Genes coding for components of the pyruvate dehydrogenase (PDH) multienzyme complex (PDHc) from Sinorhizobium meliloti, the alfalfa symbiont, have been isolated on the basis of their high expression in symbiotic bacteria. The E1p component, PDH, is encoded by two genes, pdhAα (1,047 bp) and pdhAβ (1,383 bp), a situation encountered in the α-proteobacteria Rickettsia prowazekii and Zymomonas mobilis as well as in some Gram-positive bacteria and in mitochondria. pdhAα and pdhAβ precede pdhB (1,344 bp), which encodes the E2p component, dihy-drolipoamide acetyltransferase, of the PDHc. No gene encoding the E3 component, lipoamide dehydrogenase, was found in the immediate vicinity of pdhA and pdhB genes. pdhAα, pdhAβ, and pdhB likely constitute an operon. Here, we provide evidence that pdhA expression is induced in the symbiotic stage, compared with free-living conditions. We demonstrate that symbiotic expression of pdhA genes does not depend on the fixLJ regulatory cascade that regulates nitrogen fixation and respiration gene expression in symbiotic S. meliloti cells. Induction of pdhA expression could be obtained under free-living conditions upon the addition of pyruvate to the culture medium. Induction by pyruvate and symbiotic activation of pdh gene expression take place at the same promoter.