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A stretch of 200 kilobases (kb) of DNA from the I region of the mouse major histocompatibility complex has been cloned and characterized. It contains the genes for the biochemically defined class II proteins E alpha, E beta and A beta. DNA blot analyses suggest that the I region may contain only 6-8 class II genes. Correlation of our molecular map with the genetic map of the I region confines two of the five I subregions, I-J and I-B, to less than 3.4 kb of DNA at the 3' end of the E beta gene where a hotspot for recombination has been observed. Indeed, the I-A and I-E subregions may be contiguous. If so, the I-B and I-J subregions are not encoded in the I region between the I-A and I-E subregions.

Production of NADPH in Saccharomyces cerevisiae cells grown on glucose has been attributed to glucose-6-phosphate dehydrogenase (Zwf1p) and a cytosolic aldehyde dehydrogenase (Ald6p) (Grabowska, D., and Chelstowska, A. (2003) J. Biol. Chem. 278, 13984 -13988). This was based on compensation by overexpression of Ald6p for phenotypes associated with ZWF1 gene disruption and on the apparent lethality resulting from co-disruption of ZWF1 and ALD6 genes. However, we have found that a zwf1⌬ald6⌬ mutant can be constructed by mating when tetrads are dissected on plates with a nonfermentable carbon source (lactate), a condition associated with expression of another enzymatic source of NADPH, cytosolic NADP ؉ -specific isocitrate dehydrogenase (Idp2p). We demonstrated previously that a zwf1⌬idp2⌬ mutant loses viability when shifted to medium with oleate or acetate as the carbon source, apparently because of the inadequate supply of NADPH for cellular antioxidant systems. In contrast, the zwf1⌬ald6⌬ mutant grows as well as the parental strain in similar shifts. In addition, the zwf1⌬ald6⌬ mutant grows slowly but does not lose viability when shifted to culture medium with glucose as the carbon source, and the mutant resumes growth when the glucose is exhausted from the medium. Measurements of NADP(H) levels revealed that NADPH may not be rapidly utilized in the zwf1⌬ald6⌬ mutant in glucose medium, perhaps because of a reduction in fatty acid synthesis associated with loss of Ald6p. In contrast, levels of NADP ؉ rise dramatically in the zwf1⌬idp2⌬ mutant in acetate medium, suggesting a decrease in production of NADPH reducing equivalents needed both for biosynthesis and for antioxidant functions.Reducing equivalents in the form of NADPH are required for numerous biosynthetic enzymatic reactions and antioxidant mechanisms involving glutathione and/or thioredoxin. The major cellular source of NADPH is thought to be the hexose monophosphate pathway. However, disruption of the Saccharomyces cerevisiae ZWF1 gene encoding glucose-6-phosphate dehydrogenase, the first and rate-limiting enzyme in that pathway, was found to produce relatively mild growth phenotypes including methionine auxotrophy and an increased sensitivity to exogenous oxidizing agents like hydrogen peroxide (1, 2). In studies designed to identify other crucial sources of NADPH in yeast, co-disruption of ZWF1 and the gene (IDP2) encoding cytosolic NADP ϩ -specific isocitrate dehydrogenase was found to produce a rapid loss in cell viability following shifts from medium containing glucose to medium containing either oleate or acetate as the carbon source (3, 4). This loss of viability correlated with an increase in levels of endogenous cellular oxidants. Fatty acid metabolism requires rapid flux through ␤-oxidation, a peroxisomal process in yeast that produces hydrogen peroxide in the first reaction of each cycle (5, 6), and acetate is a stringent carbon source that produces rapid flux through mitochondrial respiration, a process associated with the generation of dele...

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