4 Apr


transcriptase RNA polymerase (q.v.). Contrast with reverse transcriptase. transcription the formation of an RNA molecule upon a DNA template by complementary base pair- ing; mediated by RNA polymerase (q.v.). Transcrip- tion can proceed in opposite directions along differ- ent genes within the same chromosome. See Appendix C, Taylor, Hradecna, and Szybalski. transcription factors proteins characterized by DNA-binding segments that enable them to attach to chromosomes and regulate the transcription of specific genes. Arabidopsis has over three times more transcription factors than have been identified in Drosophila or Caenorhabditis, and of the 29 classes of transcription factors that have evolved in this spe- cies, 16 are unique to plants.

Transcription factors that contain zinc atoms are abundant in animals and fungi. For example, in Drosophila, Caenorhabditis, and Saccharomyces zinc-coordinating proteins make up 51%, 64%, and 56%, respectively, of their tran- scription factors. However, over 80% of Arabidopsis transcription factors lack zinc. See zinc finger pro- teins. transcription rate the speed at which ribonucleo- tides are polymerized into RNA chains by RNA polymerases. The transcription rate for mRNA mol- ecules in bacteria at 37°C is about 2,500 nucleotides per minute or about 14 codons per second. This transcription rate corresponds closely to the transla- tion rate (q.v.) in bacteria. Compare with replication rate. See gene expression. transcription unit the segment of DNA between the sites of initiation and termination of transcrip- tion by RNA polymerase; more than one gene may reside in a transcription unit.

A polycistronic mes- sage (q.v.) may be translated as such and the transla- tional product may be enzymatically cleaved later into two or more functional polypeptide chains. RNA is transcribed in a 5′ to 3′ direction from the template strand of the gene. However, when de- scribing the nucleotide sequence of a specific gene, the convention has been adopted to give it the same nucleotide sequence as the RNA transcript, except that each uridine is replaced by thymidine. Any ele- ment to the left of the initiation site is said to be “5′ to” or “upstream of” the gene.

Any element to the right is “3′ to” or “downstream of” the gene. Nucleo- tides are numbered starting at the initiation site and receive positive values to the right and negative val- ues to the left. Thus, in a specific gene the binding site for RNA polymerase II might include nucleo- tides −80 to −5, and the first intron might contain nucleotides +154 to +688. See Appendix C, 1967, Taylor et al.; coding strand, Miller trees, polyprotein, RNA polymerase, strand terminologies. transcriptome the totality of mRNAs being pro- duced by a cell at any given time. See metabolic con- trol levels. transcripton a unit of genetic transcription. transdetermination change in developmental fate of a cell or group of cells. See in vivo culturing of imaginal discs. transduced element the chromosomal fragment transferred during transduction. transductant a cell that has been transduced. See transduction. transduction the transfer of bacterial genetic ma- terial from one bacterium to another using a phage as a vector. In the case of restrictive or specialized transduction only a few bacterial genes are trans- ferred.

This is because the phage has a specific site of integration on the host chromosome, and only bacterial genes close to this site are transferred. In the case of generalized transduction the phage can in- tegrate at almost any position on the host chromo- some, and therefore almost any host gene can be transferred with the virus to a second bacterium. Transducing phage are usually defective in one or more normal phage functions, and may not be able to replicate in a new host cell unless aided by a nor- mal “helper” phage. See Appendix C, 1952, Zinder and Lederberg; abortive transduction. trans face See Golgi apparatus. transfection a term which is a hybrid between transformation and infection and refers to the experi- mental introduction of exogenous DNA or RNA into a cell or embryo, resulting in either a hereditary or a transient change in the affected cells. The term generally denotes one of the following. (1) The transformation (q.v.) of bacterial cells with purified viral nucleic acids, resulting in the production of the complete virus in the cells. (2) The transformation of cultured animal cells with purified DNA and in- corporation of this DNA into the cell’s genome (q.v.). In this case the term transfection has been adopted rather than transformation because the lat- ter term is used in another sense in studies involving cultured animal cells (i.e., the conversion of normal cells to a state of unregulated growth by oncogenic viruses). (3) The transformation of cells or embryos with single- or double-stranded RNA molecules, re- sulting in the expression of specific proteins or in the silencing of specific genes. Transfection with RNA


molecules produces changes that are not perma- nently transmissible. See Appendix C, 1972, Jackson, Symons and Berg; 1985, Smithies et al.; gene ther- apy, RNA interference, RNA transfection. transfectoma a myeloma cell into which immuno- globulin genes, either wild type or altered in vitro, have been transfected and expressed. Novel chim- eric immunoglobulin molecules can be produced by this technique, including unique combinations of heavy and light chains, or combinations of variable regions with different constant regions (both within and between species). Contrast with hybridoma. See immunoglobulin. transferases enzymes that catalyze the transfer of functional groups between donor and acceptor mol- ecules.

The most common molecules transferred are amino, acyl, phosphate, and glycosyl groups. transfer factor a dialyzable extract (lymphokine) from sensitized T lymphocytes that can transfer some types of cell-mediated immunity from one in- dividual to another. transferred immunity See adoptive transfer. transferrins See plasma transferrins. transfer RNA (tRNA) an RNA molecule that transfers an amino acid to a growing polypeptide chain during translation (q.v.). Transfer RNA mole- cules are among the smallest biologically active nu- cleic acids known. For example, an alanine transfer RNA isolated from yeast (shown above) contains 77 nucleotides and is folded back upon itself and kept in a “clover leaf” configuration by the characteristic pairings of the bases G to C and A to U.

All transfer RNAs attach to their amino acids by the 3′ end, which contains a terminal adenylic preceded by two cytidylic acids. The 5′ end always carries a terminal guanylic acid. The P (near 1) and the OH (near 77) show the positions of the phosphoric acid and hy- droxyl groups and the 5′ and 3′ ends of the mole- cule. Transfer RNAs contain several purines and py- rimidines not generally encountered in other RNA molecules. These rare bases (q.v.) are formed follow- ing transcription, since nuclei are known to contain enzymes that are capable of modifying certain bases on preformed RNA. The site recognized by tRNA synthetase is believed to be located in the neck re- gion adjacent to the dihydrouridine loop (see arrows).

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