tobacco See Nicotiana. tobacco mosaic virus a virus that produces lesions on the leaves of tobacco plants. An infected cell con- tains about 1 × 107 particles, and a kg of infected to- bacco leaves can yield several grams of TMV. This was the first virus to be discovered, the first to be seen with the electron microscope, the first to be pu- rified, the first shown to be built up of regularly ar-
ranged subunits, the first from which an infectious nucleic acid was obtained, and the first to have its method of assembly worked out in detail. The TMV particle is a rigid cylinder 18 nm in diameter and 300 nm long. Its genetic material is a single, positive sense strand of RNA that contains 6,400 nucleo- tides. There are four ORFs. The first two function in replication, the third facilitates movement of the virus from cell to cell, and the fourth encodes the 158 amino acids of the coat protein subunit. The protein making up each capsomere has sites that bind to the proteins of adjacent capsomeres and to the central RNA molecule.
This forms a right- handed helix of 8 nm diameter. There are about 17 protein subunits per turn of the helix, and each is attached to three nucleotides. The shell of a com- pleted virus contains 2,130 subunits. See Appendix C, 1892, Ivanovski; 1898, Beijerinck; 1935, Stanley; 1937, Bawden and Pirie; 1939, Kausche, Pfankuch, and Ruska; 1955, Fraenkel-Conrat and Williams; 1956, Gierer, Schramm, and Fraenkel-Conrat; 1959, Franklin, Caspar, and Klug; 1960, Tsugita et al.; 1982, Goelet et al.; Appendix F; Nicotiana, read- through, virus. tolerance See immunological tolerance. toluidine blue a metachromatic basic dye used in cytochemistry.
TOM See mitochondrial translocase. tomato See Lycopersicon esculentum. T24 oncogene a gene isolated from a line of hu- man bladder carcinoma cells. This oncogene appears to be homologous to the oncogene carried by the Harvey murine sarcoma virus (q.v.). The change that leads to the activation of the T24 oncogene is due to a single base substitution. See Appendix C, 1982, Reddy et al. tonofilaments synonymous with keratin filaments. See intermediate filaments, keratin. topoisomerase an enzyme that can interconvert topological isomers of DNA.
These enzymes alter DNA topology by changing the linking number of circular duplex DNAs or by interconverting knotted and catenated forms. Topoisomerase has replaced a number of earlier terms (DNA relaxing enzyme, swiv- elase, untwisting enzyme, nick-closing enzymes). To- poisomerases are divided into two classes: Type 1 en- zymes make a transient break in one strand of the duplex, whereas type 2 enzymes introduce transient double-strand breaks. During the relaxation of DNA by type 1 topoisomerases, an intact strand of the he- lix is passed through the break in its complementary strand. See mei-W68, SPO 11. topoisomerase poisons compounds such as Adriamycin (q.v.) that inhibit DNA polymerase II and cause cells to arrest at the DNA damage check- point (q.v.). Poisoned cells subsequently undergo ap- optosis (q.v.). topological isomers See linking number. topology the study of the properties of geometri- cal figures that are subjected to deformations such as bending or twisting. tormogen cell See trichogen cell. tortoiseshell cat a cat showing patches of orange and more darkly pigmented fur in its coat. The sex- linked gene O is responsible for the conversion of
eumelanin to phaeomelanin, which gives the fur an orange coloration. The O gene is epistatic to those autosomal genes, which give the coat a black or agouti color. Since the X chromosome is randomly inactivated in somatic cells during development, fe- males heterozygous for O will show the tortoiseshell phenotype. The term calico is sometimes applied to tortoiseshell females that also have patches of white fur. Such females also contain the dominant spotting gene S. totipotent descriptive of cells that have the capac- ity to differentiate into all of the cells of the adult organism. A zygote is normally totipotent, but most cells of the embryo become progressively restricted in this capacity as development progresses. See pluri- potent, stem cells. toxic shock syndrome (TSS) a rare illness caused by toxins produced by certain strains of Staphylococ- cus aureus and characterized by a sudden onset of fever, vomiting, diarrhea, sore throat, muscle aches, rash, and a drop in blood pressure. Other symptoms may appear as the illness progresses, leading to shock and, possibly, death (5% of all cases are fatal).
