title: H-NS is a novel transcriptional modulator of the ribonucleotide reductase genes in Escherichia coli.
abstract: Ribonucleotide reductases (RNRs) are essential enzymes for DNA synthesis because they are responsible for the production of the four deoxyribonucleotides (dNTPs) from their corresponding ribonucleotides. Escherichia coli contains two classes of aerobic RNRs, encoded by the nrdAB (class Ia) and nrdHIEF (class Ib) operons, and a third RNR class, which is functional under anaerobic conditions and is encoded by the nrdDG (class III) operon. Because cellular imbalances in the amounts of the four dNTPs cause an increase in the rate of mutagenesis, the activity and the expression of RNRs must be tightly regulated during bacterial chromosome replication. The transcriptional regulation of these genes requires several transcription factors (including DnaA, IciA, FIS [factor for inversion stimulation], Fnr, Fur, and NrdR), depending on the RNR class; however, the factors that dictate the expression of some RNR genes in response to different environmental conditions are not known. We show that H-NS modulates the expression of the nrdAB and nrdDG operons. H-NS represses expression both in aerobically and in anaerobically growing cells. Under aerobic conditions, repression occurs at the exponential phase of growth as well as at the transition from the exponential to the stationary phase, a period when no dNTPs are needed. Under anoxic conditions, repression occurs mainly in exponentially growing cells. Electrophoretic mobility assays performed with two DNA fragments from the regulatory region of the nrdAB operon demonstrated the direct interaction of H-NS with these sequences.
authors.affiliation: Institute for Bioengineering of Catalonia, Cellular Biotechnology, Barcelona, Spain.
authors.forename: Maria del mar
authors.initials: Mdel M
authors.lastname: Cendra
authors.forename: Antonio
authors.initials: A
authors.lastname: Juárez
authors.forename: Cristina
authors.initials: C
authors.lastname: Madrid
authors.forename: Eduard
authors.initials: E
authors.lastname: Torrents
mesh_terms: D000332:Aerobiosis; D000693:Anaerobiosis; D024202:Electrophoretic Mobility Shift Assay; D004926:Escherichia coli / Q000201:enzymology / Q000235:genetics / Q000254:growth & development* / Q000378:metabolism*; D029968:Escherichia coli Proteins / Q000096:biosynthesis / Q000235:genetics* / Q000378:metabolism; D036781:Fimbriae Proteins / Q000235:genetics / Q000378:metabolism*; D015964:Gene Expression Regulation, Bacterial; D009876:Operon / Q000235:genetics / Q000502:physiology; D012262:Ribonucleoside Diphosphate Reductase / Q000235:genetics / Q000378:metabolism; D012264:Ribonucleotide Reductases / Q000096:biosynthesis / Q000235:genetics* / Q000378:metabolism
chemical_list: D029968:Escherichia coli Proteins; C529458:FimG protein, E coli; D036781:Fimbriae Proteins; C571544:NrdB protein, E coli; C490362:NrdG protein, E coli; D012264:Ribonucleotide Reductases; C101141:anaerobic ribonucleotide reductase; C449642:NrdA protein, E coli; D012262:Ribonucleoside Diphosphate Reductase