Biochemical Genetics of the -Keto Acid Dehydrogenase Complexes of Escherichia coli K 12: Genetic Characterization and Regulatory Properties of Deletion Mutants

Abstract

Twenty-eight spontaneous auxotrophic aroP mutants with deletions in the azi-nadC-aroP-aceE-aceF-lpd region of the E. coli K12 chromosome were characterized genetically with respect to various azi, nadC, ace and lpd markers by phage P1-mediated transduction. One mutant (K.DELTA.18; aroP-lpd.DELTA.) had a deletion which extended through the aceE and aceF genes to end within the lpd gene. The polarity of the ace operon (aceE to aceF) was confirmed. Of 15 deletions, 10 generated a strict requirement for acetate terminated in the aceE gene. Of the 10, 3 mutants (K.DELTA.22, C.DELTA.41 and C.DELTA.42) synthesized detectable dihydrolipoamide acetyltransferase (the aceF gene product) and 7 were assumed to possess deletions generating polar effects on aceF gene expression. Five deletions appeared to extend into the aceF gene. A further 5 deletions, which limited the expression of the ace operon without generating an Ace- phenotype or a complete Ace- phenotype, ended closest to the aroP-proximal aceE markers. The opposite ends of all these deletions appeared to terminate before, within or extend beyond the nadC gene. There was no obvious correlation between the deletion end-points and the corresponding lipoamide dehydrogenase activities, which ranged from 30-95% of parental levels in different deletion strains. The remaining 7 deletions simply extended between the aroP and nadC genes (nad-aroP.DELTA.) without affecting expression of the ace operon. Regulation of the synthesis of the pyruvate and .alpha.-ketoglutarate dehydrogenase complexes was investigated in some of the parental and deletion strains under different physiological conditions including thiamine-deprivation. The results indicate that the syntheses of the 2 dehydrogenase complexes are independently regulated. Expression of the lpd gene appears to be coupled to complex synthesis but can be dissociated under some conditions. Mechanisms for regulating lpd gene expression are discussed, and an autogenous mechanism involving uncomplexed lipoamide dehydrogenase functioning as a negatively acting repressor at the operator site of an independent lpd gene is proposed as the simplest mechanism which is consistent with all available information.