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Chapter 10 : Ethanolamine Utilization in Salmonella
Category: History of Science; Microbial Genetics and Molecular Biology
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In 1984, only two B12- dependent enzymes were known in Salmonella, ethanolamine ammonia lyase, required for use of ethanolamine (EA) as a nitrogen and carbon source, and a methyltransferase involved in cysteine biosynthesis. A genetic analysis of ethanolamine utilization was undertaken in John Roth’s laboratory in part to explore whether ethanolamine degradation contributes to the selective forces that maintain the ability to synthesize B12. Identification of the enzymes encoded in the eut operon suggests that ethanolamine degradation is an integral part of a network of reactions involving both reduction and oxidation of ethanolamine as well as fixation of CO2. EA can be used aerobically as a carbon, energy, and nitrogen source in the presence of Ado-B12 or a suitable corrinoid precursor. Under anaerobic conditions, EA can be used as a carbon source when tetrathionate is provided as an electron acceptor. Induction of the 17-gene eut operon requires the EutR regulatory protein and two effectors: EA, the substrate for the degredative pathway, and Ado-B12, the effector for EutBC lyase, which catalyzes the first step in the pathway. Induction of the operon is under autogenous control by eutR, the regulatory gene positioned at the end of the operon transcript. The eutT gene encodes an adenosyltransferase that plays a role in maintaining levels of Ado-B12 sufficient for eutT operon induction and lyase activity.
The eut operon: gene assignments and metabolic pathway. The horizontal line represents a portion of the S. enterica chromosome. The genes are shown in italic letters below the line; point mutations are indicated by numbers above the genes; Mud insertions are shown by triangles containing the letter M; and Tn10d insertions are shown with triangles containing the letter T. Transcripts are shown with solid arrows; the thickness of the line indicates the relative rate of transcription. Open boxes indicate the extent of the material deleted by deletion mutations 302 and 333. The metabolism of EA is shown below the chromosome. The eut-38::Mud-lac insertion is outside eutR but within the Piand P n transcripts. (Modified from references 22 - 24 , 28 , 29 , and 34 .)
Ethanolamine metabolic pathway. (Modified from reference 17 .)
Characterization of Eut− mutants a
eutT encodes an adenosyltransferase
Characterization of the major (P I) and minor (P I) eut promoters a
Titration of Ado-B12 by EutBC lyase alters eut operon induction kineticsa a
Alignments of Eut proteinsa a