26 Mar

Amber suppressor

Alu family the most common dispersed, repeated DNA sequence in the human genome. There are at least 750,000 Alu elements, each consisting of about 300 base pairs, accounting for 11% of human DNA. Each element is made up of two 130 base pair se- quences joined head to tail with a 32 base pair insert in the right-hand monomer. Alu sequences are tar- geted by cohesins (q.v.). The family name is derived from the fact that these sequences are cleaved by restriction  endonuclease  Alu  I.  See  human  gene maps, repetitious DNA.

Alzheimer disease (AD)  a  multifactorial  syn- drome that causes a devastating decline in mental ability and is accompanied by the appearance of am- yloid plaques in the cerebral cortex. These deposits were first observed in 1906 by a German physician, Alois Alzheimer, in brain tissue from a woman who had died of an unusual mental illness. Amyloid plaques contain aggregates of amyloid-beta-peptides (AβPs), and these are derived from an amyloid beta precursor protein (AβPP), which is encoded by a gene on human chromosome 21. Patients with tri- somy 21 (Down syndrome, q.v.) commonly develop AD by age 40. Familial, early-onset AD is often as- sociated with mutations of genes that encode pre- senilins (PS1 and PS2). The genes for PS1 and PS2 are located at 14q24.3 and 1q42.1, respectively. The proteins they encode are made of  467 and 448 amino acids, respectively, and they contain seven to nine transmembrane domains. Both proteins are bound to membranes and play a role in cutting AβPP into AβPs, some of which are toxic. A gene homologous to the PS1 gene has been isolated from nematodes. See Appendix C, 1995, Sherrington, St. George-Hyslop et al., Schellenberg et al.; Caenorhab- iditis elegans, neuregulins (NRGs).

Amanita phalloides  a poisonous mushroom which is the source of amatoxins and phallotoxins (both of which see). See Appendix A, Fungi, Basidiomycota.

Amastigote  See undulipodium.

amatoxins  a group of bicyclic octapeptides that are among the poisons produced by Amanita phal- loides (q.v.). These poisons inhibit transcription in eukaryotic cells because of their interaction with RNA polymerase II. However, they do not affect the RNA polymerases of mitochondria or chloroplasts. Alpha amanitin (formula, above) is an amatoxin most commonly used experimentally to inhibit tran- scription. See phallotoxins, RNA polymerase.

amaurosis  blindness occurring without an obvious lesion in the eye, as from a disease of the optic nerve or brain. The term is sometimes found in the early descriptions of hereditary diseases leading to blind- ness (e.g., Leber congenital amaurosis). See Leber he- reditary optic neuropathy (LHON).

amber codon  the mRNA triplet UAG that causes termination  of  protein  translation,  one  of  three “stop” codons. The terms amber and ochre (q.v.) orig- inated from a private laboratory joke and have noth- ing to do with colors.

Amberlite  trade name for a family of ion-exchange resins.

amber mutation  a mutation in which a polypep- tide chain is terminated prematurely. Amber muta- tions are the result of a base substitution that con- verts a codon specifying an amino acid into UAG, which signals chain termination. In certain strains of E.

coli amber mutations are suppressed. These strains contain a tRNA with an AUC anticodon, which in- serts an amino acid at the UAG site and hence per- mits translation to continue. See ochre mutation, nonsense mutation.

amber suppressor  any mutant gene coding for a tRNA whose anticodon can respond to the UAG stop codon by the insertion of an amino acid that renders the gene product at least partially functional.

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