The complete set of viable deletion strains in Saccharomyces cerevisiae was screened for sensitivity of mutants to five oxidants to identify cell functions involved in resistance to oxidative stress. This screen identified a unique set of mainly constitutive functions providing the first line of defense against a particular oxidant; these functions are very dependent on the nature of the oxidant. Most of these functions are distinct from those involved in repair and recovery from damage, which are generally induced in response to stress, because there was little correlation between mutant sensitivity and the reported transcriptional response to oxidants of the relevant gene. The screen identified 456 mutants sensitive to at least one of five different types of oxidant, and these were ranked in order of sensitivity. Many genes identified were not previously known to have a role in resistance to reactive oxygen species. These encode functions including protein sorting, ergosterol metabolism, autophagy, and vacuolar acidification. Only two mutants were sensitive to all oxidants examined, only 12 were sensitive to at least four, and different oxidants had very different spectra of deletants that were sensitive. These findings highlight the specificity of cellular responses to different oxidants: No single oxidant is representative of general oxidative stress. Mitochondrial respiratory functions were overrepresented in mutants sensitive to H2O2, and vacuolar protein-sorting mutants were enriched in mutants sensitive to diamide. Core functions required for a broad range of oxidative-stress resistance include transcription, protein trafficking, and vacuolar function.

Ascertaining the impact of inhibitors on the growth phenotype of yeast mutants can be useful in elucidating the function of genes within the cell. Microtitre plates and robotics have been used to screen over 600 deletions from EUROSCARF, constructed in an FY1679 strain background, for sensitivity to various oxidants. These included the inorganic hydroperoxide, H 2 O 2 , an organic peroxide (cumene hydroperoxide) and a lipid hydroperoxide (linoleic acid hydroperoxide). These produce within the cell several different reactive oxygen species that can cause damage to DNA, proteins and lipids. Approximately 14% of deletants displayed sensitivity to at least one of the oxidants and there was also a distribution of deletants that showed sensitivity to all or different combinations of the oxidants. Deletants included genes encoding proteins involved in stress responses, heavy metal homeostasis and putative cell wall proteins. Although global mechanisms have been identified that provide general stress responses, these results imply that there are also distinct mechanisms involved in the protection of the cell against specific damage caused by different oxidants. Further analysis of these genes may reveal unknown mechanisms protecting the cell against reactive oxygen species.

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H2O2-stressed yeast cells increase superoxide radical production, dependent on the mitochondrial respiratory chain. This is protective during H2O2 stress at low levels; however, higher superoxide levels are deleterious. This hormesis may further elucidate the role of reactive oxygen species in oxidative stress and aging.

Cells treated with low doses of linoleic acid hydroperoxide (LoaOOH) exhibit a cell-cycle delay that may provide a mechanism to overcome oxidative stress. Strains sensitive to LoaOOH from the genome-wide deletion collection were screened to identify deletants in which the cell-cycle delay phenotype was reduced. Forty-seven deletants were identified that were unable to mount the normal delay response, implicating the product of the deleted gene in the oxidant-mediated cell-cycle delay of the wild-type. Of these genes, SWI6 was of particular interest due to its role in cell-cycle progression through Start. The swi6 deletant strain was delayed on entry into the cell cycle in the absence of an oxidant, and oxidant addition caused no further delay. Transforming the swi6 deletant with SWI6 on a plasmid restored the G1 arrest in response to LoaOOH, indicating that Swi6p is involved in oxidant sensing leading to cell division delay. Micro-array studies identified genes whose expression in response to LoaOOH depended on SWI6. The screening identified 77 genes that were upregulated in the wild-type strain and concurrently downregulated in the swi6 deletant treated with LoaOOH. These data show that functions such as heat shock response, and glucose transport are involved in the response.