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A nucleoprotein filament is an extended assembly containing protein and nucleic acid. In RecA-family reactions, repeated subunits organize DNA and couple neighboring nucleotide states.

A nucleotide-binding leucine-rich repeat receptor, or NLR, is an intracellular immune-receptor protein characterized by a nucleotide-binding region and leucine-rich repeats. In plants, NLRs can detect pathogen-associated molecules or their effects and initiate defensive signaling.

An NLR can work as a sensor, a signaling helper or part of a paired NLR system. Different individuals can carry different receptor inventories, so one reference genome does not fully describe the species-wide pan-NLRome.^[ref]

Replacement of a bound nucleotide by another. A changed exchange rate can alter a protein state without proving direct competition at the nucleotide-binding site.

A nucleotide transition is a nucleotide substitution between two purine-derived bases or between two pyrimidine-derived bases. In DNA, the possibilities are $A\leftrightarrow G$ and $C\leftrightarrow T$: adenine exchanges with guanine, or cytosine with thymine. The chemical class is preserved; the identity of the base changes.^[ref]

From any specified DNA base, there is exactly one possible transition target and two nucleotide transversion targets. If all three targets were equally likely, transitions would comprise $1/3$ of changes. This is a counting baseline, not a biological law about mutation rates. The Kimura two-parameter model allows the rates for these two classes to differ.^[ref]

The classification does not determine a mutation's effect on a protein. A transition can preserve an amino acid, change it or introduce a stop codon, depending on the affected codon.

Nutritional immunity is a host-defense strategy that limits the supply of nutrients available to a pathogen. Restricting access to needed metal ions is one example: a host protein can bind a metal so that its presence in tissue does not imply that invading bacteria can acquire it.

A concrete experimental example is calprotectin in Staphylococcus aureus abscesses. Brian D. Corbin and colleagues found that neutrophil-derived calprotectin inhibited bacterial growth through chelation of manganese and zinc. Abscesses in mice lacking calprotectin contained more metal and supported greater bacterial proliferation. The combination of growth inhibition and the comparison with deficient animals connects metal sequestration to a host defense, rather than merely showing that a binding protein is present at an infection site.^[ref]

A simple two-pool model makes the distinction between total amount and accessibility explicit. Write the total metal concentration as $M_T=M_{\mathrm{free}}+M_{\mathrm{bound}}$. If $M_T=1\mu \mathrm{mol}{\mathrm{L}}^{-1}$ and $99\%$ is bound, the free concentration is only $0.01\mu \mathrm{mol}{\mathrm{L}}^{-1}$: one hundredth of the value if all the metal were free. These are illustrative numbers, not measurements from the experiment. Actual microbial access also depends on whether a bacterium can obtain metal from the bound pool; total metal concentration alone therefore does not measure nutrient availability.

Occludin is an integral membrane protein concentrated at tight junctions, where neighbouring epithelial or endothelial cells regulate passage between them. It has four membrane-spanning segments and a long cytoplasmic tail. Occludin is a junction component rather than a synonym for the entire sealing structure.

In 1993, Mikio Furuse and colleagues raised monoclonal antibodies against a junction-enriched fraction from chick liver. Immunoelectron microscopy placed the recognized protein directly at membrane-contact points of tight junctions; sequence analysis suggested four membrane crossings. These combined biochemical and spatial observations identified occludin.^[ref]

Later, expression of claudin-1 or claudin-2 in fibroblasts reconstituted extensive junction strands and recruited occludin. This separates the capacity to build a strand from the presence of one particular strand-associated protein.^[ref]

An oligonucleotide is a short nucleic acid chain: an oligomer made from nucleotides, rather than from free nucleobases alone. It may be DNA or RNA. The description concerns chain length and composition, not a guarantee that the chain is single-stranded, naturally occurring or translated into a protein. IUPAC's terminology explicitly connects the word with oligo and nucleotides.^[ref]

For an unmodified linear DNA example, $5^{\prime }\text{-}\mathrm{AGTCG}\text{-}{3}^{\prime }$ has five nucleotides and four connecting phosphodiester bonds. A fully complementary strand is $3^{\prime }\text{-}\mathrm{TCAGC}\text{-}{5}^{\prime }$. Writing that partner in the conventional $5^{\prime }$-to-$3^{\prime }$ direction gives $5^{\prime }\text{-}\mathrm{CGACT}\text{-}{3}^{\prime }$, the reverse complement. Reversing the letters without complementing them would answer a different question. The five-position count also allows $4^5=1024$ possible strings over the standard four-letter DNA alphabet.

