Disease relevance of norA

• Quinolone susceptibility of norA-disrupted Staphylococcus aureus [1].
• The norA gene promoter is active in Escherichia coli HB101 [2].
• The Bacillus subtilis multidrug efflux transporter Bmr demonstrates 44% amino acid sequence identity with a product of the Staphylococcus aureus gene norA, which is responsible for clinically relevant resistance to fluoroquinolones [3].
• Rapid emergence of quinolone resistance in cirrhotic patients treated with norfloxacin to prevent spontaneous bacterial peritonitis [4].
• Comparison of the nucleotide sequences of the 150-bp fragments obtained by PCR revealed a point mutation in MTB2 leading to the substitution of histidine for aspartic acid at a position corresponding to residues involved in quinolone resistance in Escherichia coli (Asp87), Staphylococcus aureus (Glu88), and Campylobacter jejuni (Asp90) [5].

High impact information on norA

• NorC, a new efflux pump, like NorB, contributes to quinolone resistance that includes resistance to moxifloxacin and sparfloxacin in Staphylococcus aureus. norC expression, like that of norB and tet38, is negatively regulated by MgrA, and overexpression of both norC and norB contributes to the quinolone resistance phenotype of an mgrA mutant [6].
• A chromosomal norA::lacZ transcriptional fusion was constructed in different S. aureus strains, and allele replacement was used to define the relevance of promoter region sequences to norA expression [7].
• The flqB position of norA mRNA is part of a conserved imperfect inverted repeat; it is feasible that this motif could be a binding site for a norA regulatory protein [7].
• Disruption of a near-perfect inverted repeat or other putative regulatory protein binding sites did not affect norA transcription, but the repressive effect of mgrA overexpression was blunted in these mutants [7].
• We further characterized resistant strains selected with the combination of ciprofloxacin and WCK-1734 and found evidence to suggest the existence of novel mutational mechanisms for low-level quinolone resistance [8].

Chemical compound and disease context of norA

• The MIC of norfloxacin for the norA-disrupted mutant termed RDN1, obtained from quinolone-susceptible Staphylococcus aureus RN4220, was eightfold lower than that for RN4220 [1].
• Antibacterial activity of gatifloxacin (AM-1155, CG5501, BMS-206584), a newly developed fluoroquinolone, against sequentially acquired quinolone-resistant mutants and the norA transformant of Staphylococcus aureus [9].
• Fifteen strains of Escherichia coli with MICs of ciprofloxacin (CIP) between 0.015 and 256 micrograms/ml were examined for the presence of mutations in the quinolone resistance-determining region of the gyrA gene and in an analogous region of the parC gene [10].
• Emergence of quinolone resistance among clinical isolates of methicillin-resistant Staphylococcus aureus in Ontario, Canada [11].
• DX-619, a novel des-fluoro(6) quinolone manifesting low frequency of selection of resistant Staphylococcus aureus mutants: quinolone resistance beyond modification of type II topoisomerases [12].

Biological context of norA

• The norA gene of one transformant carried a single base deletion (ATACAAT to AACAAT; the deleted base is underlined) in the putative--10 Pribnow box resulting in a promoter down-regulatory mutation; a second plasmid had acquired a frameshift producing a null mutation at codon 112 [2].
• Determination of the nucleotide sequence of norA1199 revealed an encoded 388-amino-acid hydrophobic polypeptide 95% homologous with the norA-encoded protein [13].
• However, increased expression of norA can occur independently of this mutation, suggesting that a regulatory locus for this gene exists elsewhere on the chromosome [14].
• These mutations override the dual resistance-growth-inhibitory phenotype of high-copy-number norA plasmids [2].
• The results have implications for using the standard E. coli HB101 system to assess NorA function and potentially for plasmid-borne transmission of norA-mediated drug resistance [2].

Anatomical context of norA

• Increased transcription of the norA gene, leading to a greater quantity of the NorA protein within the cytoplasmic membrane, is felt to be the mechanism by which strains possessing such changes resist fluoroquinolones [15].

Associations of norA with chemical compounds

• Increased ciprofloxacin resistance correlated with norA transcript levels seen by Northern (RNA) analysis [2].
• Gatifloxacin possessed potent activity (MIC, 0.39 microg/ml) against the NorA-overproducing strain S. aureus NY12, the norA transformant, which was slightly lower than that against the parent strain SA113 [9].
• Increased expression of norA in S. aureus had no effect on the accumulation of moxifloxacin [16].
• RESULTS AND CONCLUSIONS: Exposure to several substrates significantly increased norA expression whereas salicylate and osmotic stress had no effect and no stable soluble factor affecting norA expression was detectable [17].
• The uptake of a hydrophilic quinolone, enoxacin, by S. aureus harboring a plasmid carrying the norA gene was about 50% that by the parent strain lacking the plasmid, but it increased to almost the same level as that by the latter strain with carbonyl cyanide m-chlorophenyl hydrazone [18].

Other interactions of norA

• Those properties of gatifloxacin likely explain its good activity against quinolone-resistant clinical isolates of S. aureus harboring the grlA, gyrA, and/or norA mutations [9].
• Two genes previously associated with fluoroquinolone resistance, the gyrA gene of DNA gyrase and the norA gene (associated with decreased drug accumulation), were localized to the G and D fragments, respectively [19].
• Expression studies with an arlS mutant revealed that the norA promoter is ArlS dependent [20].
• No mutations were observed in either the gyrB or norA genes [21].
• Overexpression of norR in sarA and agr mutants did not alter quinolone sensitivity or levels of norA transcription, indicating that the presence of these two global regulatory systems is necessary for NorR to affect the expression of norA [22].

Analytical, diagnostic and therapeutic context of norA

• Introduction of this mutation into the chromosome of a fluoroquinolone-susceptible strain by plasmid integration resulted in the minimum inhibitory concentrations of NorA substrates being increased, fluoroquinolone uptake being reduced, and norA expression being enhanced [23].
• The norA gene was amplified by PCR and cloned in the expression vector pTrcHis2 [24].
• Results from sequence analysis of the quinolone resistance-determining region of GyrA and of the corresponding region of GrlA, the DNA topoisomerase IV subunit, showed an alteration of Ser-80 to Tyr (corresponding to Ser-83 of Escherichia coli GyrA) or Glu-84 to Lys in GrlA of both low- and high-level quinolone-resistant mutants [25].
• Molecular epidemiology of quinolone resistance and comparative in vitro activities of new quinolones against European Staphylococcus aureus isolates [26].
• Pulsed field gel electrophoresis showed isolates related to USA 300, but methicillin-resistant strains had unusually high rates of quinolone resistance [27].

References