Mutations in trans which affect the anaerobic expression of a formate dehydrogenase (fdhF) structural gene

Abstract

An operon fusion was constructed in which the chloramphenicol acetyltransferase gene (cat) is under the transcriptional control of the anaerobically-activated formate dehydrogenase (fdhF) gene promoter. It was used as a screening system for mutations in trans which prevent the formate-dependent anaerobic induction of fdhF gene expression. Five classes of mutants were identified. The defect in class I mutants was complemented by a plasmid (pBA11) or subclones thereof, which harbor genes of the Escherichia coli 58 min hyd (hydrogenase) gene cluster. They may comprise regulatory gene mutants. The phenotype of class II mutants was reversed by supplementing the medium with 100 μM MoO ₄ ^2- ; WO ₄ ^2- could substitute for MoO ₄ ^2- in restoring anerobic induction by formate. Similarly, class III mutants were phenotypically suppressed by inclusion of 500 μM Ni²⁺ in the medium; these mutants were shown to carry a defective fnr gene. The mutant of class IV had a defect in a formate dehydrogenase structural gene and that of class V was unable to grow under fermentative conditions while maintaining the capability to grow anaerobically in the presence of electron acceptors.