Chemical synthesis makes this sequence-level control experimentally useful. In 1994, researchers prepared three-nucleotide building blocks representing codons for all 20 amino acids and incorporated them during automated DNA synthesis. Adding whole triplets allowed mixed oligonucleotides to vary at the codon level rather than only one base at a time.^[ref] This separates designing an individual chain from designing the distribution of sequences in a collection.

oligosaccharyltransferase transfers an assembled oligosaccharide from a lipid-linked donor to a protein acceptor. Recognition of an accessible acceptor sequence is distinct from synthesis of the donor.

O-linked glycosylation attaches a sugar through an oxygen atom on the acceptor, often a protein hydroxyl group. Several different pathways and donor specificities fall under this description.

A bacterial outer-membrane protein studied by NMR in optimized lipid nanodiscs.^[ref]

A gene whose altered activity can promote tumour development, commonly through activating or expression-increasing changes that retain a useful protein function.

An opioid receptor is a G protein-coupled receptor recognizing opioid ligands, including endogenous peptides and exogenous drugs. Major receptor classes differ in distribution and ligand responses. Their signalling commonly engages inhibitory G proteins, linking receptor activation to changes in cyclic-AMP production and ion-channel activity. ^[ref]

The same bound ligand need not produce identical effects in every assay. Receptor abundance, coupling efficiency and downstream amplification affect observed efficacy. Experiments comparing candidate opioid agonists showed why low intrinsic G-protein efficacy can help explain apparently favourable signalling profiles, rather than assuming that every difference establishes a uniquely selective pathway. ^[ref]

A light-sensitive receptor protein associated with a chromophore. Sequence, expression, chromophore and neural processing all contribute to visual performance.

An opsonin is a substance that coats a target and promotes recognition and uptake by a phagocyte. The name describes a biological role: IgG and complement system fragments such as C3b and iC3b can supply different molecular handles for phagocytosis.^[ref]

A 1983 experiment separated uptake from another response. Cultured monocytes in contact with appropriate fibronectin-containing surfaces could ingest complement-coated targets without the substantial hydrogen peroxide release seen in the IgG comparison.^[ref]

Thus, coating, receptor engagement, phagocytosis and oxidant production are distinguishable steps. Calling a protein an opsonin does not imply that it digests the target itself, or that every receptor-triggered response accompanies uptake. A useful assay measures binding and internalization separately: a particle attached to a cell membrane is not necessarily inside the biological cell.

The maintained combination of membrane composition, resident proteins, lumenal conditions, geometry and trafficking functions that distinguishes a compartment. Its molecules can turn over while this functional organization persists.

A test of whether a protein crosses a specified organelle boundary, with controls distinguishing import from surface binding or membrane rupture.

Organelle protein import brings externally synthesized proteins into an organelle. Many plastid proteins made from nuclear genes carry targeting information recognized by machinery that transports them across the envelope membranes. This delivery enables nuclear genes to supply functions inside a plastid, but the presence of targeting information is not itself evidence that a gene was acquired through endosymbiotic gene transfer. ^[ref]

For the endoplasmic reticulum, cotranslational protein translocation illustrates how organelle protein import can begin before a polypeptide chain is complete. Import into the ER lumen and insertion as a transmembrane protein are different outcomes; neither alone establishes subsequent secretion from the biological cell.

A protein complex that participates in recruitment of factors for replication origin licensing in eukaryotes.

Orthoretrovirinae is the subfamily of Retroviridae containing Lentivirus and several other retroviral genera. It includes human immunodeficiency virus and feline immunodeficiency virus. The family placement describes a replication strategy; it does not imply one host range or one disease outcome for all members.^[ref]

A typical virion packages two copies of its RNA genome. After entry and release of the viral core into a host cell, reverse transcription produces DNA that can become an integrated provirus. Proteolytic processing of a viral polyprotein also releases functional components during maturation.^[ref][ref]

An invented counting example clarifies the two-copy description. If each packaged RNA molecule has 10,000 nucleotides, the pair contains 20,000 RNA nucleotides in total. The length of one genome copy is still 10,000 nucleotides. A corresponding 10,000-base-pair double-stranded DNA molecule contains 20,000 DNA nucleotides, but its length is conventionally reported in base pairs. Copy count, nucleotide count and base-pair length answer different questions.

The subfamily's genera are distinguished in part by phylogenetic analysis of conserved reverse-transcriptase regions. A shared enzyme mechanism can coexist with differences in accessory proteins, host distribution and cellular targets. Observing the shared mechanism therefore provides a classification clue while leaving those other biological properties to be determined.^[ref]

Protein excretion that increases when upright and is reduced or absent in a recumbent sample. The positional pattern distinguishes it from persistent protein loss throughout the day.