Identification and Characterization of a Suppressor Mutation That Relieves the Enzymic and Growth Defects of Sod-Deficient Escherichia Coli
Maringanti, Sujatha
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https://hdl.handle.net/2142/86748
Description
Title
Identification and Characterization of a Suppressor Mutation That Relieves the Enzymic and Growth Defects of Sod-Deficient Escherichia Coli
Author(s)
Maringanti, Sujatha
Issue Date
1999
Doctoral Committee Chair(s)
Imlay, James A.
Department of Study
Microbiology
Discipline
Microbiology
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Biology, Molecular
Language
eng
Abstract
Mutants of Escherichia coli that lack cytoplasmic superoxide dismutase exhibit auxotrophies for sulfur-containing, branched-chain, and aromatic amino acids and cannot catabolize non-fermentable carbon sources. A secondary-site mutation substantially relieved these growth defects. This suppression was mediated at least in part by the restoration of activity to the oxidatively labile dehydratases, evidently without changing the internal concentration of O2-. Cloning, complementation and sequence analysis identified the suppressor mutation to be in dapD, which encodes tetrahydrodipicolinate succinylase, an enzyme involved in diaminopimelate/lysine biosynthesis. A block in dapB, which encodes dihydrodipicolinate reductase in the same pathway, conferred similar protection. Genetic analysis indicated that the accumulation of tetrahydro- or dihydrodipicolinate mediates protection. Expression of dipicolinate synthase, an enzyme from Bacillus subtilis that generates dipicolinate, also protected the E. coli SOD mutants. The SOD-deficient strains are somewhat starved for iron, and the iron-binding ability of these dipicolinates caused the Fur (iron uptake) regulon to be derepressed in the suppressed strains. However, a fur mutation only partially relieved the auxotrophies, indicating that Fur derepression assists but is not sufficient for suppression. The connection between changes in iron metabolism and the increased dehydratase activities of the suppressor strains is explored. This study provides circumstantial evidence that the sulfur and aromatic auxotrophies are directly or indirectly linked to cluster damage.
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