MUTANTS OF ESCHERICHIA-COLI K-12 THAT ARE RESISTANT TO A SELENIUM ANALOG OF LIPOIC ACID IDENTIFY UNKNOWN GENES IN LIPOATE METABOLISM

Lipoic acid is a disulfide-containing cofactor required for the reacti ons catalyzed by alpha-ketoacid dehydrogenase enzyme complexes. We rep ort the chemical synthesis and biological properties of lipoic acid an alogs in which one or both sulfur atoms were replaced by selenium. Rep lacement of either the C-6 or the C-8 sulfur atom with selenium result s in lipoic acid derivatives with apparently unaltered biological prop erties. However, simultaneous replacement of both sulfur atoms gave an analog (selenolipoic acid) that inhibited growth of wild-type Escheri chia coil when present in minimal glucose medium at 50 ng/ml. This gro wth inhibition was reversed by the addition of either excess lipoic ac id or acetate plus succinate. Labeling experiments with [Se-75]selenol ipoic acid showed that this compound was efficiently incorporated into the alpha-ketoacid dehydrogenase complexes of growing cells. Spontane ously arising selenolipoic acid-resistant (slr) mutants were isolated. Two of these isolates resistant to high levels of selenolipoic acid w ere studied in detail. The slr-1 mutation, which was mapped to min 99. 6 of the E. coil chromosome, increased the lipoate requirement of lipA strains by 4-fold and appeared to define a gene encoding a lipoate-pr otein ligase. The slr-7 mutation, which was mapped to min 15.25 of the chromosome, completely suppressed the lipoate requirement of lipA str ains and defined a gene of unknown function in the synthesis of lipoic acid.