Gene-Protein Relationships of the -Keto Acid Dehydrogenase Complexes of Escherichia coli K12: Chromosomal Location of the Lipoamide Dehydrogenase Gene

Two representative lipoamide dehydrogenase mutants (lpd1 and lpd4) were used to locate the lpd gene in the linkage map of Escherichia coli by conjugation and detailed transductional analysis with phage P1. Time of entry mapping indicated that the lpd gene was between leu and proC and very close to leu. Average cotransduction frequencies between lpd and other markers were: aceF (97 %), aceE (93 %), aroP (84 %), nadC (71 %), azi (64 %), leu (8 to 41 %), ara (38 %), pan (21 %), tonA (17 %) and thr, nad and gal (< 1 %). These values, together with the contransduction frequencies for many other pairs of markers in this region and the results of three-factor crosses, established the gene order leu-azi-nadC-aroP-aceE-aceF-lpd-pan. The very close proximity of aceF and lpd suggested that they may be contiguous and that the lpd gene may be the distal gene in the pyruvate dehydrogenase operon. In this case, the expression of the lpd gene may be governed by a secondary transcriptional promoter in the aceF-lpd region in order to generate lipoamide dehydrogenase components for assembly of the α-ketoglutarate dehydrogenase complex. Other possible mechanisms are also discussed.