiments with G2 arrested cells. None of the carcinogens appeared to be activated by S9 for either DEL recombination or ICR induction. Furthermore, we only detect cytochrome P-450 in dividing but not in arrested cells, arguing that possible differences in the ability to metabolize the compounds does not explain the observed differences for DEL recombination induction in the different cell cycle phases. We discuss these data in terms of the mechanism of induced DEL recombination and the possible biological activities of these carcinogens.