Nongenetic reactivation See reactivation and complementation

Nonidet P40 A non-ionic detergent comprising octylphenol ethylene condensate. Used to disrupt cells and viral membranes.

non-ionic detergent A detergent with no net surface charge, e.g. the triton series, Nonidet P40.

non-neutralizable fraction Neutralization by antibody is often not complete, a small fraction of the original infectivity

'Norwalk-like viruses'

resisting neutralization. This may be due to dissociation of the virus-antibody union or to the formation of infective complexes. The addition of anti-antibody neutralizes such complexes. See neutralization.

Synonym: persistent fraction.

non-nucleoside reverse transcriptase inhibitors (NNRTIs) Compounds which inhibit reverse transcriptase by directly binding to the enzyme at an allosteric site that influences the catalytic site. All NNRTIs studied to date rapidly give rise to a high level of resistance both in patients and in vitro, but in combination with a nucleoside inhibitor such as AZT, they appear to be much more effective. Detailed structural analysis has shown that these inhibitors bind to tyrosine residues on the enzyme surface, and the principal mutation to resistance is Y181C. See delaviridine, nevirapine.

non-permissive cells Cells in which a virus will not replicate. They may be permissive for one virus but not for another. The fact that they are non-permissive for virus replication may make them very suitable for demonstrating transformation.

non-producer (NP) cells Cells usually transformed, carrying all or part of a viral genome but not producing infective virus particles. In the case of Rous sarcoma virus-transformed NP cells, non-infective virus may be produced. Such cells are called 'L-R cells'.

nonsense codons Codons which do not code for an amino acid. They are UAA, UAG and UGA, sometimes referred to as the 'ochre', 'amber' and 'opal' codons, respectively. They are chain-terminating signals which can be introduced by a nonsense mutation, converting a sense codon into a stop codon. The nonsense mutation can arise by base substitution or frameshifting. It has been reported that in some contexts the UGA codon may encode the amino acid seleno-cysteine.

Synonym: stop codon.

Brenner S et al (1961) Nature 190, 576 Taylor EW (1994) J Med Chem 37, 2637

non-structural viral proteins Proteins coded for by the viral genome but not incorporated into the viral particle. They have a functional role during viral replication.

North Clett virus (NCLV) A serotype of Great Island virus in the genus Orbivirus. A member of the Kemerovo antigenic group.

North End virus (NEDV) A serotype of Great Island virus in the genus Orbivirus. A member of the Kemorovo antigenic group.

Northern blotting A procedure analogous to Southern blotting but involving the transfer of RNA rather than DNA on to nitrocellulose or activated paper sheets.

northern pike herpesvirus Synonym for esocid herpesvirus 1.

Northway virus (NORV) A serotype of Bunyamwera virus in the genus Bunyavirus. Isolated from Aedes sp in Alaska. Not known to cause disease in humans.

Norvir An inhibitor of HIV protease, licensed for human use. See ritonavir.

'Norwalk-like viruses' A genus in the family Caliciviridae, with only one species, Norwalk virus. There are a number of recognized strains, which form a distinct clade within the family Caliciviridae. Formerly called 'small round structured virus (SRSV) particles'. Associated with epidemic gastroenteritis, their importance has been increasingly recognized since the application of PCR-based diagnostic assays in investigations of outbreaks worldwide, and it is estimated that they cause 95% of cases of non-bacterial gastroenteritis in the USA. On 18 September 1998, the first documented case of transmission of a gastroenteritis virus between players on a football field occurred during a match between Duke and Florida State University teams. Duke lost 62-13, but Duke players, vomiting after a box lunch, infected several of their opponents.

Becker KM et al (2000) N Engl J Med 343,1223 Green J (2000) Virus Genes 20 , 227

Norwalk virus (NV)

Norwalk virus (NV) A species in the genus 'Norwalk-like viruses', the prototype of a group of related viruses, 27 nm in diameter, with a positive-strand RNA genome, found in the feces of patients with gastroenteritis. There are seven recognized strains: the Desert Shield, Hawaii, Lordsdale, Mexico, Norwalk, Snow Mountain and Southampton agents. The viruses cannot be propagated in cell cultures and no experimental animals have been found susceptible to the virus, although chimpanzees undergo subclinical infection and shed antigen in the feces. Swine cali-civirus is also closely related genetically and is a tentative species in the genus. The genome of Norwalk virus is 7.75kb in length with three open reading frames (ORFs): ORF-1 encodes a polyprotein precursor of the non-structural proteins including a replicase; ORF-2 encodes the major capsid protein; and ORF-3 encodes a small protein of unknown function. Norwalk and related viruses are a major cause of acute non-bacterial gastroenteritis. It is presently unclear whether protective immunity develops after Norwalk virus infection, and no specific treatment or antiviral therapy is available.

Synonym: epidemic gastroenteritis virus.

Berke T et al (1997) J Med Virol 52,419 Green J et al (2000) Virus Genes 20, 227 Jiang X et al (1993) Virology 195, 51 Lambden PR et al (1993) Science 259, 516

nosocomial infections Hospital-acquired infections, usually applied to patients, but includes hospital personnel as well.

Novirhabdovirus A genus in the family Rhabdoviridae comprised of fish viruses. Optimum virus replication temperatures are 15-28°C. The genome negative single-stranded RNA is 11.1 kb in length with six genes in the order 3'-N-P-M-G-NV-L-5'. The virus appears to be transmitted horizontally by waterborne virus. There are three species in the genus: Hirame rhabdovirus, Infectious hematopoietic necrosis virus (the type species) and Viral hemorrhagic septicemia virus (Egtved virus).

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