An alternate form, called Streptococcal toxic shock syn- drome (STSS) is caused by Streptococcus pyogenes and appears most often following a skin infection caused by these bacteria. See Staphylococcus, Streptococcus. toxin a poisonous substance elaborated by a mi- croorganism, as well as some fungal, plant, and ani- mal species. An example is alpha amanatin (q.v.). toxoid a poisonous protein that has been detoxi- fied without harm to its antigenic properties. TP53 the symbol for the tumor suppressor gene that encodes the p53 protein. TP53 contains 11 ex- ons and is located at 17p13.1. Two promoters have been identified at the 5′ end of TP53. The first is 100-250 bp upstream of exon 1, and the second is in intron 1. The gene was first isolated in 1983 from mouse cells and subsequently from human, rat, rhe- sus monkey, cat, sheep, horse, rabbit, hamster, chicken, and Xenopus. Mutations of TP53 occur in about half of all human cancers.
All of the small-cell lung carcinomas and the cancers found in patients with Li-Fraumeni syndrome (q.v.) have TP53 muta- tions. When introduced into cultured cells from hu- man cancers, the TP53 genes suppress their prolifer- ation. See Appendix C, 1983, Oren and Levine; 1990, Baker et al.; anti-oncogenes. T phages virulent viruses that attack Escherichia coli and other enterobacteria. The chromosome is contained in a large capsule of protein, and it is in- jected into the host through a hollow, tubular tail. The T-even viruses (T2, T4, and T6) have heads 80 × 110 nm, while the T-odd phages (T1, T3, T5, and T7) have isometric heads about 60 nm in diame- ter. T2, T4, and T6 phages differ in the cell wall receptors to which they bind. Their dsDNAs are lin- ear, cyclically permuted, and terminally redundant. The DNA contains 5-hydroxymethylcytosine (q.v.), rather than cytosine.
T2 was the first phage to be observed under the electron microscope (1942, Lu- ria and Anderson), and it was utilized in the famous Hershey-Chase experiment (1952). T4, with a ge- nome of 166 kbp, is the best-known of all the T phages. About 300 of its genes have been character- ized. Of the 43 phage-encoded proteins, 16 are used in constructing the head and 27 for the tail. T4 was the subject of the classic studies by Benzer (1955), Crick, Brenner et al. (1961), Brenner, Stretton, and Kaplan (1965), and Edgar and Wood (1966). In the T7 phage DNA replication involves a concatenation (q.v.) of multiple head-to-tail copies of the genome. See Appendix C, 1949, Hershey and Rotman; 1961, Rubinstein, Thomas, and Hershey; Appendix F; bac- teriophages, cyclically permuted sequences, rII, triplet code T4 RNA ligase, T7 RNA polymerase, viruses. tracer See labeled compound, radioactive isotope. traction fiber one of the fibers connecting the cen- tromeres of the various chromosomes to either cen- triole. See mitotic apparatus. trailer sequence a nontranslated segment at the 3′ end of mRNA following the signal that terminates translation, but exclusive of the poly-A tail. The trailer contains the binding site for the polyadenylat- ing enzyme. Some mRNAs contain blocks of nucleo- tides in their trailers that bind to receptor molecules that are localized within specific regions of cells.
See Appendix C, 1988, Macdonald and Stuhl; bicoid, exon, leader sequence, polyadenylation. trait See character. trans See cis-trans configuration. trans-acting locus a genetic element, such as a regulator gene (q.v.) that encodes a diffusible prod- uct that can influence the activity of other genes. Trans-acting genes can be on different DNA mole- cules from the genes they control. Contrast with cis- acting locus. transcribed spacer that part of a primary rRNA transcript that is discarded during the formation of functional RNAs of the ribosome (q.v.). transcript a length of R