Genetic evidence that genes fdhD and fdhE do not control synthesis of formate dehydrogenase-N in Escherichia coli K-12.

Enterobacteria synthesize two formate dehydrogenases, formate dehydrogenase-N (encoded by fdnGHI) and formate dehydrogenase H (encoded by fdhF). Previous work has identified two rha-linked Salmonella typhimurium genes, fdnB and fdnC, which are required primarily for formate dehydrogenase-N activity. Analogous mutants, termed fdhD and fdhE, have been isolated in Escherichia coli. We used gene fusions between fdnG, the structural gene for the large subunit of formate dehydrogenase-N, and lacZ, the structural gene for beta-galactosidase, to examine E. coli fdnGHI operon expression in fdhD and fdhE insertion mutants. Expression of the phi (fdnG-lacZ) gene fusions was little affected by these insertions, suggesting that fdhD and fdhE do not control transcription or UGA decoding of the formate dehydrogenase-N structural genes. Our complementation tests, with cloned E. coli fdhD and fdhE genes, indicate that the S. typhimurium fdnC and fdnB genes are functionally homologous to the E. coli fdhD and fdhE genes, respectively.