The E1p enzyme is an essential part of the pyruvate dehydrogenase complex (PDHC) and catalyzes the oxidative decarboxylation of pyruvate with concomitant acetylation of the E2p enzyme within the complex. We analyzed the Corynebacterium glutamicum aceE gene, encoding the E1p enzyme, and constructed and characterized an E1p-deficient mutant. Sequence analysis of the C. glutamicum aceE gene and adjacent regions revealed that aceE is not flanked by genes encoding other enzymes of the PDHC. Transcriptional analysis revealed that aceE from C. glutamicum is monocistronic and that its transcription is initiated 121 nucleotides upstream of the translational start site. Inactivation of the chromosomal aceE gene led to the inability to grow on glucose and to the absence of PDHC and E1p activities, indicating that only a single E1p enzyme is present in C. glutamicum and that the PDHC is essential for the growth of this organism on carbohydrate substrates. Surprisingly, the E1p enzyme of C. glutamicum showed up to 51% identity to homodimeric E1p proteins from gram-negative bacteria but no similarity to E1 ␣-or ␤-subunits of heterotetrameric E1p enzymes which are generally assumed to be typical for gram-positives. To investigate the distribution of E1p enzymes in bacteria, we compiled and analyzed the phylogeny of 46 homodimeric E1p proteins and of 58 ␣-subunits of heterotetrameric E1p proteins deposited in public databases. The results revealed that the distribution of homodimeric and heterotetrameric E1p subunits in bacteria is not in accordance with the rRNA-based phylogeny of bacteria and is more heterogeneous than previously assumed.The pyruvate dehydrogenase complex (PDHC) represents a member of a multienzyme complex family that also comprises the 2-oxoglutarate dehydrogenase complex (OGDHC) and the branched-chain 2-oxoacid dehydrogenase complex (BCOADHC). These enzymes catalyze the oxidative decarboxylation of pyruvate, 2-oxoglutarate, and the 2-oxo acids of the branched-chain amino acids L-leucine, L-valine, and L-isoleucine, respectively. In general, the multienzyme complexes are composed of multiple copies of three different enzymes, a thiamine pyrophosphate (TPP) containing 2-oxoacid decarboxylase (E1), a lipoic acid-containing dihydrolipoamide acyltransferase (E2), and the flavin-containing lipoamide dehydrogenase (LPD). The E1 enzyme catalyzes the irreversible, TPP-dependent oxidative decarboxylation of the 2-oxoacid, followed by the acylation of the lipoyl prosthetic group covalently attached to the E2 chain. The E2 component catalyzes the transfer of the acyl group from the lipoyl group to coenzyme A (CoA). The resulting dihydrolipoyl group is reoxidized by LPD, generating NADH and H ϩ from NAD ϩ (for a recent review, see reference 11). The E1 and E2 enzymes are specific for each of the three multienzyme complexes and therefore specified as E1p and E2p in the PDHC, E1o and E2o in the OGDHC, and E1b, and E2b in the BCOADHC. In contrast, the LPD component is common in the three multienzyme complexes in most organ...

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