Mutants of Escherichia coli K12 Unable to use Fumarate as an Anaerobic Electron Acceptor

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Abstract
Mutants of E. coli K12 strain WGAS-GF+/LF+ were selected for their inability to use fumarate as terminal electron acceptor for supporting growth on glycerol or lactate in an atmosphere of H2 plus 5% CO2. Mutants (83) were grouped into 7 different categories according to their ability to grow on different media and their ability to produce gas during glucose fermentation. Enzymological and genetic studies indicated that the major class (type I), representing nearly 70% of the isolates, lacked fumarate reductase and corresponded to the frdA mutants studied previously. Members of a 2nd class (type II) were phenotypically similar to men mutants, blocked in menaquinone biosynthesis. They differed from menA mutants in having lesions in the 44-51 min region of the chromosome rather than at 87 min. Fumarate reductase and menaquinone are essential for anaerobic growth when fumarate serves as electron acceptor but not when nitrate performs this function. Fumarate reductase and menaquinone are also essential for H2-dependent growth on fumarate. Type III mutants, originally frdB, were designated fnr for they were defective in fumarate and nitrate reduction and impaired in their ability to produce gas. The fnr gene was located at 28.5 min by its contransducibility with pyrF (5.7-9.2%) and trpA (2.7-5.7%) and the gene order fnr-qmeA-pyrF-trpA was established. Specific metabolic lesions were not assigned to the fnr mutants nor to the remaining classes, which all exhibited pleiotropic phenotypes. Functional or organizational relationships apparently exist between the fumarate reductase system, nitrate reduction and H2 production.

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