Escherichia coli response to nitrosative stress
Vine, Claire Elizabeth (2012)
Ph.D. thesis, University of Birmingham.
Previous transcriptomic experiments have revealed that various Escherichia coli K-12 genes encoding proteins of unknown function are highly expressed during anaerobic growth in the presence of nitrate, or especially nitrite. Products of some of these genes, especially YeaR-YoaG, YgbA, YibIH and the hybrid cluster protein, Hcp, have been implicated in the response to nitrosative stress. The aims of this study were to investigate sources of nitrosative stress, and the possible roles of some of these proteins in protection against nitric oxide. The YtfE protein has been implicated in the repair of iron centres, especially in iron-sulphur proteins. The previously unexplained anaerobic growth defect of the ytfE strain LMS 4209 was shown to be due to a secondary 126-gene deletion rather than to the deletion of ytfE.
At the start of the project, the transcription factor, NsrR, was known to respond to low concentrations of intracellular NO, and to repress expression of ytfE, hcp-hcr encoding the hybrid cluster protein and its reductase, and hmp that encodes the flavohaemoglobin, Hmp. In this work, a biochemical assay of hcp promoter activity was developed as a reporter of intracellular NO generation. This assay was used in combination with a range of mutants to show that the major source of intracellular NO is the reduction of nitrite by the cytoplasmic nitrate reductase, NarG. Although the periplasmic cytochrome c nitrite reductase, NrfAB, and the cytoplasmic nitrite reductase, NirBD, decrease nitrosative stress by reducing nitrite to ammonia, at least one additional source of NO production from nitrite remains unidentified.
An assay for NO reduction using a Clark-Type electrode was validated. Rates of NO reduction were induced 2-fold in the presence of nitrate. Lowest rates of NO reduction were found in a mutant defective in nsrR. A quadruple mutant defective in Hmp, the flavorubredoxin, NorV, NrfAB and NirBD still reduced NO at more than half the rate of the parent. This residual activity was not due to YibIH, YeaR-YoaG or YgbA. Various hcp-hcr derivatives of strains defective in NO reductases revealed a severe growth defect under conditions of nitrosative stress, but Hcp was eliminated as a possible additional NO reductase. This growth defect was substantially suppressed by a further mutation in ytfE. Growth experiments with isogenic sets of mutant defective in all combinations of hcp and ytfE in addition to deletions in hmp, norVW and nrfAB implicated both YtfE and Hcp in repair of nitrosative damage. However, weaker phenotypes of these strains in absence of nitrate or nitrite are consistent with more general roles for these proteins in the repair of damage to protein iron centres.
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