Twenty-eight spontaneous auxotrophic mutants with deletions in the region of the K 12 chromosome were characterized genetically with respect to various and markers by P1-mediated transduction. One mutant (K δ18; δ) had a deletion which extended through the and genes to end within the gene. The polarity of the operon ( to ) was confirmed. It was concluded that 10 out of 15 deletions generating a strict requirement for acetate terminated in the gene. Of the ten, three mutants (K δ22, cδ41 and cδ42) synthesized detectable dihydrolipoamide acetyltransferase (the gene product) and seven were assumed to possess deletions generating polar effects on gene expression. Five deletions appeared to extend into the gene. A further five deletions, which limited the expression of the operon without generating an Ace phenotype or a complete Ace phenotype, ended closest to the -proximal markers. The opposite ends of all these deletions appeared to terminate before (10), within (2) or extend beyond (9) the gene. There was no obvious correlation between the deletion end-points and the corresponding lipoamide dehydrogenase activities, which ranged from 30 to 95% of parental levels in different deletion strains. The remaining seven deletions simply extended between the and genes (δ) without affecting expression of the operon.

Regulation of the synthesis of the pyruvate and α-ketoglutarate dehydrogenase complexes was investigated in some of the parental and deletion strains under different physiological conditions including thiamin-deprivation. The results indicate that the syntheses of the two dehydrogenase complexes are independently regulated. Expression of the gene appears to be coupled to complex synthesis but can be dissociated under some conditions. Mechanisms for regulating 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 gene is proposed as the simplest mechanism which is consistent with all available information.


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