HEAT-SHOCK CHANGES THE RESPONSE OF THE PSO3 MUTANT OF SACCHAROMYCES-CEREVISIAE TO 8-METHOXYPSORALEN PHOTOADDITION

A putative tolerance, induced by heat shock (HS), to the lethal and mu tagenic effects of 8-methoxypsoralen (8-MOP) photoaddition and hyperth ermia was analyzed in Saccharomyces cerevisiae using the wild-type str ain N123 and the isogenic DNA repair-deficient mutant pso3-1. In wild- type cells, the HS (38 degrees C for 1 h) did not modify either the su rvival or the mutation frequency observed after 8-MOP photoaddition, e ven though it conferred protection against the lethal effect of hypert hermia (50 degrees C). In the pso3-1 mutant, HS induced an increase of the survival, and a decrease of the mutation frequency, after 8-MOP p hotoaddition and it also protected against the lethal effect of hypert hermia. The responses induced by HS were specific for 8-MOP photoaddit ion, since they were not observed after 254 nm ultraviolet-light damag e. These results indicate that the protection conferred by HS depends of the type of lesion, and operates through the induction of different repair processes. In the pso3-1 mutant, HS could channel the repair i ntermediates to and error-free repair pathway.