Symbiotic induction of pyruvate dehydrogenase genes from Sinorhizobium meliloti

Genes coding for components of the pyruvate dehydrogenase (PDH) multienzyme complex (PDHc) from Sinorhizobium meliloti, the alfalfa symbiont, have bee n isolated on the basis of their high expression in symbiotic bacteria. The E1p component, PDH, is encoded by two genes, pdhA alpha (1,047 bp) and pdh A beta (1,383 bp), a situation encountered in the alpha-proteobacteria Rick ettsia prowazekii and Zymomonas mobilis as well as in some Gram-positive ba cteria and in mitochondria, pdhA alpha and pdhA beta precede pdhB (1,344 bp ), which encodes the E2p component, dihydrolipoamide acetyltransferase, of the PDHc No gene encoding the E3 component, lipoamide dehydrogenase, was fo und in the immediate vicinity of pdhA and pdhB genes, pdhA alpha; pdhA alph a and pdh beta likely constitute an operon, Here, we provide evidence that pdhA expression is induced in the symbiotic stage, compared with free-livin g conditions. We demonstrate that symbiotic expression of pdhA genes does n ot depend on the fixLJ regulatory cascade that regulates nitrogen fixation and respiration gene expression in symbiotic S, meliloti cells, Induction o f pdhA expression could be obtained under free-living conditions upon the a ddition of pyruvate to the culture medium. Induction by pyruvate and symbio tic activation of pdh gene expression take place at the same promoter.